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<h2 class="chapter-name chapter-name-short">Declarations</h2>
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Module Scope
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Code Blocks
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Import Declaration
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Constant Declaration
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Enumeration Declaration
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Extension Declaration
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Subscript Declaration
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Operator Declaration
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<section class="section">
<p class="para">
A <em>declaration</em> introduces a new name or construct into your program. For example, you use declarations to introduce functions and methods, variables and constants, and to define new, named enumeration, structure, class, and protocol types. You can also use a declaration to extend the the behavior of an existing named type and to import symbols into your program that are declared elsewhere.
</p>
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In Swift, most declarations are also definitions in the sense that they are implemented or initialized at the same time they are declared. That said, because protocols don’t implement their members, most protocol members are declarations only. For convenience and because the distinction isn’t that important in Swift, the term <em>declaration</em> covers both declarations and definitions.
</p>
<div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of a declaration
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/declaration"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/import-declaration" href="#//apple_ref/swift/grammar/import-declaration">import-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_478"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/constant-declaration" href="#//apple_ref/swift/grammar/constant-declaration">constant-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_479"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-declaration" href="#//apple_ref/swift/grammar/variable-declaration">variable-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_480"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/typealias-declaration" href="#//apple_ref/swift/grammar/typealias-declaration">typealias-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_481"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/function-declaration" href="#//apple_ref/swift/grammar/function-declaration">function-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_482"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/enum-declaration" href="#//apple_ref/swift/grammar/enum-declaration">enum-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_483"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/struct-declaration" href="#//apple_ref/swift/grammar/struct-declaration">struct-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_484"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/class-declaration" href="#//apple_ref/swift/grammar/class-declaration">class-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_485"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/protocol-declaration" href="#//apple_ref/swift/grammar/protocol-declaration">protocol-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_486"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/initializer-declaration" href="#//apple_ref/swift/grammar/initializer-declaration">initializer-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_487"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/deinitializer-declaration" href="#//apple_ref/swift/grammar/deinitializer-declaration">deinitializer-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_488"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/extension-declaration" href="#//apple_ref/swift/grammar/extension-declaration">extension-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_489"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/subscript-declaration" href="#//apple_ref/swift/grammar/subscript-declaration">subscript-declaration</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_490"></a>
<span class="syntax-def-name">
declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/operator-declaration" href="#//apple_ref/swift/grammar/operator-declaration">operator-declaration</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/declarations"></a>
<span class="syntax-def-name">
declarations
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/declaration" href="#//apple_ref/swift/grammar/declaration">declaration</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/declarations" href="#//apple_ref/swift/grammar/declarations">declarations</a></span><sub class="subscript">opt</sub></span>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/declaration-specifiers"></a>
<span class="syntax-def-name">
declaration-specifiers
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/declaration-specifier" href="#//apple_ref/swift/grammar/declaration-specifier">declaration-specifier</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/declaration-specifiers" href="#//apple_ref/swift/grammar/declaration-specifiers">declaration-specifiers</a></span><sub class="subscript">opt</sub></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/declaration-specifier"></a>
<span class="syntax-def-name">
declaration-specifier
</span>
<span class="arrow">
→
</span><span class="alternative">
<code class="literal">class</code>
</span><span class="alternative">
<code class="literal">mutating</code>
</span><span class="alternative">
<code class="literal">nonmutating</code>
</span><span class="alternative">
<code class="literal">override</code>
</span><span class="alternative">
<code class="literal">static</code>
</span><span class="alternative">
<code class="literal">unowned</code>
</span><span class="alternative">
<code class="literal">unowned(safe)</code>
</span><span class="alternative">
<code class="literal">unowned(unsafe)</code>
</span><span class="alternative">
<code class="literal">weak</code>
</span>
</p>
</div>
</div>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_494"></a>
<h3 tabindex="0" class="section-name">Module Scope</h3>
<p class="para">
The module scope defines the code that’s visible to other code in Swift source files that are part of the same module. The top-level code in a Swift source file consists of zero or more statements, declarations, and expressions. Variables, constants, and other named declarations that are declared at the top-level of a source file are visible to code in every source file that is part of the same module.
