Solidity is a statically typed language, which means that the type of each variable (state and local) needs to be specified (or at least known - see Type Deduction below) at compile-time. Solidity provides several elementary types which can be combined to complex types.

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

Division always truncates (it just maps to the DIV opcode of the EVM), but it does not truncate if both operators are literals (or literal expressions).


address: Holds a 20 byte value (size of an Ethereum address). Address types also have members(see [Functions on addresses](#functions-on-addresses)) and serve as base for all contracts.


  • <=, <, ==, !=, >= and >

Members of Addresses

  • balance and send

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

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


If x is a contract address, its code (more specifically: its fallback function, if present) will be executed together with the send call (this is a limitation 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. In this case, send returns false.

  • call, callcode and delegatecall

Furthermore, to interface with contracts that do not adhere to the ABI, the function call is provided which takes an arbitrary number of arguments of any type. These arguments are padded to 32 bytes and concatenated. One exception is the case where the first argument is encoded to exactly four bytes. In this case, it is not padded to allow the use of function signatures here.

address nameReg = 0x72ba7d8e73fe8eb666ea66babc8116a41bfb10e2;
nameReg.call("register", "MyName");
nameReg.call(bytes4(sha3("fun(uint256)")), a);

call returns a boolean indicating whether the invoked function terminated (true) or caused an EVM exception (false). It is not possible to access the actual data returned (for this we would need to know the encoding and size in advance).

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.

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


All contracts inherit the members of address, so it is possible to query the balance of the current contract using this.balance.

Fixed-size byte arrays

bytes1, bytes2, bytes3, ..., bytes32. byte is an alias for bytes1.


  • Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)
  • Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation)
  • Index access: If x is of type bytesI, then x[k] for 0 <= k < I returns the k th byte (read-only).


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

Dynamically-sized byte array

Dynamically-sized byte array, see Arrays. Not a value-type!
Dynamically-sized UTF8-encoded string, see Arrays. Not a value-type!

As a rule of thumb, 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 bytes1 to bytes32 because they are much cheaper.

Integer Literals

Integer Literals are arbitrary precision integers until they are used together with a non-literal. In var x = 1 - 2;, for example, the value of 1 - 2 is -1, which is assigned to x and thus x receives the type int8 – the smallest type that contains -1, although the natural types of 1 and 2 are actually uint8.

It is even possible to temporarily exceed the maximum of 256 bits as long as only integer literals are used for the computation: var x = (0xffffffffffffffffffff * 0xffffffffffffffffffff) * 0; Here, x will have the value 0 and thus the type uint8.


Divison on integer literals used to truncate in earlier versions, but it will actually convert into a rational number in the future, i.e. 1/2 is not equal to 0, but to 0.5.

String Literals

String Literals are written with double quotes (“abc”). As with integer literals, their type can vary, but they are implicitly convertible to bytes• if they fit, to bytes and to string.


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.

contract test {
    enum ActionChoices { GoLeft, GoRight, GoStraight, SitStill }
    ActionChoices choice;
    ActionChoices constant defaultChoice = ActionChoices.GoStraight;
    function setGoStraight()
        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. The integer type used is just
    // large enough to hold all enum values, i.e. if you have more values,
    // `uint16` will be used and so on.
    function getChoice() returns (ActionChoices)
        return choice;
    function getDefaultChoice() returns (uint)
        return uint(defaultChoice);

Reference Types

Complex types, i.e. types which do not always fit into 256 bits have to be handled more carefully than the value-types we have already seen. Since copying them can be quite expensive, we have to think about whether we want them to be stored in memory (which is not persisting) or storage (where the state variables are held).

Data location

Every complex type, i.e. arrays and structs, has an additional annotation, the “data location”, about whether it is stored in memory or in storage. Depending on the context, there is always a default, but it can be overridden by appending either storage or memory to the type. The default for function parameters (including return parameters) is memory, the default for local variables is storage and the location is forced to storage for state variables (obviously).

There is also a third data location, “calldata”, which is a non-modifyable non-persistent area where function arguments are stored. Function parameters (not return parameters) of external functions are forced to “calldata” and it behaves mostly like memory.

