structure-of-a-contract.rst

.. index:: contract, state variable, function, event, struct, enum, function;modifier

Structure of a Contract

Contracts in Solidity are similar to classes in object-oriented languages. Each contract can contain declarations of :ref:`structure-state-variables`, :ref:`structure-functions`, :ref:`structure-function-modifiers`, :ref:`structure-events`, :ref:`structure-errors`, :ref:`structure-struct-types` and :ref:`structure-enum-types`. Furthermore, contracts can inherit from other contracts.

There are also special kinds of contracts called :ref:`libraries<libraries>` and :ref:`interfaces<interfaces>`.

The section about :ref:`contracts<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.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;

contract SimpleStorage {
    uint storedData; // State variable
    // ...
}

See the :ref:`types` section for valid state variable types and :ref:`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.

// 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;
}

:ref:`function-calls` can happen internally or externally and have different levels of :ref:`visibility<visibility-and-getters>` towards other contracts. :ref:`Functions<functions>` accept :ref:`parameters and return variables<function-parameters-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 :ref:`modifiers` in the contracts section).

Overloading, that is, having the same modifier name with different parameters, is not possible.

Like functions, modifiers can be :ref:`overridden <modifier-overriding>`.

// 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

Events are convenience interfaces with the EVM logging facilities.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.21 <0.9.0;

contract SimpleAuction {
    event HighestBidIncreased(address bidder, uint amount); // Event

    function bid() public payable {
        // ...
        emit HighestBidIncreased(msg.sender, msg.value); // Triggering event
    }
}

See :ref:`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 :ref:`revert statements <revert-statement>`. 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.

// 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;
        // ...
    }
}

See :ref:`errors` in the contracts section for more information.

Struct Types

Structs are custom defined types that can group several variables (see :ref:`structs` in types section).

// 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

Enums can be used to create custom types with a finite set of 'constant values' (see :ref:`enums` in types section).

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;

contract Purchase {
    enum State { Created, Locked, Inactive } // Enum
}