Solidity est un langage statiquement typé, ce qui signifie que le type de chaque variable (état et locale) doit être spécifié. Solidity fournit plusieurs types élémentaires qui peuvent être combinés pour former des types complexes.

De plus, les types peuvent interagir entre eux dans des expressions contenant des opérateurs. Pour une référence rapide des différents opérateurs, voir Ordre de Préséance des Opérateurs.

Le concept de valeurs « indéfinies » ou « nulles » n’existe pas dans Solidity, mais les variables nouvellement déclarées ont toujours une valeur par défaut dépendant de son type. Pour gérer toute valeur inattendue, vous devez utiliser la fonction revert pour annuler toute la transaction, ou retourner un tuple avec une seconde valeur bool indiquant le succès.

Value Types

The following types are also called value types because variables of these types will always be passed by value, i.e. they are always copied when they are used as function arguments or in assignments.


bool: The possible values are constants true and false.


  • ! (logical negation)

  • && (logical conjunction, « and »)

  • || (logical disjunction, « or »)

  • == (equality)

  • != (inequality)

The operators || and && apply the common short-circuiting rules. This means that in the expression f(x) || g(y), if f(x) evaluates to true, g(y) will not be evaluated even if it may have side-effects.


int / uint: Signed and unsigned integers of various sizes. Keywords uint8 to uint256 in steps of 8 (unsigned of 8 up to 256 bits) and int8 to int256. uint and int are aliases for uint256 and int256, respectively.


  • Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)

  • Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation)

  • Shift operators: << (left shift), >> (right shift)

  • Arithmetic operators: +, -, unary - (only for signed integers), *, /, % (modulo), ** (exponentiation)

For an integer type X, you can use type(X).min and type(X).max to access the minimum and maximum value representable by the type.


Integers in Solidity are restricted to a certain range. For example, with uint32, this is 0 up to 2**32 - 1. There are two modes in which arithmetic is performed on these types: The « wrapping » or « unchecked » mode and the « checked » mode. By default, arithmetic is always « checked », which mean that if the result of an operation falls outside the value range of the type, the call is reverted through a failing assertion. You can switch to « unchecked » mode using unchecked { ... }. More details can be found in the section about unchecked.


The value of a comparison is the one obtained by comparing the integer value.

Bit operations

Bit operations are performed on the two’s complement representation of the number. This means that, for example ~int256(0) == int256(-1).


The result of a shift operation has the type of the left operand, truncating the result to match the type. The right operand must be of unsigned type, trying to shift by a signed type will produce a compilation error.

Shifts can be « simulated » using multiplication by powers of two in the following way. Note that the truncation to the type of the left operand is always performed at the end, but not mentioned explicitly.

  • x << y is equivalent to the mathematical expression x * 2**y.

  • x >> y is equivalent to the mathematical expression x / 2**y, rounded towards negative infinity.


Before version 0.5.0 a right shift x >> y for negative x was equivalent to the mathematical expression x / 2**y rounded towards zero, i.e., right shifts used rounding up (towards zero) instead of rounding down (towards negative infinity).


Overflow checks are never performed for shift operations as they are done for arithmetic operations. Instead, the result is always truncated.

Addition, Subtraction and Multiplication

Addition, subtraction and multiplication have the usual semantics, with two different modes in regard to over- and underflow:

By default, all arithmetic is checked for under- or overflow, but this can be disabled using the unchecked block, resulting in wrapping arithmetic. More details can be found in that section.

The expression -x is equivalent to (T(0) - x) where T is the type of x. It can only be applied to signed types. The value of -x can be positive if x is negative. There is another caveat also resulting from two’s complement representation:

If you have int x = type(int).min;, then -x does not fit the positive range. This means that unchecked { assert(-x == x); } works, and the expression -x when used in checked mode will result in a failing assertion.


Since the type of the result of an operation is always the type of one of the operands, division on integers always results in an integer. In Solidity, division rounds towards zero. This means that int256(-5) / int256(2) == int256(-2).

Note that in contrast, division on literals results in fractional values of arbitrary precision.


Division by zero causes a Panic error. This check can not be disabled through unchecked { ... }.


The expression type(int).min / (-1) is the only case where division causes an overflow. In checked arithmetic mode, this will cause a failing assertion, while in wrapping mode, the value will be type(int).min.


The modulo operation a % n yields the remainder r after the division of the operand a by the operand n, where q = int(a / n) and r = a - (n * q). This means that modulo results in the same sign as its left operand (or zero) and a % n == -(-a % n) holds for negative a:

  • int256(5) % int256(2) == int256(1)

  • int256(5) % int256(-2) == int256(1)

  • int256(-5) % int256(2) == int256(-1)

  • int256(-5) % int256(-2) == int256(-1)


Modulo with zero causes a Panic error. This check can not be disabled through unchecked { ... }.


Exponentiation is only available for unsigned types in the exponent. The resulting type of an exponentiation is always equal to the type of the base. Please take care that it is large enough to hold the result and prepare for potential assertion failures or wrapping behaviour.


In checked mode, exponentiation only uses the comparatively cheap exp opcode for small bases. For the cases of x**3, the expression x*x*x might be cheaper. In any case, gas cost tests and the use of the optimizer are advisable.


Note that 0**0 is defined by the EVM as 1.

Fixed Point Numbers


Fixed point numbers are not fully supported by Solidity yet. They can be declared, but cannot be assigned to or from.

fixed / ufixed: Signed and unsigned fixed point number of various sizes. Keywords ufixedMxN and fixedMxN, where M represents the number of bits taken by the type and N represents how many decimal points are available. M must be divisible by 8 and goes from 8 to 256 bits. N must be between 0 and 80, inclusive. ufixed and fixed are aliases for ufixed128x18 and fixed128x18, respectively.


  • Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)

  • Arithmetic operators: +, -, unary -, *, /, % (modulo)


The main difference between floating point (float and double in many languages, more precisely IEEE 754 numbers) and fixed point numbers is that the number of bits used for the integer and the fractional part (the part after the decimal dot) is flexible in the former, while it is strictly defined in the latter. Generally, in floating point almost the entire space is used to represent the number, while only a small number of bits define where the decimal point is.


The address type comes in two flavours, which are largely identical:

  • address: Holds a 20 byte value (size of an Ethereum address).

  • address payable: Same as address, but with the additional members transfer and send.

The idea behind this distinction is that address payable is an address you can send Ether to, while a plain address cannot be sent Ether.

Type conversions:

Implicit conversions from address payable to address are allowed, whereas conversions from address to address payable must be explicit via payable(<address>).

Explicit conversions to and from address are allowed for uint160, integer literals, bytes20 and contract types.

Only expressions of type address and contract-type can be converted to the type address payable via the explicit conversion payable(...). For contract-type, this conversion is only allowed if the contract can receive Ether, i.e., the contract either has a receive or a payable fallback function. Note that payable(0) is valid and is an exception to this rule.


If you need a variable of type address and plan to send Ether to it, then declare its type as address payable to make this requirement visible. Also, try to make this distinction or conversion as early as possible.


  • <=, <, ==, !=, >= and >


If you convert a type that uses a larger byte size to an address, for example bytes32, then the address is truncated. To reduce conversion ambiguity version 0.4.24 and higher of the compiler force you make the truncation explicit in the conversion. Take for example the 32-byte value 0x111122223333444455556666777788889999AAAABBBBCCCCDDDDEEEEFFFFCCCC.

You can use address(uint160(bytes20(b))), which results in 0x111122223333444455556666777788889999aAaa, or you can use address(uint160(uint256(b))), which results in 0x777788889999AaAAbBbbCcccddDdeeeEfFFfCcCc.


