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.. index:: contract, state variable, function, event, struct, enum, function;modifier
.. _contract_structure:
***********************
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-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.
.. _structure-state-variables:
State Variables
===============
State variables are variables whose values are permanently stored in contract
storage.
::
pragma solidity >=0.4.0 <0.6.0;
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.
.. _structure-functions:
Functions
=========
Functions are the executable units of code within a contract.
::
pragma solidity >=0.4.0 <0.6.0;
contract SimpleAuction {
function bid() public payable { // Function
// ...
}
}
: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.
.. _structure-function-modifiers:
Function Modifiers
==================
Function modifiers can be used to amend the semantics of functions in a declarative way
(see :ref:`modifiers` in the contracts section).
::
pragma solidity >=0.4.22 <0.6.0;
contract Purchase {
address public seller;
modifier onlySeller() { // Modifier
require(
msg.sender == seller,
"Only seller can call this."
);
_;
}
function abort() public view onlySeller { // Modifier usage
// ...
}
}
.. _structure-events:
Events
======
Events are convenience interfaces with the EVM logging facilities.
::
pragma solidity >=0.4.21 <0.6.0;
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.
.. _structure-struct-types:
Struct Types
=============
Structs are custom defined types that can group several variables (see
:ref:`structs` in types section).
::
pragma solidity >=0.4.0 <0.6.0;
contract Ballot {
struct Voter { // Struct
uint weight;
bool voted;
address delegate;
uint vote;
}
}
.. _structure-enum-types:
Enum Types
==========
Enums can be used to create custom types with a finite set of 'constant values' (see
:ref:`enums` in types section).
::
pragma solidity >=0.4.0 <0.6.0;
contract Purchase {
enum State { Created, Locked, Inactive } // Enum
}
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