</p><div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of a top-level declaration
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/top-level-declaration"></a>
<span class="syntax-def-name">
top-level-declaration
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/statements" href="Statements.html#//apple_ref/swift/grammar/statements">statements</a></span><sub class="subscript">opt</sub></span>
</p>
</div>
</div>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_497"></a>
<h3 tabindex="0" class="section-name">Code Blocks</h3>
<p class="para">
A <em>code block</em> is used by a variety of declarations and control structures to group statements together. It has the following form:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice">{</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
The <em>statements</em> inside a code block include declarations, expressions, and other kinds of statements and are executed in order of their appearance in source code.
</p><div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of a code block
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/code-block"></a>
<span class="syntax-def-name">
code-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/statements" href="Statements.html#//apple_ref/swift/grammar/statements">statements</a></span><sub class="subscript">opt</sub></span><code class="literal">}</code>
</p>
</div>
</div>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_500"></a>
<h3 tabindex="0" class="section-name">Import Declaration</h3>
<p class="para">
An <em>import declaration</em> lets you access symbols that are declared outside the current file. The basic form imports the entire module; it consists of the <code class="code-voice">import</code> keyword followed by a module name:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">import</span> <em class="variable-text">module</em></pre></li>
</ul>
</div><p class="para">
Providing more detail limits which symbols are imported—you can specify a specific submodule or a specific declaration within a module or submodule. When this detailed form is used, only the imported symbol (and not the module that declares it) is made available in the current scope.
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">import</span> <em class="variable-text">import kind</em> <em class="variable-text">module</em>.<em class="variable-text">symbol name</em></pre></li><li><pre class="code-voice"><span class="kt">import</span> <em class="variable-text">module</em>.<em class="variable-text">submodule</em></pre></li>
</ul>
</div><div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of an import declaration
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/import-declaration"></a>
<span class="syntax-def-name">
import-declaration
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">import</code><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/import-kind" href="#//apple_ref/swift/grammar/import-kind">import-kind</a></span><sub class="subscript">opt</sub></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/import-path" href="#//apple_ref/swift/grammar/import-path">import-path</a></span>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/import-kind"></a>
<span class="syntax-def-name">
import-kind
</span>
<span class="arrow">
→
</span><span class="alternative">
<code class="literal">typealias</code>
</span><span class="alternative">
<code class="literal">struct</code>
</span><span class="alternative">
<code class="literal">class</code>
</span><span class="alternative">
<code class="literal">enum</code>
</span><span class="alternative">
<code class="literal">protocol</code>
</span><span class="alternative">
<code class="literal">var</code>
</span><span class="alternative">
<code class="literal">func</code>
</span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/import-path"></a>
<span class="syntax-def-name">
import-path
</span>
<span class="arrow">
→
</span><span class="alternative">
<span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/import-path-identifier" href="#//apple_ref/swift/grammar/import-path-identifier">import-path-identifier</a></span>
</span><span class="alternative">
<span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/import-path-identifier" href="#//apple_ref/swift/grammar/import-path-identifier">import-path-identifier</a></span><code class="literal">.</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/import-path" href="#//apple_ref/swift/grammar/import-path">import-path</a></span>
</span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/import-path-identifier"></a>
<span class="syntax-def-name">
import-path-identifier
</span>
<span class="arrow">
→
</span><span class="alternative">
<span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/identifier" href="Lexical Structure.html#//apple_ref/swift/grammar/identifier">identifier</a></span>
</span><span class="alternative">
<span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/operator" href="Lexical Structure.html#//apple_ref/swift/grammar/operator">operator</a></span>
</span>
</p>
</div>
</div>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_506"></a>
<h3 tabindex="0" class="section-name">Constant Declaration</h3>
<p class="para">
A <em>constant declaration</em> introduces a constant named value into your program. Constant declarations are declared using the keyword <code class="code-voice">let</code> and have the following form:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">let</span> <em class="variable-text">constant name</em>: <em class="variable-text">type</em> = <em class="variable-text">expression</em></pre></li>
</ul>
</div><p class="para">
A constant declaration defines an immutable binding between the <em>constant name</em> and the value of the initializer <em>expression</em>; after the value of a constant is set, it cannot be changed. That said, if a constant is initialized with a class object, the object itself can change, but the binding between the constant name and the object it refers to can’t.
