Structure of a Contract

Structure of a Contract
Contracts in Solidity are similar to classes in object-oriented languages. Each contract can contain declarations of State Variables, Functions, Function Modifiers, Events, Errors, Struct Types and Enum Types. Furthermore, contracts can inherit from other contracts.
There are also special kinds of contracts called libraries and interfaces.
The section about contracts contains more details than this section, which serves to provide a quick overview.
State Variables
State variables are variables whose values are permanently stored in contract storage.
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity >=0.4.0 <0.9.0;
3
​
4
contract SimpleStorage {
5
    uint storedData; // State variable
6
    // ...
7
}
Copied!
See the Types section for valid state variable types and Visibility and Getters for possible choices for visibility.
Functions
Functions are the executable units of code. Functions are usually defined inside a contract, but they can also be defined outside of contracts.
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity >=0.7.1 <0.9.0;
3
​
4
contract SimpleAuction {
5
    function bid() public payable { // Function
6
        // ...
7
    }
8
}
9
​
10
// Helper function defined outside of a contract
11
function helper(uint x) pure returns (uint) {
12
    return x * 2;
13
}
Copied!
​Function Calls can happen internally or externally and have different levels of visibility towards other contracts. Functions accept parameters and return variables to pass parameters and values between them.
Function Modifiers
Function modifiers can be used to amend the semantics of functions in a declarative way (see Function Modifiers in the contracts section).
Overloading, that is, having the same modifier name with different parameters, is not possible.
Like functions, modifiers can be overridden.
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity >=0.4.22 <0.9.0;
3
​
4
contract Purchase {
5
    address public seller;
6
​
7
    modifier onlySeller() { // Modifier
8
        require(
9
            msg.sender == seller,
10
            "Only seller can call this."
11
        );
12
        _;
13
    }
14
​
15
    function abort() public view onlySeller { // Modifier usage
16
        // ...
17
    }
18
}
Copied!
Events
Events are convenience interfaces with the EVM logging facilities.
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity >=0.4.21 <0.9.0;
3
​
4
contract SimpleAuction {
5
    event HighestBidIncreased(address bidder, uint amount); // Event
6
​
7
    function bid() public payable {
8
        // ...
9
        emit HighestBidIncreased(msg.sender, msg.value); // Triggering event
10
    }
11
}
Copied!
See Events in contracts section for information on how events are declared and can be used from within a dapp.
Errors
Errors allow you to define descriptive names and data for failure situations. Errors can be used in revert statements. In comparison to string descriptions, errors are much cheaper and allow you to encode additional data. You can use NatSpec to describe the error to the user.
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity ^0.8.4;
3
​
4
/// Not enough funds for transfer. Requested `requested`,
5
/// but only `available` available.
6
error NotEnoughFunds(uint requested, uint available);
7
​
8
contract Token {
9
    mapping(address => uint) balances;
10
    function transfer(address to, uint amount) public {
11
        uint balance = balances[msg.sender];
12
        if (balance < amount)
13
            revert NotEnoughFunds(amount, balance);
14
        balances[msg.sender] -= amount;
15
        balances[to] += amount;
16
        // ...
17
    }
18
}
Copied!
See Errors and the Revert Statement in the contracts section for more information.
Struct Types
Structs are custom defined types that can group several variables (see Structs in types section).
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity >=0.4.0 <0.9.0;
3
​
4
contract Ballot {
5
    struct Voter { // Struct
6
        uint weight;
7
        bool voted;
8
        address delegate;
9
        uint vote;
10
    }
11
}
Copied!
Enum Types
Enums can be used to create custom types with a finite set of ‘constant values’ (see Enums in types section).
1
// SPDX-License-Identifier: GPL-3.0
2
pragma solidity >=0.4.0 <0.9.0;
3
​
4
contract Purchase {
5
    enum State { Created, Locked, Inactive } // Enum
6
}
Copied!
​Next ​ Previous
Solidarity - Previous
Ethereum
Next - NFT's
NFT's
Last modified 1mo ago
Copy link
Contents
Structure of a Contract