The distinction between address and address payable was introduced with version 0.5.0. Also starting from that version, contracts do not derive from the address type, but can still be explicitly converted to address or to address payable, if they have a receive or payable fallback function.

Members of Addresses

For a quick reference of all members of address, see Membres des types d’adresses.

  • balance and transfer

It is possible to query the balance of an address using the property balance and to send Ether (in units of wei) to a payable address using the transfer function:

address payable x = payable(0x123);
address myAddress = address(this);
if (x.balance < 10 && myAddress.balance >= 10) x.transfer(10);

The transfer function fails if the balance of the current contract is not large enough or if the Ether transfer is rejected by the receiving account. The transfer function reverts on failure.


If x is a contract address, its code (more specifically: its Fonction de réception d’Ether, if present, or otherwise its Fonction de repli, if present) will be executed together with the transfer call (this is a feature of the EVM and cannot be prevented). If that execution runs out of gas or fails in any way, the Ether transfer will be reverted and the current contract will stop with an exception.

  • send

Send is the low-level counterpart of transfer. If the execution fails, the current contract will not stop with an exception, but send will return false.


There are some dangers in using send: The transfer fails if the call stack depth is at 1024 (this can always be forced by the caller) and it also fails if the recipient runs out of gas. So in order to make safe Ether transfers, always check the return value of send, use transfer or even better: use a pattern where the recipient withdraws the money.

  • call, delegatecall and staticcall

In order to interface with contracts that do not adhere to the ABI, or to get more direct control over the encoding, the functions call, delegatecall and staticcall are provided. They all take a single bytes memory parameter and return the success condition (as a bool) and the returned data (bytes memory). The functions abi.encode, abi.encodePacked, abi.encodeWithSelector and abi.encodeWithSignature can be used to encode structured data.


bytes memory payload = abi.encodeWithSignature("register(string)", "MyName");
(bool success, bytes memory returnData) = address(nameReg).call(payload);


All these functions are low-level functions and should be used with care. Specifically, any unknown contract might be malicious and if you call it, you hand over control to that contract which could in turn call back into your contract, so be prepared for changes to your state variables when the call returns. The regular way to interact with other contracts is to call a function on a contract object (x.f()).


Previous versions of Solidity allowed these functions to receive arbitrary arguments and would also handle a first argument of type bytes4 differently. These edge cases were removed in version 0.5.0.

It is possible to adjust the supplied gas with the gas modifier:

address(nameReg).call{gas: 1000000}(abi.encodeWithSignature("register(string)", "MyName"));

Similarly, the supplied Ether value can be controlled too:

address(nameReg).call{value: 1 ether}(abi.encodeWithSignature("register(string)", "MyName"));

Lastly, these modifiers can be combined. Their order does not matter:

address(nameReg).call{gas: 1000000, value: 1 ether}(abi.encodeWithSignature("register(string)", "MyName"));

In a similar way, the function delegatecall can be used: the difference is that only the code of the given address is used, all other aspects (storage, balance, …) are taken from the current contract. The purpose of delegatecall is to use library code which is stored in another contract. The user has to ensure that the layout of storage in both contracts is suitable for delegatecall to be used.


Prior to homestead, only a limited variant called callcode was available that did not provide access to the original msg.sender and msg.value values. This function was removed in version 0.5.0.

Since byzantium staticcall can be used as well. This is basically the same as call, but will revert if the called function modifies the state in any way.

All three functions call, delegatecall and staticcall are very low-level functions and should only be used as a last resort as they break the type-safety of Solidity.

The gas option is available on all three methods, while the value option is only available on call.


It is best to avoid relying on hardcoded gas values in your smart contract code, regardless of whether state is read from or written to, as this can have many pitfalls. Also, access to gas might change in the future.


All contracts can be converted to address type, so it is possible to query the balance of the current contract using address(this).balance.

Contract Types

Every contract defines its own type. You can implicitly convert contracts to contracts they inherit from. Contracts can be explicitly converted to and from the address type.

Explicit conversion to and from the address payable type is only possible if the contract type has a receive or payable fallback function. The conversion is still performed using address(x). If the contract type does not have a receive or payable fallback function, the conversion to address payable can be done using payable(address(x)). You can find more information in the section about the address type.


Before version 0.5.0, contracts directly derived from the address type and there was no distinction between address and address payable.

If you declare a local variable of contract type (MyContract c), you can call functions on that contract. Take care to assign it from somewhere that is the same contract type.

You can also instantiate contracts (which means they are newly created). You can find more details in the “Contracts via new” section.

The data representation of a contract is identical to that of the address type and this type is also used in the ABI.

Contracts do not support any operators.

The members of contract types are the external functions of the contract including any state variables marked as public.

For a contract C you can use type(C) to access type information about the contract.

Fixed-size byte arrays

The value types bytes1, bytes2, bytes3, …, bytes32 hold a sequence of bytes from one to up to 32.


  • Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)

  • Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation)

  • Shift operators: << (left shift), >> (right shift)

  • Index access: If x is of type bytesI, then x[k] for 0 <= k < I returns the k th byte (read-only).

The shifting operator works with unsigned integer type as right operand (but returns the type of the left operand), which denotes the number of bits to shift by. Shifting by a signed type will produce a compilation error.


  • .length yields the fixed length of the byte array (read-only).


The type bytes1[] is an array of bytes, but due to padding rules, it wastes 31 bytes of space for each element (except in storage). It is better to use the bytes type instead.


Prior to version 0.8.0, byte used to be an alias for bytes1.

Dynamically-sized byte array


Dynamically-sized byte array, see Arrays. Not a value-type!


Dynamically-sized UTF-8-encoded string, see Arrays. Not a value-type!

Address Literals

Hexadecimal literals that pass the address checksum test, for example 0xdCad3a6d3569DF655070DEd06cb7A1b2Ccd1D3AF are of address type. Hexadecimal literals that are between 39 and 41 digits long and do not pass the checksum test produce an error. You can prepend (for integer types) or append (for bytesNN types) zeros to remove the error.


The mixed-case address checksum format is defined in EIP-55.

Rational and Integer Literals

Integer literals are formed from a sequence of digits in the range 0-9. They are interpreted as decimals. For example, 69 means sixty nine. Octal literals do not exist in Solidity and leading zeros are invalid.

Decimal fractional literals are formed by a . with at least one number on one side. Examples include 1., .1 and 1.3.

Scientific notation in the form of 2e10 is also supported, where the mantissa can be fractional but the exponent has to be an integer. The literal MeE is equivalent to M * 10**E. Examples include 2e10, -2e10, 2e-10, 2.5e1.

Underscores can be used to separate the digits of a numeric literal to aid readability. For example, decimal 123_000, hexadecimal 0x2eff_abde, scientific decimal notation 1_2e345_678 are all valid. Underscores are only allowed between two digits and only one consecutive underscore is allowed. There is no additional semantic meaning added to a number literal containing underscores, the underscores are ignored.

Number literal expressions retain arbitrary precision until they are converted to a non-literal type (i.e. by using them together with a non-literal expression or by explicit conversion). This means that computations do not overflow and divisions do not truncate in number literal expressions.

For example, (2**800 + 1) - 2**800 results in the constant 1 (of type uint8) although intermediate results would not even fit the machine word size. Furthermore, .5 * 8 results in the integer 4 (although non-integers were used in between).

Any operator that can be applied to integers can also be applied to number literal expressions as long as the operands are integers. If any of the two is fractional, bit operations are disallowed and exponentiation is disallowed if the exponent is fractional (because that might result in a non-rational number).