Data locations are important because they change how assignments behave: Assignments between storage and memory and also to a state variable (even from other state variables) always create an independent copy. Assignments to local storage variables only assign a reference though, and this reference always points to the state variable even if the latter is changed in the meantime. On the other hand, assignments from a memory stored reference type to another memory-stored reference type does not create a copy.

contract c {
  uint[] x; // the data location of x is storage
  // the data location of memoryArray is memory
  function f(uint[] memoryArray) {
    x = memoryArray; // works, copies the whole array to storage
    var y = x; // works, assigns a pointer, data location of y is storage
    y[7]; // fine, returns the 8th element
    y.length = 2; // 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 storageArray) internal {}
  function h(uint[] memoryArray) {}


Forced data location:
  • parameters (not return) of external functions: calldata
  • state variables: storage
Default data location:
  • parameters (also return) of functions: memory
  • all other local variables: storage


Arrays can have a compile-time fixed size or they can be dynamic. For storage arrays, the element type can be arbitrary (i.e. also other arrays, mappings or structs). For memory arrays, it cannot be a mapping and has to be an ABI type if it is an argument of a publicly-visible function.

An array of fixed size k and element type T is written as T[k], an array of dynamic size as T[]. As an example, an array of 5 dynamic arrays of uint is uint[][5] (note that the notation is reversed when compared to some other languages). To access the second uint in the third dynamic array, you use x[2][1] (indices are zero-based and access works in the opposite way of the declaration, i.e. x[2] shaves off one level in the type from the right).

Variables of type bytes and string are special arrays. A bytes is similar to byte[], but it is packed tightly in calldata. string is equal to bytes but does not allow length or index access (for now).

So bytes should always be preferred over byte[] because it is 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!


Arrays have a length member to hold their number of elements. Dynamic arrays can be resized in storage (not in memory) by changing the .length member. This does not happen automatically when attempting to access elements outside the current length. The size 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 can be used to append an element at the end of the array. The function returns the new length.


It is not yet possible to use arrays of arrays in external functions.


Due to limitations of the EVM, it is not possible to return dynamic content from external function calls. The function f in contract C { function f() returns (uint[]) { ... } } will return something if called from web3.js, but not if called from Solidity.

The only workaround for now is to use large statically-sized arrays.

contract ArrayContract {
  uint[2**20] m_aLotOfIntegers;
  // Note that the following is not a pair of arrays but an array of pairs.
  bool[2][] m_pairsOfFlags;
  // newPairs is stored in memory - the default for function arguments
  function setAllFlagPairs(bool[2][] newPairs) {
    // assignment to a storage array replaces the complete array
    m_pairsOfFlags = newPairs;
  function setFlagPair(uint index, bool flagA, bool flagB) {
    // access to a non-existing index will throw an exception
    m_pairsOfFlags[index][0] = flagA;
    m_pairsOfFlags[index][1] = flagB;
  function changeFlagArraySize(uint newSize) {
    // if the new size is smaller, removed array elements will be cleared
    m_pairsOfFlags.length = newSize;
  function clear() {
    // these clear the arrays completely
    delete m_pairsOfFlags;
    delete m_aLotOfIntegers;
    // identical effect here
    m_pairsOfFlags.length = 0;
  bytes m_byteData;
  function byteArrays(bytes data) {
    // byte arrays ("bytes") are different as they are stored without padding,
    // but can be treated identical to "uint8[]"
    m_byteData = data;
    m_byteData.length += 7;
    m_byteData[3] = 8;
    delete m_byteData[2];
  function addFlag(bool[2] flag) returns (uint) {
    return m_pairsOfFlags.push(flag);
  function createMemoryArray(uint size) returns (bytes) {
    // Dynamic memory arrays are created using `new`:
    uint[2][] memory arrayOfPairs = new uint[2][](size);
    // Create a dynamic byte array:
    bytes memory b = new bytes(200);
    for (uint i = 0; i < b.length; i++)
      b[i] = byte(i);
    return b;


