Structs
Structs can be declared outside of a contract and imported in another contract.
Sometimes you need a more complex data type.
For this, Solidity provides structs, allowing you to create more complicated data types that have multiple properties.
struct People {
uint256 favoriteNumber;
string name;
}
// create a New Person:
People public person = People({favoriteNumber: 7, name: "Eridian"});
Declaring and Importing Struct
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;
// This is saved as 'StructDeclaration.sol'
struct Todo {
string text;
bool completed;
}
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;
import "./StructDeclaration.sol";
contract Todos {
// An array of 'Todo' structs
Todo[] public todos;
}
Passing structs as arguments
You can pass a storage pointer to a struct as an argument to a
private
orinternal
function.This is useful, for example, for passing around our
Zombie
structs between functions.
function _doStuff(People storage _person) internal {
// do stuff with _person
}
This way we can pass a reference to our person into a function instead of passing in a person ID and looking it up.
Struct packing to save gas
There are other types of
uint
s:uint8
,uint16
,uint32
, etc.Normally there's no benefit to using these sub-types because Solidity reserves 256 bits of storage regardless of the
uint
size.For example, using
uint8
instead ofuint (uint256)
won't save you any gas.
But there's an exception to this: inside
struct
s.If you have multiple
uint
s inside a struct, using a smaller-sizeduint
when possible will allow Solidity to pack these variables together to take up less storage.
struct NormalStruct {
uint a;
uint b;
uint c;
}
struct MiniMe {
uint32 a;
uint32 b;
uint c;
}
// `mini` will cost less gas than `normal` because of struct packing
NormalStruct normal = NormalStruct(10, 20, 30);
MiniMe mini = MiniMe(10, 20, 30);
For this reason, inside a struct you'll want to use the smallest integer sub-types you can get away with.
You'll also want to cluster identical data types together (i.e. put them next to each other in the struct) so that Solidity can minimize the required storage space.
For example, a struct with fields
uint c; uint32 a; uint32 b;
will cost less gas than a struct with fieldsuint32 a; uint c; uint32 b;
because theuint32
fields are clustered together.
Example
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;
contract Todos {
struct Todo {
string text;
bool completed;
}
// An array of 'Todo' structs
Todo[] public todos;
function create(string calldata _text) public {
// 3 ways to initialize a struct
// - calling it like a function
todos.push(Todo(_text, false));
// key value mapping
todos.push(Todo({text: _text, completed: false}));
// initialize an empty struct and then update it
Todo memory todo;
todo.text = _text;
// todo.completed initialized to false
todos.push(todo);
}
// Solidity automatically created a getter for 'todos' so you don't actually need this function.
function get(uint _index) public view returns (string memory text, bool completed) {
Todo storage todo = todos[_index];
return (todo.text, todo.completed);
}
// update text
function updateText(uint _index, string calldata _text) public {
Todo storage todo = todos[_index];
todo.text = _text;
}
// update completed
function toggleCompleted(uint _index) public {
Todo storage todo = todos[_index];
todo.completed = !todo.completed;
}
}
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