Shifts and exponentiation with literal numbers as left (or base) operand and integer types as the right (exponent) operand are always performed in the uint256 (for non-negative literals) or int256 (for a negative literals) type, regardless of the type of the right (exponent) operand.


Division on integer literals used to truncate in Solidity prior to version 0.4.0, but it now converts into a rational number, i.e. 5 / 2 is not equal to 2, but to 2.5.


Solidity has a number literal type for each rational number. Integer literals and rational number literals belong to number literal types. Moreover, all number literal expressions (i.e. the expressions that contain only number literals and operators) belong to number literal types. So the number literal expressions 1 + 2 and 2 + 1 both belong to the same number literal type for the rational number three.


Number literal expressions are converted into a non-literal type as soon as they are used with non-literal expressions. Disregarding types, the value of the expression assigned to b below evaluates to an integer. Because a is of type uint128, the expression 2.5 + a has to have a proper type, though. Since there is no common type for the type of 2.5 and uint128, the Solidity compiler does not accept this code.

uint128 a = 1;
uint128 b = 2.5 + a + 0.5;

String Literals and Types

String literals are written with either double or single-quotes ("foo" or 'bar'), and they can also be split into multiple consecutive parts ("foo" "bar" is equivalent to "foobar") which can be helpful when dealing with long strings. They do not imply trailing zeroes as in C; "foo" represents three bytes, not four. As with integer literals, their type can vary, but they are implicitly convertible to bytes1, …, bytes32, if they fit, to bytes and to string.

For example, with bytes32 samevar = "stringliteral" the string literal is interpreted in its raw byte form when assigned to a bytes32 type.

String literals can only contain printable ASCII characters, which means the characters between and including 0x20 .. 0x7E.

Additionally, string literals also support the following escape characters:

  • \<newline> (escapes an actual newline)

  • \\ (backslash)

  • \' (single quote)

  • \" (double quote)

  • \n (newline)

  • \r (carriage return)

  • \t (tab)

  • \xNN (hex escape, see below)

  • \uNNNN (unicode escape, see below)

\xNN takes a hex value and inserts the appropriate byte, while \uNNNN takes a Unicode codepoint and inserts an UTF-8 sequence.


Until version 0.8.0 there were three additional escape sequences: \b, \f and \v. They are commonly available in other languages but rarely needed in practice. If you do need them, they can still be inserted via hexadecimal escapes, i.e. \x08, \x0c and \x0b, respectively, just as any other ASCII character.

The string in the following example has a length of ten bytes. It starts with a newline byte, followed by a double quote, a single quote a backslash character and then (without separator) the character sequence abcdef.


Any Unicode line terminator which is not a newline (i.e. LF, VF, FF, CR, NEL, LS, PS) is considered to terminate the string literal. Newline only terminates the string literal if it is not preceded by a \.

Unicode Literals

While regular string literals can only contain ASCII, Unicode literals – prefixed with the keyword unicode – can contain any valid UTF-8 sequence. They also support the very same escape sequences as regular string literals.

string memory a = unicode"Hello 😃";

Hexadecimal Literals

Hexadecimal literals are prefixed with the keyword hex and are enclosed in double or single-quotes (hex"001122FF", hex'0011_22_FF'). Their content must be hexadecimal digits which can optionally use a single underscore as separator between byte boundaries. The value of the literal will be the binary representation of the hexadecimal sequence.

Multiple hexadecimal literals separated by whitespace are concatenated into a single literal: hex"00112233" hex"44556677" is equivalent to hex"0011223344556677"

Hexadecimal literals behave like string literals and have the same convertibility restrictions.


Enums are one way to create a user-defined type in Solidity. They are explicitly convertible to and from all integer types but implicit conversion is not allowed. The explicit conversion from integer checks at runtime that the value lies inside the range of the enum and causes a Panic error otherwise. Enums require at least one member, and its default value when declared is the first member. Enums cannot have more than 256 members.

The data representation is the same as for enums in C: The options are represented by subsequent unsigned integer values starting from 0.

Using type(NameOfEnum).min and type(NameOfEnum).max you can get the smallest and respectively largest value of the given enum.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.8;

contract test {
    enum ActionChoices { GoLeft, GoRight, GoStraight, SitStill }
    ActionChoices choice;
    ActionChoices constant defaultChoice = ActionChoices.GoStraight;

    function setGoStraight() public {
        choice = ActionChoices.GoStraight;

    // Since enum types are not part of the ABI, the signature of "getChoice"
    // will automatically be changed to "getChoice() returns (uint8)"
    // for all matters external to Solidity.
    function getChoice() public view returns (ActionChoices) {
        return choice;

    function getDefaultChoice() public pure returns (uint) {
        return uint(defaultChoice);

    function getLargestValue() public pure returns (ActionChoices) {
        return type(ActionChoices).max;

    function getSmallestValue() public pure returns (ActionChoices) {
        return type(ActionChoices).min;


Enums can also be declared on the file level, outside of contract or library definitions.

User Defined Value Types

A user defined value type allows creating a zero cost abstraction over an elementary value type. This is similar to an alias, but with stricter type requirements.

A user defined value type is defined using type C is V, where C is the name of the newly introduced type and V has to be a built-in value type (the « underlying type »). The function C.wrap is used to convert from the underlying type to the custom type. Similarly, the function C.unwrap is used to convert from the custom type to the underlying type.

The type C does not have any operators or bound member functions. In particular, even the operator == is not defined. Explicit and implicit conversions to and from other types are disallowed.

The data-representation of values of such types are inherited from the underlying type and the underlying type is also used in the ABI.

The following example illustrates a custom type UFixed256x18 representing a decimal fixed point type with 18 decimals and a minimal library to do arithmetic operations on the type.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.8;

// Represent a 18 decimal, 256 bit wide fixed point type using a user defined value type.
type UFixed256x18 is uint256;

/// A minimal library to do fixed point operations on UFixed256x18.
library FixedMath {
    uint constant multiplier = 10**18;

    /// Adds two UFixed256x18 numbers. Reverts on overflow, relying on checked
    /// arithmetic on uint256.
    function add(UFixed256x18 a, UFixed256x18 b) internal pure returns (UFixed256x18) {
        return UFixed256x18.wrap(UFixed256x18.unwrap(a) + UFixed256x18.unwrap(b));
    /// Multiplies UFixed256x18 and uint256. Reverts on overflow, relying on checked
    /// arithmetic on uint256.
    function mul(UFixed256x18 a, uint256 b) internal pure returns (UFixed256x18) {
        return UFixed256x18.wrap(UFixed256x18.unwrap(a) * b);
    /// Take the floor of a UFixed256x18 number.
    /// @return the largest integer that does not exceed `a`.
    function floor(UFixed256x18 a) internal pure returns (uint256) {
        return UFixed256x18.unwrap(a) / multiplier;
    /// Turns a uint256 into a UFixed256x18 of the same value.
    /// Reverts if the integer is too large.
    function toUFixed256x18(uint256 a) internal pure returns (UFixed256x18) {
        return UFixed256x18.wrap(a * multiplier);

Notice how UFixed256x18.wrap and FixedMath.toUFixed256x18 have the same signature but perform two very different operations: The UFixed256x18.wrap function returns a UFixed256x18 that has the same data representation as the input, whereas toUFixed256x18 returns a UFixed256x18 that has the same numerical value.

Function Types

Function types are the types of functions. Variables of function type can be assigned from functions and function parameters of function type can be used to pass functions to and return functions from function calls. Function types come in two flavours - internal and external functions:

Internal functions can only be called inside the current contract (more specifically, inside the current code unit, which also includes internal library functions and inherited functions) because they cannot be executed outside of the context of the current contract. Calling an internal function is realized by jumping to its entry label, just like when calling a function of the current contract internally.

