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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-structs-types` and :ref:`structure-enum-types`.
Furthermore, contracts can inherit from other contracts.
.. _structure-state-variables:
State Variables
===============
State variables are values which are permanently stored in contract storage.
::
contract SimpleStorage {
uint storedData; // State variable
// ...
}
See the :ref:`types` section for valid state variable types and
:ref:`visibility-and-accessors` for possible choices for
visibility.
.. _structure-functions:
Functions
=========
Functions are the executable units of code within a contract.
::
contract SimpleAuction {
function bid() { // Function
// ...
}
}
:ref:`function-calls` can happen internally or externally
and have different levels of visibility (:ref:`visibility-and-accessors`)
towards other contracts.
.. _structure-function-modifiers:
Function Modifiers
==================
Function modifiers can be used to amend the semantics of functions in a declarative way
(see :ref:`modifiers` in contracts section).
::
contract Purchase {
address public seller;
modifier onlySeller() { // Modifier
if (msg.sender != seller) throw;
_
}
function abort() onlySeller { // Modifier usage
// ...
}
}
in the section on contracts for a more in-depth explanation.
.. _structure-events:
Events
======
Events are convenience interfaces with the EVM logging facilities.
::
contract SimpleAuction {
event HighestBidIncreased(address bidder, uint amount); // Event
function bid() {
// ...
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-structs-types:
Structs Types
=============
Structs are custom defined types that can group several variables (see
:ref:`structs` in types section).
::
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 values (see
:ref:`enums` in types section).
::
contract Purchase {
enum State { Created, Locked, Inactive } // Enum
}
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