solidity/docs/types/reference-types.rst

.. index:: ! type;reference, ! reference type, storage, memory, location, array, struct

.. _reference-types:

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 a function call), ``storage`` (the location where the state variables are stored)
or ``calldata`` (special data location that contains the function arguments,
only available for external function call parameters).

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:

Data location
-------------

Every reference type, i.e. *arrays* and *structs*, has an additional
annotation, the "data location", about where it is stored. There are three data locations:
``memory``, ``storage`` and ``calldata``. Calldata is only valid for parameters of external contract
functions and is required for this type of parameter. Calldata is a non-modifiable,
non-persistent area where function arguments are stored, and behaves mostly like memory.


.. note::
    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-assignment:

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.

::

    pragma solidity >=0.4.0 <0.7.0;

    contract C {
        uint[] x; // the data location of x is storage

        // 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.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) internal pure {}
        function h(uint[] memory) public pure {}
    }

.. index:: ! array

.. _arrays:

Arrays
------

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] x memory``, you access the
second ``uint`` in the third dynamic array using ``x[2][1]``, 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 :ref:`ABI types <ABI>`.

It is possible to mark state variable arrays ``public`` and have Solidity create a :ref:`getter <visibility-and-getters>`.
The numeric index becomes a required parameter for the getter.

Accessing an array past its end causes a failing assertion. You can use the ``.push()`` method to append a new element at the end or assign to the ``.length`` :ref:`member <array-members>` to change the size (see below for caveats).
method or increase the ``.length`` :ref:`member <array-members>` to add elements.

``bytes`` and ``strings`` as Arrays
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Variables of type ``bytes`` and ``string`` are special arrays. A ``bytes`` is similar to ``byte[]``,
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 ``abi.encodePacked(s1, s2)``.

You should use ``bytes`` over ``byte[]`` because it is cheaper, since ``byte[]`` adds 31 padding bytes between the elements. 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.

.. note::
    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.

.. index:: ! array;allocating, new

Allocating Memory Arrays
^^^^^^^^^^^^^^^^^^^^^^^^

You must use the ``new`` keyword to create arrays with a runtime-dependent length in memory.
As opposed to storage arrays, it is **not** possible to resize memory arrays (e.g. by assigning to
the ``.length`` member). You either have to calculate the required size in advance
or create a new memory array and copy every element.

::

    pragma solidity >=0.4.16 <0.7.0;

    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;
        }
    }

.. index:: ! array;literals, ! inline;arrays

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)]``. There must be a
common type all elements can be implicitly converted to. This is the elementary
type of the array.

Array literals are always statically-sized memory arrays.

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``.

::

    pragma solidity >=0.4.16 <0.7.0;

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

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

::

    pragma solidity >=0.4.0 <0.7.0;

    // 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.

.. index:: ! array;length, length, push, pop, !array;push, !array;pop

.. _array-members:

Array Members
^^^^^^^^^^^^^

**length**:
    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.
    For dynamically-sized arrays (only available for storage), this member can be assigned to resize the array.
    Accessing elements outside the current length does not automatically resize the array and instead causes a failing assertion.
    Increasing the length adds new zero-initialised elements to the array.
    Reducing the length performs an implicit :ref:``delete`` on each of the
    removed elements. If you try to resize a non-dynamic array that isn't in
    storage, you receive a ``Value must be an lvalue`` error.
**push**:
     Dynamic storage arrays and ``bytes`` (not ``string``) have a member function called ``push`` that you can use to append an element at the end of the array. The element will be zero-initialised. The function returns the new length.
**pop**:
     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 :ref:``delete`` on the removed element.

.. warning::
    If you use ``.length--`` on an empty array, it causes an underflow and
    thus sets the length to ``2**256-1``.

.. note::
    Increasing the length of a storage array has constant gas costs because
    storage is assumed to be zero-initialised, while decreasing
    the length has at least linear cost (but in most cases worse than linear),
    because it includes explicitly clearing the removed
    elements similar to calling :ref:``delete`` on them.

.. note::
    It is not yet possible to use arrays of arrays in external functions
    (but they are supported in public functions).

.. note::
    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.

::

    pragma solidity >=0.4.16 <0.7.0;

    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 {
            // if the new size is smaller, removed array elements will be cleared
            m_pairsOfFlags.length = newSize;
        }

        function clear() public {
            // these clear the arrays completely
            delete m_pairsOfFlags;
            delete m_aLotOfIntegers;
            // identical effect here
            m_pairsOfFlags.length = 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;
            m_byteData.length += 7;
            m_byteData[3] = 0x08;
            delete m_byteData[2];
        }

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

        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] = byte(uint8(i));
            return b;
        }
    }


.. index:: ! struct, ! type;struct

.. _structs:

Structs
-------

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

::

    pragma solidity >=0.4.11 <0.7.0;

    contract CrowdFunding {
        // Defines a new type with two fields.
        struct Funder {
            address addr;
            uint amount;
        }

        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
            // Creates new struct in memory and copies it to storage.
            // We leave out the mapping type, because it is not valid in memory.
            // If structs are copied (even from storage to storage), mapping types
            // are always omitted, because they cannot be enumerated.
            campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0);
        }

        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;
            c.beneficiary.transfer(amount);
            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
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``.