External functions consist of an address and a function signature and they can be passed via and returned from external function calls.

Function types are notated as follows:

function (<parameter types>) {internal|external} [pure|view|payable] [returns (<return types>)]

In contrast to the parameter types, the return types cannot be empty - if the function type should not return anything, the whole returns (<return types>) part has to be omitted.

By default, function types are internal, so the internal keyword can be omitted. Note that this only applies to function types. Visibility has to be specified explicitly for functions defined in contracts, they do not have a default.


A function type A is implicitly convertible to a function type B if and only if their parameter types are identical, their return types are identical, their internal/external property is identical and the state mutability of A is more restrictive than the state mutability of B. In particular:

  • pure functions can be converted to view and non-payable functions

  • view functions can be converted to non-payable functions

  • payable functions can be converted to non-payable functions

No other conversions between function types are possible.

The rule about payable and non-payable might be a little confusing, but in essence, if a function is payable, this means that it also accepts a payment of zero Ether, so it also is non-payable. On the other hand, a non-payable function will reject Ether sent to it, so non-payable functions cannot be converted to payable functions.

If a function type variable is not initialised, calling it results in a Panic error. The same happens if you call a function after using delete on it.

If external function types are used outside of the context of Solidity, they are treated as the function type, which encodes the address followed by the function identifier together in a single bytes24 type.

Note that public functions of the current contract can be used both as an internal and as an external function. To use f as an internal function, just use f, if you want to use its external form, use this.f.

A function of an internal type can be assigned to a variable of an internal function type regardless of where it is defined. This includes private, internal and public functions of both contracts and libraries as well as free functions. External function types, on the other hand, are only compatible with public and external contract functions. Libraries are excluded because they require a delegatecall and use a different ABI convention for their selectors. Functions declared in interfaces do not have definitions so pointing at them does not make sense either.


External (or public) functions have the following members:

  • .address returns the address of the contract of the function.

  • .selector returns the ABI function selector


External (or public) functions used to have the additional members .gas(uint) and .value(uint). These were deprecated in Solidity 0.6.2 and removed in Solidity 0.7.0. Instead use {gas: ...} and {value: ...} to specify the amount of gas or the amount of wei sent to a function, respectively. See External Function Calls for more information.

Example that shows how to use the members:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.4 <0.9.0;

contract Example {
    function f() public payable returns (bytes4) {
        assert(this.f.address == address(this));
        return this.f.selector;

    function g() public {
        this.f{gas: 10, value: 800}();

Example that shows how to use internal function types:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

library ArrayUtils {
    // internal functions can be used in internal library functions because
    // they will be part of the same code context
    function map(uint[] memory self, function (uint) pure returns (uint) f)
        returns (uint[] memory r)
        r = new uint[](self.length);
        for (uint i = 0; i < self.length; i++) {
            r[i] = f(self[i]);

    function reduce(
        uint[] memory self,
        function (uint, uint) pure returns (uint) f
        returns (uint r)
        r = self[0];
        for (uint i = 1; i < self.length; i++) {
            r = f(r, self[i]);

    function range(uint length) internal pure returns (uint[] memory r) {
        r = new uint[](length);
        for (uint i = 0; i < r.length; i++) {
            r[i] = i;

contract Pyramid {
    using ArrayUtils for *;

    function pyramid(uint l) public pure returns (uint) {
        return ArrayUtils.range(l).map(square).reduce(sum);

    function square(uint x) internal pure returns (uint) {
        return x * x;

    function sum(uint x, uint y) internal pure returns (uint) {
        return x + y;

Another example that uses external function types:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.22 <0.9.0;

contract Oracle {
    struct Request {
        bytes data;
        function(uint) external callback;

    Request[] private requests;
    event NewRequest(uint);

    function query(bytes memory data, function(uint) external callback) public {
        requests.push(Request(data, callback));
        emit NewRequest(requests.length - 1);

    function reply(uint requestID, uint response) public {
        // Here goes the check that the reply comes from a trusted source

contract OracleUser {
    Oracle constant private ORACLE_CONST = Oracle(address(0x00000000219ab540356cBB839Cbe05303d7705Fa)); // known contract
    uint private exchangeRate;

    function buySomething() public {
        ORACLE_CONST.query("USD", this.oracleResponse);

    function oracleResponse(uint response) public {
            msg.sender == address(ORACLE_CONST),
            "Only oracle can call this."
        exchangeRate = response;


Lambda or inline functions are planned but not yet supported.

Reference Types

Values of reference type can be modified through multiple different names. Contrast this with value types where you get an independent copy whenever a variable of value type is used. Because of that, reference types have to be handled more carefully than value types. Currently, reference types comprise structs, arrays and mappings. If you use a reference type, you always have to explicitly provide the data area where the type is stored: memory (whose lifetime is limited to an external function call), storage (the location where the state variables are stored, where the lifetime is limited to the lifetime of a contract) or calldata (special data location that contains the function arguments).

An assignment or type conversion that changes the data location will always incur an automatic copy operation, while assignments inside the same data location only copy in some cases for storage types.

Data location

Every reference type has an additional annotation, the « data location », about where it is stored. There are three data locations: memory, storage and calldata. Calldata is a non-modifiable, non-persistent area where function arguments are stored, and behaves mostly like memory.


If you can, try to use calldata as data location because it will avoid copies and also makes sure that the data cannot be modified. Arrays and structs with calldata data location can also be returned from functions, but it is not possible to allocate such types.


Prior to version 0.6.9 data location for reference-type arguments was limited to calldata in external functions, memory in public functions and either memory or storage in internal and private ones. Now memory and calldata are allowed in all functions regardless of their visibility.


Prior to version 0.5.0 the data location could be omitted, and would default to different locations depending on the kind of variable, function type, etc., but all complex types must now give an explicit data location.

Data location and assignment behaviour

Data locations are not only relevant for persistency of data, but also for the semantics of assignments:

  • Assignments between storage and memory (or from calldata) always create an independent copy.

  • Assignments from memory to memory only create references. This means that changes to one memory variable are also visible in all other memory variables that refer to the same data.

  • Assignments from storage to a local storage variable also only assign a reference.

  • All other assignments to storage always copy. Examples for this case are assignments to state variables or to members of local variables of storage struct type, even if the local variable itself is just a reference.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.9.0;

contract C {
    // The data location of x is storage.
    // This is the only place where the
    // data location can be omitted.
    uint[] x;

    // The data location of memoryArray is memory.
    function f(uint[] memory memoryArray) public {
        x = memoryArray; // works, copies the whole array to storage
        uint[] storage y = x; // works, assigns a pointer, data location of y is storage
        y[7]; // fine, returns the 8th element
        y.pop(); // fine, modifies x through y
        delete x; // fine, clears the array, also modifies y
        // The following does not work; it would need to create a new temporary /
        // unnamed array in storage, but storage is "statically" allocated:
        // y = memoryArray;
        // This does not work either, since it would "reset" the pointer, but there
        // is no sensible location it could point to.
        // delete y;
        g(x); // calls g, handing over a reference to x
        h(x); // calls h and creates an independent, temporary copy in memory

    function g(uint[] storage) internal pure {}
    function h(uint[] memory) public pure {}


Arrays can have a compile-time fixed size, or they can have a dynamic size.

The type of an array of fixed size k and element type T is written as T[k], and an array of dynamic size as T[].

For example, an array of 5 dynamic arrays of uint is written as uint[][5]. The notation is reversed compared to some other languages. In Solidity, X[3] is always an array containing three elements of type X, even if X is itself an array. This is not the case in other languages such as C.