</p><p class="para">
When a constant is declared at global scope, it must be initialized with a value. When a constant declaration occurs in the context of a class or structure declaration, it is considered a <em>constant property</em>. Constant declarations are not computed properties and therefore do not have getters or setters.
</p><p class="para">
If the <em>constant name</em> of a constant declaration is a tuple pattern, the name of each item in the tuple is bound to the corresponding value in the initializer <em>expression</em>.
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="kt">let</span> (<span class="vc">firstNumber</span>, <span class="vc">secondNumber</span>) = (<span class="m">10</span>, <span class="m">42</span>)</code></li>
</ul>
</div>
</section><p class="para">
In this example, <code class="code-voice">firstNumber</code> is a named constant for the value <code class="code-voice">10</code>, and <code class="code-voice">secondNumber</code> is a named constant for the value <code class="code-voice">42</code>. Both constants can now be used independently:
</p><section class="code-listing">
<span class="caption"></span>
<div class="code-sample">
<ul class="code-lines">
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"The first number is </span>\(<span class="vc">firstNumber</span>)<span class="s">."</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "The first number is 10."</span></code></li>
<li><code class="code-voice"><span class="vc">println</span>(<span class="s">"The second number is </span>\(<span class="vc">secondNumber</span>)<span class="s">."</span>)</code></li>
<li><code class="code-voice"><span class="c">// prints "The second number is 42."</span></code></li>
</ul>
</div>
</section><p class="para">
The type annotation (<code class="code-voice">:</code> <em>type</em>) is optional in a constant declaration when the type of the <em>constant name</em> can be inferred, as described in <span class="x-name"><a data-id="TP40014097-CH31-XID_1029" href="Types.html#TP40014097-CH31-XID_1029">Type Inference</a></span>.
</p><p class="para">
To declare a static constant property, mark the declaration with the <code class="code-voice">static</code> keyword. Static properties are discussed in <span class="x-name"><a data-id="TP40014097-CH14-XID_339" href="Properties.html#TP40014097-CH14-XID_339">Type Properties</a></span>.
</p><p class="para">
For more information about constants and for guidance about when to use them, see <span class="x-name"><a data-id="TP40014097-CH5-XID_400" href="The Basics.html#TP40014097-CH5-XID_400">Constants and Variables</a></span> and <span class="x-name"><a data-id="TP40014097-CH14-XID_324" href="Properties.html#TP40014097-CH14-XID_324">Stored Properties</a></span>.
</p><div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of a constant declaration
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/constant-declaration"></a>
<span class="syntax-def-name">
constant-declaration
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/declaration-specifiers" href="#//apple_ref/swift/grammar/declaration-specifiers">declaration-specifiers</a></span><sub class="subscript">opt</sub></span><code class="literal">let</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/pattern-initializer-list" href="#//apple_ref/swift/grammar/pattern-initializer-list">pattern-initializer-list</a></span>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/pattern-initializer-list"></a>
<span class="syntax-def-name">
pattern-initializer-list
</span>
<span class="arrow">
→
</span><span class="alternative">
<span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/pattern-initializer" href="#//apple_ref/swift/grammar/pattern-initializer">pattern-initializer</a></span>
</span><span class="alternative">
<span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/pattern-initializer" href="#//apple_ref/swift/grammar/pattern-initializer">pattern-initializer</a></span><code class="literal">,</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/pattern-initializer-list" href="#//apple_ref/swift/grammar/pattern-initializer-list">pattern-initializer-list</a></span>
</span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/pattern-initializer"></a>
<span class="syntax-def-name">
pattern-initializer
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/pattern" href="Patterns.html#//apple_ref/swift/grammar/pattern">pattern</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/initializer" href="#//apple_ref/swift/grammar/initializer">initializer</a></span><sub class="subscript">opt</sub></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/initializer"></a>
<span class="syntax-def-name">
initializer
</span>
<span class="arrow">
→
</span><code class="literal">=</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/expression" href="Expressions.html#//apple_ref/swift/grammar/expression">expression</a></span>
</p>
</div>
</div>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_512"></a>
<h3 tabindex="0" class="section-name">Variable Declaration</h3>
<p class="para">
A <em>variable declaration</em> introduces a variable named value into your program and is declared using the keyword <code class="code-voice">var</code>.
