1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
|
.. 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.
.. _structure-state-variables:
State Variables
===============
State variables are values which are permanently stored in contract storage.
::
pragma solidity ^0.4.0;
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;
contract SimpleAuction {
function bid() public payable { // Function
// ...
}
}
:ref:`function-calls` can happen internally or externally
and have different levels of visibility (:ref:`visibility-and-getters`)
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).
::
pragma solidity ^0.4.11;
contract Purchase {
address public seller;
modifier onlySeller() { // Modifier
require(
msg.sender == seller,
"Only seller can call this."
);
_;
}
function abort() public onlySeller { // Modifier usage
// ...
}
}
.. _structure-events:
Events
======
Events are convenience interfaces with the EVM logging facilities.
::
pragma solidity ^0.4.21;
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;
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).
::
pragma solidity ^0.4.0;
contract Purchase {
enum State { Created, Locked, Inactive } // Enum
}
|