Indices are zero-based, and access is in the opposite direction of the declaration.

For example, if you have a variable uint[][5] memory x, you access the seventh uint in the third dynamic array using x[2][6], and to access the third dynamic array, use x[2]. Again, if you have an array T[5] a for a type T that can also be an array, then a[2] always has type T.

Array elements can be of any type, including mapping or struct. The general restrictions for types apply, in that mappings can only be stored in the storage data location and publicly-visible functions need parameters that are ABI types.

It is possible to mark state variable arrays public and have Solidity create a getter. The numeric index becomes a required parameter for the getter.

Accessing an array past its end causes a failing assertion. Methods .push() and .push(value) can be used to append a new element at the end of the array, where .push() appends a zero-initialized element and returns a reference to it.

bytes and string as Arrays

Variables of type bytes and string are special arrays. The bytes type is similar to bytes1[], but it is packed tightly in calldata and memory. string is equal to bytes but does not allow length or index access.

Solidity does not have string manipulation functions, but there are third-party string libraries. You can also compare two strings by their keccak256-hash using keccak256(abi.encodePacked(s1)) == keccak256(abi.encodePacked(s2)) and concatenate two strings using bytes.concat(bytes(s1), bytes(s2)).

You should use bytes over bytes1[] because it is cheaper, since using bytes1[] in memory adds 31 padding bytes between the elements. Note that in storage, the padding is absent due to tight packing, see bytes and string. As a general rule, use bytes for arbitrary-length raw byte data and string for arbitrary-length string (UTF-8) data. If you can limit the length to a certain number of bytes, always use one of the value types bytes1 to bytes32 because they are much cheaper.


If you want to access the byte-representation of a string s, use bytes(s).length / bytes(s)[7] = 'x';. Keep in mind that you are accessing the low-level bytes of the UTF-8 representation, and not the individual characters.

bytes.concat function

You can concatenate a variable number of bytes or bytes1 ... bytes32 using bytes.concat. The function returns a single bytes memory array that contains the contents of the arguments without padding. If you want to use string parameters or other types, you need to convert them to bytes or bytes1/…/bytes32 first.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.4;

contract C {
    bytes s = "Storage";
    function f(bytes calldata c, string memory m, bytes16 b) public view {
        bytes memory a = bytes.concat(s, c, c[:2], "Literal", bytes(m), b);
        assert((s.length + c.length + 2 + 7 + bytes(m).length + 16) == a.length);

If you call bytes.concat without arguments it will return an empty bytes array.

Allocating Memory Arrays

Memory arrays with dynamic length can be created using the new operator. As opposed to storage arrays, it is not possible to resize memory arrays (e.g. the .push member functions are not available). You either have to calculate the required size in advance or create a new memory array and copy every element.

As all variables in Solidity, the elements of newly allocated arrays are always initialized with the default value.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

contract C {
    function f(uint len) public pure {
        uint[] memory a = new uint[](7);
        bytes memory b = new bytes(len);
        assert(a.length == 7);
        assert(b.length == len);
        a[6] = 8;

Array Literals

An array literal is a comma-separated list of one or more expressions, enclosed in square brackets ([...]). For example [1, a, f(3)]. The type of the array literal is determined as follows:

It is always a statically-sized memory array whose length is the number of expressions.

The base type of the array is the type of the first expression on the list such that all other expressions can be implicitly converted to it. It is a type error if this is not possible.

It is not enough that there is a type all the elements can be converted to. One of the elements has to be of that type.

In the example below, the type of [1, 2, 3] is uint8[3] memory, because the type of each of these constants is uint8. If you want the result to be a uint[3] memory type, you need to convert the first element to uint.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

contract C {
    function f() public pure {
        g([uint(1), 2, 3]);
    function g(uint[3] memory) public pure {
        // ...

The array literal [1, -1] is invalid because the type of the first expression is uint8 while the type of the second is int8 and they cannot be implicitly converted to each other. To make it work, you can use [int8(1), -1], for example.

Since fixed-size memory arrays of different type cannot be converted into each other (even if the base types can), you always have to specify a common base type explicitly if you want to use two-dimensional array literals:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

contract C {
    function f() public pure returns (uint24[2][4] memory) {
        uint24[2][4] memory x = [[uint24(0x1), 1], [0xffffff, 2], [uint24(0xff), 3], [uint24(0xffff), 4]];
        // The following does not work, because some of the inner arrays are not of the right type.
        // uint[2][4] memory x = [[0x1, 1], [0xffffff, 2], [0xff, 3], [0xffff, 4]];
        return x;

Fixed size memory arrays cannot be assigned to dynamically-sized memory arrays, i.e. the following is not possible:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;

// This will not compile.
contract C {
    function f() public {
        // The next line creates a type error because uint[3] memory
        // cannot be converted to uint[] memory.
        uint[] memory x = [uint(1), 3, 4];

It is planned to remove this restriction in the future, but it creates some complications because of how arrays are passed in the ABI.

If you want to initialize dynamically-sized arrays, you have to assign the individual elements:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

contract C {
    function f() public pure {
        uint[] memory x = new uint[](3);
        x[0] = 1;
        x[1] = 3;
        x[2] = 4;

Array Members


Arrays have a length member that contains their number of elements. The length of memory arrays is fixed (but dynamic, i.e. it can depend on runtime parameters) once they are created.


Dynamic storage arrays and bytes (not string) have a member function called push() that you can use to append a zero-initialised element at the end of the array. It returns a reference to the element, so that it can be used like x.push().t = 2 or x.push() = b.


Dynamic storage arrays and bytes (not string) have a member function called push(x) that you can use to append a given element at the end of the array. The function returns nothing.


Dynamic storage arrays and bytes (not string) have a member function called pop that you can use to remove an element from the end of the array. This also implicitly calls delete on the removed element.


Increasing the length of a storage array by calling push() has constant gas costs because storage is zero-initialised, while decreasing the length by calling pop() has a cost that depends on the « size » of the element being removed. If that element is an array, it can be very costly, because it includes explicitly clearing the removed elements similar to calling delete on them.


To use arrays of arrays in external (instead of public) functions, you need to activate ABI coder v2.


In EVM versions before Byzantium, it was not possible to access dynamic arrays return from function calls. If you call functions that return dynamic arrays, make sure to use an EVM that is set to Byzantium mode.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;

contract ArrayContract {
    uint[2**20] m_aLotOfIntegers;
    // Note that the following is not a pair of dynamic arrays but a
    // dynamic array of pairs (i.e. of fixed size arrays of length two).
    // Because of that, T[] is always a dynamic array of T, even if T
    // itself is an array.
    // Data location for all state variables is storage.
    bool[2][] m_pairsOfFlags;

