mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
60 lines
2.4 KiB
ReStructuredText
60 lines
2.4 KiB
ReStructuredText
.. index:: !mapping
|
|
.. _mapping-types:
|
|
|
|
Mapping Types
|
|
=============
|
|
|
|
You declare mapping types with the syntax ``mapping(_KeyType => _ValueType)``.
|
|
The ``_KeyType`` can be any elementary type. This means it can be any of
|
|
the built-in value types plus ``bytes`` and ``string``. User-defined
|
|
or complex types like contract types, enums, mappings, structs and any array type
|
|
apart from ``bytes`` and ``string`` are not allowed.
|
|
``_ValueType`` can be any type, including mappings.
|
|
|
|
You can think of mappings as `hash tables <https://en.wikipedia.org/wiki/Hash_table>`_, which are virtually initialised
|
|
such that every possible key exists and is mapped to a value whose
|
|
byte-representation is all zeros, a type's :ref:`default value <default-value>`. The similarity ends there, the key data is not stored in a
|
|
mapping, only its ``keccak256`` hash is 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, and therefore cannot be erased without extra information
|
|
regarding the assigned keys (see :ref:`clearing-mappings`).
|
|
|
|
Mappings can only have a data location of ``storage`` and thus
|
|
are allowed for state variables, as storage reference types
|
|
in functions, or as parameters for library functions.
|
|
They cannot be used as parameters or return parameters
|
|
of contract functions that are publicly visible.
|
|
|
|
You can mark state variables of mapping type as ``public`` and Solidity creates a
|
|
:ref:`getter <visibility-and-getters>` for you. The ``_KeyType`` becomes a
|
|
parameter for the getter. If ``_ValueType`` is a value type or a struct,
|
|
the getter returns ``_ValueType``.
|
|
If ``_ValueType`` is an array or a mapping, the getter has one parameter for
|
|
each ``_KeyType``, recursively. For example with a mapping:
|
|
|
|
::
|
|
|
|
pragma solidity >=0.4.0 <0.7.0;
|
|
|
|
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();
|
|
m.update(100);
|
|
return m.balances(address(this));
|
|
}
|
|
}
|
|
|
|
|
|
.. note::
|
|
Mappings are not iterable, but it is possible to implement a data structure
|
|
on top of them. For an example, see `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_.
|