</p><p class="para">
Variable declarations have several forms that declare different kinds of named, mutable values, including stored and computed variables and properties, stored variable and property observers, and static variable properties. The appropriate form to use depends on the scope at which the variable is declared and the kind of variable you intend to declare.
</p><div class="note">
<a name="TP40014097-CH34-XID_513"></a>
<aside class="aside">
<p class="aside-title">Note
</p>
<p class="para">You can also declare properties in the context of a protocol declaration, as described in <span class="x-name"><a data-id="TP40014097-CH34-XID_610" href="#TP40014097-CH34-XID_610">Protocol Property Declaration</a></span>.
</p>
</aside>
</div><p class="para">
You can override a property in a subclass by prefixing the subclass’s property declaration with the <code class="code-voice">override</code> keyword, as described in <span class="x-name"><a data-id="TP40014097-CH17-XID_257" href="Inheritance.html#TP40014097-CH17-XID_257">Overriding</a></span>.
</p>
<section class="section">
<a name="TP40014097-CH34-XID_514"></a>
<h3 tabindex="0" class="section-name">Stored Variables and Stored Variable Properties</h3>
<p class="para">
The following form declares a stored variable or stored variable property:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">var</span> <em class="variable-text">variable name</em>: <em class="variable-text">type</em> = <em class="variable-text">expression</em></pre></li>
</ul>
</div><p class="para">
You define this form of a variable declaration at global scope, the local scope of a function, or in the context of a class or structure declaration. When a variable declaration of this form is declared at global scope or the local scope of a function, it is referred to as a <em>stored variable</em>. When it is declared in the context of a class or structure declaration, it is referred to as a <em>stored variable property</em>.
</p><p class="para">
The initializer <em>expression</em> can’t be present in a protocol declaration, but in all other contexts, the initializer <em>expression</em> is optional. That said, if no initializer <em>expression</em> is present, the variable declaration must include an explicit type annotation (<code class="code-voice">:</code> <em>type</em>).
</p><p class="para">
As with constant declarations, if the <em>variable name</em> is a tuple pattern, the name of each item in the tuple is bound to the corresponding value in the initializer <em>expression</em>.
</p><p class="para">
As their names suggest, the value of a stored variable or a stored variable property is stored in memory.
</p>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_515"></a>
<h3 tabindex="0" class="section-name">Computed Variables and Computed Properties</h3>
<p class="para">
The following form declares a computed variable or computed property:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">var</span> <em class="variable-text">variable name</em>: <em class="variable-text">type</em> {</pre></li><li><pre class="code-voice"><span class="kt">get</span> {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li><li><pre class="code-voice"><span class="kt">set</span>(<em class="variable-text">setter name</em>) {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
You define this form of a variable declaration at global scope, the local scope of a function, or in the context of a class, structure, enumeration, or extension declaration. When a variable declaration of this form is declared at global scope or the local scope of a function, it is referred to as a <em>computed variable</em>. When it is declared in the context of a class, structure, or extension declaration, it is referred to as a <em>computed property</em>.
</p><p class="para">
The getter is used to read the value, and the setter is used to write the value. The setter clause is optional, and when only a getter is needed, you can omit both clauses and simply return the requested value directly, as described in <span class="x-name"><a data-id="TP40014097-CH14-XID_331" href="Properties.html#TP40014097-CH14-XID_331">Read-Only Computed Properties</a></span>. But if you provide a setter clause, you must also provide a getter clause.
</p><p class="para">
The <em>setter name</em> and enclosing parentheses is optional. If you provide a setter name, it is used as the name of the parameter to the setter. If you do not provide a setter name, the default parameter name to the setter is <code class="code-voice">newValue</code>, as described in <span class="x-name"><a data-id="TP40014097-CH14-XID_330" href="Properties.html#TP40014097-CH14-XID_330">Shorthand Setter Declaration</a></span>.
</p><p class="para">
Unlike stored named values and stored variable properties, the value of a computed named value or a computed property is not stored in memory.
</p><p class="para">
For more information and to see examples of computed properties, see <span class="x-name"><a data-id="TP40014097-CH14-XID_329" href="Properties.html#TP40014097-CH14-XID_329">Computed Properties</a></span>.