    // newPairs is stored in memory - the only possibility
    // for public contract function arguments
    function setAllFlagPairs(bool[2][] memory newPairs) public {
        // assignment to a storage array performs a copy of ``newPairs`` and
        // replaces the complete array ``m_pairsOfFlags``.
        m_pairsOfFlags = newPairs;

    struct StructType {
        uint[] contents;
        uint moreInfo;
    StructType s;

    function f(uint[] memory c) public {
        // stores a reference to ``s`` in ``g``
        StructType storage g = s;
        // also changes ``s.moreInfo``.
        g.moreInfo = 2;
        // assigns a copy because ``g.contents``
        // is not a local variable, but a member of
        // a local variable.
        g.contents = c;

    function setFlagPair(uint index, bool flagA, bool flagB) public {
        // access to a non-existing index will throw an exception
        m_pairsOfFlags[index][0] = flagA;
        m_pairsOfFlags[index][1] = flagB;

    function changeFlagArraySize(uint newSize) public {
        // using push and pop is the only way to change the
        // length of an array
        if (newSize < m_pairsOfFlags.length) {
            while (m_pairsOfFlags.length > newSize)
        } else if (newSize > m_pairsOfFlags.length) {
            while (m_pairsOfFlags.length < newSize)

    function clear() public {
        // these clear the arrays completely
        delete m_pairsOfFlags;
        delete m_aLotOfIntegers;
        // identical effect here
        m_pairsOfFlags = new bool[2][](0);

    bytes m_byteData;

    function byteArrays(bytes memory data) public {
        // byte arrays ("bytes") are different as they are stored without padding,
        // but can be treated identical to "uint8[]"
        m_byteData = data;
        for (uint i = 0; i < 7; i++)
        m_byteData[3] = 0x08;
        delete m_byteData[2];

    function addFlag(bool[2] memory flag) public returns (uint) {
        return m_pairsOfFlags.length;

    function createMemoryArray(uint size) public pure returns (bytes memory) {
        // Dynamic memory arrays are created using `new`:
        uint[2][] memory arrayOfPairs = new uint[2][](size);

        // Inline arrays are always statically-sized and if you only
        // use literals, you have to provide at least one type.
        arrayOfPairs[0] = [uint(1), 2];

        // Create a dynamic byte array:
        bytes memory b = new bytes(200);
        for (uint i = 0; i < b.length; i++)
            b[i] = bytes1(uint8(i));
        return b;

Array Slices

Array slices are a view on a contiguous portion of an array. They are written as x[start:end], where start and end are expressions resulting in a uint256 type (or implicitly convertible to it). The first element of the slice is x[start] and the last element is x[end - 1].

If start is greater than end or if end is greater than the length of the array, an exception is thrown.

Both start and end are optional: start defaults to 0 and end defaults to the length of the array.

Array slices do not have any members. They are implicitly convertible to arrays of their underlying type and support index access. Index access is not absolute in the underlying array, but relative to the start of the slice.

Array slices do not have a type name which means no variable can have an array slices as type, they only exist in intermediate expressions.


As of now, array slices are only implemented for calldata arrays.

Array slices are useful to ABI-decode secondary data passed in function parameters:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.8.5 <0.9.0;
contract Proxy {
    /// @dev Address of the client contract managed by proxy i.e., this contract
    address client;

    constructor(address _client) {
        client = _client;

    /// Forward call to "setOwner(address)" that is implemented by client
    /// after doing basic validation on the address argument.
    function forward(bytes calldata _payload) external {
        bytes4 sig = bytes4(_payload[:4]);
        // Due to truncating behaviour, bytes4(_payload) performs identically.
        // bytes4 sig = bytes4(_payload);
        if (sig == bytes4(keccak256("setOwner(address)"))) {
            address owner = abi.decode(_payload[4:], (address));
            require(owner != address(0), "Address of owner cannot be zero.");
        (bool status,) = client.delegatecall(_payload);
        require(status, "Forwarded call failed.");


Solidity provides a way to define new types in the form of structs, which is shown in the following example:

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;

// Defines a new type with two fields.
// Declaring a struct outside of a contract allows
// it to be shared by multiple contracts.
// Here, this is not really needed.
struct Funder {
    address addr;
    uint amount;

contract CrowdFunding {
    // Structs can also be defined inside contracts, which makes them
    // visible only there and in derived contracts.
    struct Campaign {
        address payable beneficiary;
        uint fundingGoal;
        uint numFunders;
        uint amount;
        mapping (uint => Funder) funders;

    uint numCampaigns;
    mapping (uint => Campaign) campaigns;

    function newCampaign(address payable beneficiary, uint goal) public returns (uint campaignID) {
        campaignID = numCampaigns++; // campaignID is return variable
        // We cannot use "campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0)"
        // because the right hand side creates a memory-struct "Campaign" that contains a mapping.
        Campaign storage c = campaigns[campaignID];
        c.beneficiary = beneficiary;
        c.fundingGoal = goal;

    function contribute(uint campaignID) public payable {
        Campaign storage c = campaigns[campaignID];
        // Creates a new temporary memory struct, initialised with the given values
        // and copies it over to storage.
        // Note that you can also use Funder(msg.sender, msg.value) to initialise.
        c.funders[c.numFunders++] = Funder({addr: msg.sender, amount: msg.value});
        c.amount += msg.value;

    function checkGoalReached(uint campaignID) public returns (bool reached) {
        Campaign storage c = campaigns[campaignID];
        if (c.amount < c.fundingGoal)
            return false;
        uint amount = c.amount;
        c.amount = 0;
        return true;

The contract does not provide the full functionality of a crowdfunding contract, but it contains the basic concepts necessary to understand structs. Struct types can be used inside mappings and arrays and they can themselves contain mappings and arrays.

It is not possible for a struct to contain a member of its own type, although the struct itself can be the value type of a mapping member or it can contain a dynamically-sized array of its type. This restriction is necessary, as the size of the struct has to be finite.

Note how in all the functions, a struct type is assigned to a local variable with data location storage. This does not copy the struct but only stores a reference so that assignments to members of the local variable actually write to the state.

Of course, you can also directly access the members of the struct without assigning it to a local variable, as in campaigns[campaignID].amount = 0.


Until Solidity 0.7.0, memory-structs containing members of storage-only types (e.g. mappings) were allowed and assignments like campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0) in the example above would work and just silently skip those members.

Type Mapping

Les types de mappage utilisent la syntaxe mapping(_KeyType => _ValueType) et des variables de type mapping sont déclarés en utilisant la syntaxe mapping(_KeyType => _ValueType) _VariableName. Le _KeyType peut être n’importe quel type de valeur intégré, bytes, string, ou tout type de contrat ou d’énumération. Autre défini par l’utilisateur ou les types complexes, tels que les mappages, les structures ou les types de tableau ne sont pas autorisés. _ValueType peut être n’importe quel type, y compris les mappages, les tableaux et les structures.

Vous pouvez considérer les mappages comme des tables de hachage, qui sont virtuellement initialisées telle que chaque clé possible existe et est mappée à une valeur dont byte-representation n’est que des zéros, la default value d’un type. La similitude s’arrête là, les données clés ne sont pas stockées dans un mappage, seul son hachage keccak256 est utilisé pour rechercher la valeur.

Pour cette raison, les mappages n’ont pas de longueur ou de concept de clé ou valeur définie et ne peut donc pas être effacée sans informations supplémentaires concernant les clés attribuées (voir Effacement des mappages).

Les mappages ne peuvent avoir qu’un emplacement de données: le storage et donc sont autorisés que pour les variables d’état (State), en tant que types de référence de stockage (storage) dans les fonctions ou comme paramètres pour les fonctions de la bibliothèque. Ils ne peuvent pas être utilisés comme paramètres ou paramètres de retour (return) des fonctions contractuelles qui sont publiquement visibles. Ces restrictions s’appliquent également aux tableaux et structures contenant des mappages.

Vous pouvez marquer les variables d’état de type mappage comme public et Solidity crée un getter pour vous. Le _KeyType devient un paramètre pour le getter. Si _ValueType est un type valeur ou une structure, le getter renvoie _ValueType. Si _ValueType est un tableau ou un mappage, le getter a un paramètre pour chaque _KeyType, récursivement.