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

contract CrowdFunding {
  // Defines a new type with two fields.
  struct Funder {
    address addr;
    uint amount;
  struct Campaign {
    address beneficiary;
    uint fundingGoal;
    uint numFunders;
    uint amount;
    mapping (uint => Funder) funders;
  uint numCampaigns;
  mapping (uint => Campaign) campaigns;
  function newCampaign(address beneficiary, uint goal) returns (uint campaignID) {
    campaignID = numCampaigns++; // campaignID is return variable
    // Creates new struct and saves in storage. We leave out the mapping type.
    campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0);
  function contribute(uint campaignID) {
    Campaign 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) returns (bool reached) {
    Campaign c = campaigns[campaignID];
    if (c.amount < c.fundingGoal)
      return false;
    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 itself 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. 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 (of the default storage data location). 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.


Mapping types are declared as mapping _KeyType => _ValueType, where _KeyType can be almost any type except for a mapping and _ValueType can actually be any type, including mappings.

Mappings can be seen as hashtables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros. The similarity ends here, though: The key data is not actually stored in a mapping, only its sha3 hash used to look up the value.

Because of this, mappings do not have a length or a concept of a key or value being “set”.

Mappings are only allowed for state variables (or as storage reference types in internal functions).

Operators Involving LValues

If a is an LValue (i.e. a variable or something that can be assigned to), the following operators are available as shorthands:

a += e is equivalent to a = a + e. The operators -=, *=, /=, %=, a |=, &= and ^= are defined accordingly. a++ and a– are equivalent to a += 1 / a -= 1 but the expression itself still has the previous value of a. In contrast, –a and ++a have the same effect on a but return the value after the change.


delete a assigns the initial value for the type to a. I.e. for integers it is equivalent to a = 0, but it can also be used on arrays, where it assigns a dynamic array of length zero or a static array of the same length with all elements reset. For structs, it assigns a struct with all members reset.

delete has no effect on whole mappings (as the keys of mappings may be arbitrary and are generally unknown). So if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings. However, individual keys and what they map to can be deleted.

It is important to note that delete a really behaves like an assignment to a, i.e. it stores a new object in a.

contract DeleteExample {
  uint data;
  uint[] dataArray;
  function f() {
    uint x = data;
    delete x; // sets x to 0, does not affect data
    delete data; // sets data to 0, does not affect x which still holds a copy
    uint[] y = dataArray;
    delete dataArray; // this sets dataArray.length to zero, but as uint[] is a complex object, also
    // y is affected which is an alias to the storage object
    // On the other hand: "delete y" is not valid, as assignments to local variables
    // referencing storage objects can only be made from existing storage objects.

Conversions between Elementary Types

Implicit Conversions

If an operator is applied to different types, the compiler tries to implicitly convert one of the operands to the type of the other (the same is true for assignments). In general, an implicit conversion between value-types is possible if it makes sense semantically and no information is lost: uint8 is convertible to uint16 and int128 to int256, but int8 is not convertible to uint256 (because uint256 cannot hold e.g. -1). Furthermore, unsigned integers can be converted to bytes of the same or larger size, but not vice-versa. Any type that can be converted to uint160 can also be converted to address.

Explicit Conversions

If the compiler does not allow implicit conversion but you know what you are doing, an explicit type conversion is sometimes possible:

int8 y = -3;
uint x = uint(y);

At the end of this code snippet, x will have the value 0xfffff..fd (64 hex characters), which is -3 in two’s complement representation of 256 bits.

If a type is explicitly converted to a smaller type, higher-order bits are cut off:

uint32 a = 0x12345678;
uint16 b = uint16(a); // b will be 0x5678 now

Type Deduction

For convenience, it is not always necessary to explicitly specify the type of a variable, the compiler automatically infers it from the type of the first expression that is assigned to the variable:

uint20 x = 0x123;
var y = x;

Here, the type of y will be uint20. Using var is not possible for function parameters or return parameters.


The type is only deduced from the first assignment, so the loop in the following snippet is infinite, as i will have the type uint8 and any value of this type is smaller than 2000. for (var i = 0; i < 2000; i++) { ... }