Contract Structure
Contract structure
State and local variables
State variables
are variables declared at the beginning of the contract, outside the scope of local variables
declared in function
.
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract SimpleStorage {
uint public storedData; // State variable
// ...
function updateStoredData(uint newData) public {
uint formattedData = newData * 2; // formattedData is local variable
storedData = formattedData;
}
function getFormattedData() public view returns (uint) {
return formattedData; // failed can't compile because formattedData is local scope to the other function
}
function getStoredData() public view returns (uint) {
return storedData; // can compile because global variable
}
}
Functions
Function
are functions declared to perform calculations, change the value of variables, etc. A sample function
is given below.
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.1 <0.9.0;
contract SimpleAuction {
function bid() public payable { // Function
// ...
}
}
// Helper function defined outside of a contract
function helper(uint x) pure returns (uint) {
return x * 2;
}
Function modifiers
Function modifier
are declarations for function
to create conditions for running actions of that function
.
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.22 <0.9.0;
contract Purchase {
address public seller;
modifier onlySeller() { // Modifier
require(
msg.sender == seller,
"Only seller can call this."
);
_;
}
function abort() public view onlySeller { // Modifier usage
// ...
}
}
Events
event
is a feature for recording smart contract activities. event
is often used in building interactive UI with smart contracts.
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.22;
event HighestBidIncreased(address bidder, uint amount); // Event
contract SimpleAuction {
function bid() public payable {
// ...
emit HighestBidIncreased(msg.sender, msg.value); // Triggering event
}
}
Errors
error
is used to inform the user why the action failed, and error
has a lower gas
cost than returning string
.
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.4;
/// Not enough funds for transfer. Requested `requested`,
/// but only `available` available.
error NotEnoughFunds(uint requested, uint available);
contract Token {
mapping(address => uint) balances;
function transfer(address to, uint amount) public {
uint balance = balances[msg.sender];
if (balance < amount)
revert NotEnoughFunds(amount, balance);
balances[msg.sender] -= amount;
balances[to] += amount;
// ...
}
}
Struct types
struct
is used to declare a type
of object
.
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract Ballot {
struct Voter { // Struct
uint weight;
bool voted;
address delegate;
uint vote;
}
}
Enum types
enum
is used to declare a type
whose values ββare constant
.
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract Purchase {
enum State { Created, Locked, Inactive } // Enum
}
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