Dans l’exemple ci-dessous, le contrat MappingExample définit un balances public mappage, avec le type de clé une adresse, et un type de valeur un uint, map une adresse Ethereum à une valeur entière non signée. Comme uint est un type valeur, le getter renvoie une valeur qui correspond au type, que vous pouvez voir dans le MappingUser contrat qui renvoie la valeur à l’adresse spécifiée.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;

contract MappingExample {
    mapping(address => uint) public balances;

    function update(uint newBalance) public {
        balances[msg.sender] = newBalance;

contract MappingUser {
    function f() public returns (uint) {
        MappingExample m = new MappingExample();
        return m.balances(address(this));

L’exemple ci-dessous est une version simplifiée d’un Jeton ERC20. _allowances est un exemple de type de mappage à l’intérieur d’un autre type de mappage. L’exemple ci-dessous utilise _allowances pour enregistrer le montant que quelqu’un d’autre est autorisé à retirer de votre compte.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.22 <0.9.0;

contract MappingExample {

    mapping (address => uint256) private _balances;
    mapping (address => mapping (address => uint256)) private _allowances;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowances[owner][spender];

    function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
        require(_allowances[sender][msg.sender] >= amount, "ERC20: Allowance not high enough.");
        _allowances[sender][msg.sender] -= amount;
        _transfer(sender, recipient, amount);
        return true;

    function approve(address spender, uint256 amount) public returns (bool) {
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;

    function _transfer(address sender, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");
        require(_balances[sender] >= amount, "ERC20: Not enough funds.");

        _balances[sender] -= amount;
        _balances[recipient] += amount;
        emit Transfer(sender, recipient, amount);

Mapping itérables

Vous ne pouvez pas itérer les mappages, c’est-à-dire que vous ne pouvez pas énumérer leurs clés. Il est cependant possible d’implémenter une structure de données par dessus d’eux et itérer dessus. Par exemple, le code ci-dessous implémente un bibliothèque IterableMapping que le contrat User ajoute également des données, et la fonction sum effectue une itération pour additionner toutes les valeurs.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.8 <0.9.0;

struct IndexValue { uint keyIndex; uint value; }
struct KeyFlag { uint key; bool deleted; }

struct itmap {
    mapping(uint => IndexValue) data;
    KeyFlag[] keys;
    uint size;

library IterableMapping {
    function insert(itmap storage self, uint key, uint value) internal returns (bool replaced) {
        uint keyIndex = self.data[key].keyIndex;
        self.data[key].value = value;
        if (keyIndex > 0)
            return true;
        else {
            keyIndex = self.keys.length;
            self.data[key].keyIndex = keyIndex + 1;
            self.keys[keyIndex].key = key;
            return false;

    function remove(itmap storage self, uint key) internal returns (bool success) {
        uint keyIndex = self.data[key].keyIndex;
        if (keyIndex == 0)
            return false;
        delete self.data[key];
        self.keys[keyIndex - 1].deleted = true;
        self.size --;

    function contains(itmap storage self, uint key) internal view returns (bool) {
        return self.data[key].keyIndex > 0;

    function iterate_start(itmap storage self) internal view returns (uint keyIndex) {
        return iterate_next(self, type(uint).max);

    function iterate_valid(itmap storage self, uint keyIndex) internal view returns (bool) {
        return keyIndex < self.keys.length;

    function iterate_next(itmap storage self, uint keyIndex) internal view returns (uint r_keyIndex) {
        while (keyIndex < self.keys.length && self.keys[keyIndex].deleted)
        return keyIndex;

    function iterate_get(itmap storage self, uint keyIndex) internal view returns (uint key, uint value) {
        key = self.keys[keyIndex].key;
        value = self.data[key].value;

// Comme l'utiliser
contract User {
    // Juste un struct contenant nos données
    itmap data;
    // Appliquez les fonctions de la bibliothèque au type de données.
    using IterableMapping for itmap;

    // Ajouter quelque chose
    function insert(uint k, uint v) public returns (uint size) {
        // Appel IterableMapping.insert(data, k, v)
        data.insert(k, v);
        // Nous pouvons toujours accéder aux membres de la struct,
        // mais nous devons faire attention de ne pas jouer avec eux.
        return data.size;

    // Calcule la somme de toutes les données stockées.
    function sum() public view returns (uint s) {
        for (
            uint i = data.iterate_start();
            i = data.iterate_next(i)
        ) {
            (, uint value) = data.iterate_get(i);
            s += value;

Les opérateurs (arithmetique)

Les opérateurs arithmétiques et binaires peuvent être appliqués même si les deux opérandes n’ont pas le même type. Par exemple, vous pouvez calculer y = x + z, où x est un uint8 et z a le type int32. Dans ces cas, le mécanisme suivant sera utilisé pour déterminer le type dans lequel l’opération est calculée (c’est important en cas de débordement) et le type du résultat de l’opérateur :

  1. Si le type de l’opérande droit peut être implicitement converti en type de l’opérande gauche utilisez le type de l’opérande de gauche,

  2. Si le type de l’opérande gauche peut être implicitement converti en type de l’opérande droite utilisez le type de l’opérande de droite,

  3. Sinon, l’opération n’est pas autorisée.

Dans le cas où l’un des opérandes est un literal number il est d’abord converti en son « type mobile », qui est le plus petit type pouvant contenir la valeur (les types non signés de même largeur de bit sont considérés comme « plus petits » que les types signés). Si les deux sont des nombres littéraux, l’opération est calculée avec une précision arbitraire.

Le type de résultat de l’opérateur est le même que le type dans lequel l’opération est effectuée, sauf pour les opérateurs de comparaison où le résultat est toujours bool.

Les opérateurs ** (exponentiation), << and >> utilisent le type du opérande de gauche pour l’opération et le résultat.

Opérateurs composés et d’incrémentation/décrémentation

Si a est une LValue (c’est-à-dire une variable ou quelque chose qui peut être assignée), les opérateurs suivants sont disponibles comme raccourcis :

a += e est équivalent à a = a + e. Les opérations -=, *=, /=, %=, |=, &=, ^=, <<= and >>= sont définis en conséquence. a++ and a-- est équivalent à a += 1 / a -= 1 mais l’expression elle-même a toujours la valeur précédente de a. En revanche, --a et ++a ont le même effet sur a main retourne la valeur après le changement.


delete a affecte la valeur initiale du type à a. C’est à dire. pour les entiers c’est équivalent à a = 0, mais il peut aussi être utilisé sur des tableaux, où il assigne une dynamique tableau de longueur zéro ou un tableau statique de même longueur avec tous les éléments mis à leur valeur initiale. delete a[x] supprime l’élément à l’index x du tableau et laisse tous les autres éléments et la longueur du tableau intacts. Cela signifie surtout qu’il laisse une lacune dans le tableau. Si vous envisagez de supprimer des éléments, un mapping est probablement un meilleur choix.

Pour les structures, il attribue une structure avec tous les membres réinitialisés. Autrement dit, la valeur de a après delete a est la même que si a était déclaré sans affectation, avec la mise en garde suivante :

delete n’a aucun effet sur les mapping (car les clés des mappages peuvent être arbitraires et sont généralement inconnus). Donc, si vous supprimez une structure, elle réinitialisera tous les membres qui ne sont pas des mapping et se récursent également dans les membres à moins qu’il ne s’agisse de mapping. Cependant, les clés individuelles et ce à quoi elles correspondent peuvent être supprimées : si a est un mapping, alors delete a[x] supprimera la valeur stockée à x.

