Variables are declared as either storage
, memory
or calldata
to explicitly specify the location of the data.
storage
- variable is a state variable (store on blockchain)memory
- variable is in memory and it exists while a function is being calledcalldata
- special data location that contains function arguments, only available for external
functions// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;
contract DataLocations {
uint[] public arr;
mapping(uint => address) map;
struct MyStruct {
uint foo;
}
mapping(uint => MyStruct) myStructs;
function f() public {
// call _f with state variables
_f(arr, map, myStructs[1]);
// get a struct from a mapping
MyStruct storage myStruct = myStructs[1];
// create a struct in memory
MyStruct memory myMemStruct = MyStruct(0);
}
function _f(
uint[] storage _arr,
mapping(uint => address) storage _map,
MyStruct storage _myStruct
) internal {
// do something with storage variables
}
// You can return memory variables
function g(uint[] memory _arr) public returns (uint[] memory) {
// do something with memory array
}
function h(uint[] calldata _arr) external {
// do something with calldata array
}
}
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