Il est important de noter que delete a se comporte vraiment comme un affectation à a, c’est-à-dire qu’il stocke un nouvel objet dans a. Cette distinction est visible lorsque a est une variable de référence : ne réinitialisera que a lui-même, pas le valeur à laquelle il se référait précédemment.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;

contract DeleteExample {
    uint data;
    uint[] dataArray;

    function f() public {
        uint x = data;
        delete x; // définit x sur 0, n'affecte pas les données
        delete data; // définit les données sur 0, n'affecte pas x
        uint[] storage y = dataArray;
        delete dataArray; // cela définit dataArray.length à zéro, mais comme uint[] est un objet complexe, aussi
        // il est affecté qui est un alias de l'objet de stockage
        // Par contre : "delete y" n'est pas valide, car les affectations aux variables locales
        // les objets de stockage de référence ne peuvent être créés qu'à partir d'objets de stockage existants.
        assert(y.length == 0);

Conversions entre types élémentaires

Conversions implicites

Une conversion de type implicite est automatiquement appliquée par le compilateur dans certains cas lors des affectations, lors du passage d’arguments aux fonctions et lors de l’application d’opérateurs. En général, une conversion implicite entre les types de valeur est possible si elle est sémantique et qu’aucune information n’est perdue.

Par exemple, uint8 est convertible en uint16 et int128 en int256, mais int8 n’est pas convertible en uint256, car uint256 ne peut pas contenir de valeurs telles que -1.

Si un opérateur est appliqué à différents types, le compilateur essaie implicitement convertir l’un des opérandes dans le type de l’autre (il en va de même pour les affectations). Cela signifie que les opérations sont toujours effectuées dans le type de l’un des opérandes.

Pour plus de détails sur les conversions implicites possibles, veuillez consulter les sections sur les types eux-mêmes.

Dans l’exemple ci-dessous, y et z, les opérandes de l’addition, n’ont pas le même type, mais uint8 peut être implicitement converti en uint16 et non l’inverse. À cause de ça, y est converti dans le type de z avant que l’addition ne soit effectuée dans le type uint16. Le type résultant de l’expression y + z est uint16. Parce qu’il est assigné à une variable de type uint32 une autre conversion implicite est effectué après l’addition.

uint8 y;
uint16 z;
uint32 x = y + z;

Conversions explicites

Si le compilateur n’autorise pas la conversion implicite mais que vous êtes sûr qu’une conversion fonctionnera, une conversion de type explicite est parfois possible. Ceci peut entraîner un comportement inattendu et vous permet de contourner certaines mesures de sécurité fonctionnalités du compilateur, assurez-vous donc de tester que le le résultat est ce que vous voulez et attendez!

Prenons l’exemple suivant qui convertit un int négatif en un uint :

int  y = -3;
uint x = uint(y);

A la fin de cet extrait de code, x aura la valeur 0xfffff..fd (64 hex caractères), qui est -3 dans la représentation en complément à deux de 256 bits (Ce qui deonnera une erreur).

Si un entier est explicitement converti en un type plus petit, les bits d’ordre supérieur sont couper:

uint32 a = 0x12345678;
uint16 b = uint16(a); // b sera maintenant égale à 0x5678

Si un entier (integer) est explicitement converti en un type plus grand, il est rempli à gauche (c’est-à-dire à l’extrémité d’ordre supérieur). Le résultat de la conversion sera égal à l’entier d’origine :

uint16 a = 0x1234;
uint32 b = uint32(a); // b sera maintenant égale à 0x00001234
assert(a == b);

Les types d’octets de taille fixe se comportent différemment lors des conversions. Ils peuvent être considérés comme séquences d’octets individuels et la conversion en un type plus petit coupera le séquence:

bytes2 a = 0x1234;
bytes1 b = bytes1(a); // b sera égale à 0x12

Si un type d’octets de taille fixe est explicitement converti en un type plus grand, il est rempli sur la droite. L’accès à l’octet à un index fixe se traduira par la même valeur avant et après la conversion (si l’indice est toujours dans la plage):

bytes2 a = 0x1234;
bytes4 b = bytes4(a); // b sera égale à 0x12340000
assert(a[0] == b[0]);
assert(a[1] == b[1]);

Étant donné que les entiers et les tableaux d’octets de taille fixe se comportent différemment lors de la troncature ou du padding, les conversions explicites entre entiers et tableaux d’octets de taille fixe ne sont autorisées, si les deux ont la même taille. Si vous voulez convertir entre des nombres entiers et des tableaux d’octets de taille fixe de taille différente, vous devez utiliser des conversions intermédiaires qui font la troncature et le padding souhaités Règles explicites :

bytes2 a = 0x1234;
uint32 b = uint16(a); // b sera égale à 0x00001234
uint32 c = uint32(bytes4(a)); // c sera égale à 0x12340000
uint8 d = uint8(uint16(a)); // d sera égale à 0x34
uint8 e = uint8(bytes1(a)); // e sera égale à 0x12

Les tableaux bytes et les tranches de calldata bytes peuvent être convertis explicitement en types d’octets fixes (bytes1/…/bytes32). Si le tableau est plus long que le type d’octets fixes cible, une troncature à la fin se produira. Si le tableau est plus court que le type cible, il sera complété par des zéros à la fin.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.5;

contract C {
    bytes s = "abcdefgh";
    function f(bytes calldata c, bytes memory m) public view returns (bytes16, bytes3) {
        require(c.length == 16, "");
        bytes16 b = bytes16(m);  // si la longueur de m est supérieure à 16, la troncature se produira
        b = bytes16(s);  // rembourré à droite, donc le résultat est "abcdefgh\0\0\0\0\0\0\0\0"
        bytes3 b1 = bytes3(s); // tronqué, b1 est égal à "abc"
        b = bytes16(c[:8]);  // également rempli de zéros
        return (b, b1);

Conversions entre littéraux et types élémentaires

Types entiers (Integer)

Les littéraux décimaux et hexadécimaux peuvent être implicitement convertis en n’importe quel type entier suffisamment grand pour le représenter sans troncature :

uint8 a = 12; // Pas d'erreurs
uint32 b = 1234; // Pas d'erreurs
uint16 c = 0x123456; // Échec, car il faudrait tronquer à 0x3456


Avant la version 0.8.0, tous les littéraux décimaux ou hexadécimaux pouvaient être explicitement converti en un type entier. Depuis la version 0.8.0, ces conversions explicites sont aussi strictes qu’implicites conversions, c’est-à-dire qu’elles ne sont autorisées que si le littéral correspond à la plage résultante.

Tableaux d’octets de taille fixe

Les littéraux décimaux ne peuvent pas être implicitement convertis en tableaux d’octets de taille fixe. Hexadécimal les littéraux numériques peuvent être, mais seulement si le nombre de chiffres hexadécimaux correspond exactement à la taille des octets taper. Exceptionnellement, les littéraux décimaux et hexadécimaux qui ont une valeur de zéro peuvent être converti en n’importe quel type d’octets de taille fixe :

bytes2 a = 54321; // Interdit
bytes2 b = 0x12; // Interdit
bytes2 c = 0x123; // Interdit
bytes2 d = 0x1234; // OK
bytes2 e = 0x0012; // OK
bytes4 f = 0; // OK
bytes4 g = 0x0; // OK

Les littéraux de chaîne et les littéraux de chaîne hexadécimaux peuvent être implicitement convertis en tableaux d’octets de taille fixe, si leur nombre de caractères correspond à la taille du type d’octets :

bytes2 a = hex"1234"; // OK
bytes2 b = "xy"; // OK
bytes2 c = hex"12"; // Interdit
bytes2 d = hex"123"; // Interdit
bytes2 e = "x"; // Interdit
bytes2 f = "xyz"; // Interdit


Comme décrit dans Address Literals, les littéraux hexadécimaux de la taille correcte qui passent la somme de contrôle test sont de type addresse. Aucun autre littéral ne peut être implicitement converti en type addresse.

Les conversions explicites de bytes20 ou de n’importe quel type d’entier en address résultent en address payable.

Une address a peut être convertie en address payable via payable(a).