</p>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_516"></a>
<h3 tabindex="0" class="section-name">Stored Variable Observers and Property Observers</h3>
<p class="para">
You can also declare a stored variable or property with <code class="code-voice">willSet</code> and <code class="code-voice">didSet</code> observers. A stored variable or property declared with observers has the following form:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">var</span> <em class="variable-text">variable name</em>: <em class="variable-text">type</em> = <em class="variable-text">expression</em> {</pre></li><li><pre class="code-voice"><span class="kt">willSet</span>(<em class="variable-text">setter name</em>) {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li><li><pre class="code-voice"><span class="kt">didSet</span>(<em class="variable-text">setter name</em> {</pre></li><li><pre class="code-voice"> <em class="variable-text">statements</em></pre></li><li><pre class="code-voice">}</pre></li><li><pre class="code-voice">}</pre></li>
</ul>
</div><p class="para">
You define this form of a variable declaration at global scope, the local scope of a function, or in the context of a class or structure declaration. When a variable declaration of this form is declared at global scope or the local scope of a function, the observers are referred to as <em>stored variable observers</em>. When it is declared in the context of a class or structure declaration, the observers are referred to as <em>property observers</em>.
</p><p class="para">
You can add property observers to any stored property. You can also add property observers to any inherited property (whether stored or computed) by overriding the property within a subclass, as described in <span class="x-name"><a data-id="TP40014097-CH17-XID_263" href="Inheritance.html#TP40014097-CH17-XID_263">Overriding Property Observers</a></span>.
</p><p class="para">
The initializer <em>expression</em> is optional in the context of a class or structure declaration, but required elsewhere. The type annotation is required in all variable declarations that include observers, regardless of the context in which they are declared.
</p><p class="para">
The <code class="code-voice">willSet</code> and <code class="code-voice">didSet</code> observers provide a way to observe (and to respond appropriately) when the value of a variable or property is being set. The observers are not called when the variable or property is first initialized. Instead, they are called only when the value is set outside of an initialization context.
</p><p class="para">
A <code class="code-voice">willSet</code> observer is called just before the value of the variable or property is set. The new value is passed to the <code class="code-voice">willSet</code> observer as a constant, and therefore it can’t be changed in the implementation of the <code class="code-voice">willSet</code> clause. The <code class="code-voice">didSet</code> observer is called immediately after the new value is set. In contrast to the <code class="code-voice">willSet</code> observer, the old value of the variable or property is passed to the <code class="code-voice">didSet</code> observer in case you still need access to it. That said, if you assign a value to a variable or property within its own <code class="code-voice">didSet</code> observer clause, that new value that you assign will replace the one that was just set and passed to the <code class="code-voice">willSet</code> observer.
</p><p class="para">
The <em>setter name</em> and enclosing parentheses in the <code class="code-voice">willSet</code> and <code class="code-voice">didSet</code> clauses are optional. If you provide setter names, they are used as the parameter names to the <code class="code-voice">willSet</code> and <code class="code-voice">didSet</code> observers. If you do not provide setter names, the default parameter name to the <code class="code-voice">willSet</code> observer is <code class="code-voice">newValue</code> and the default parameter name to the <code class="code-voice">didSet</code> observer is <code class="code-voice">oldValue</code>.
</p><p class="para">
The <code class="code-voice">didSet</code> clause is optional when you provide a <code class="code-voice">willSet</code> clause. Likewise, the <code class="code-voice">willSet</code> clause is optional when you provide a <code class="code-voice">didSet</code> clause.
</p><p class="para">
For more information and to see an example of how to use property observers, see <span class="x-name"><a data-id="TP40014097-CH14-XID_333" href="Properties.html#TP40014097-CH14-XID_333">Property Observers</a></span>.
</p>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_517"></a>
<h3 tabindex="0" class="section-name">Class and Static Variable Properties</h3>
<p class="para">
To declare a class computed property, mark the declaration with the <code class="code-voice">class</code> keyword. To declare a static variable property, mark the declaration with the <code class="code-voice">static</code> keyword. Class and static properties are discussed in <span class="x-name"><a data-id="TP40014097-CH14-XID_339" href="Properties.html#TP40014097-CH14-XID_339">Type Properties</a></span>.
</p><div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of a variable declaration
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/variable-declaration"></a>
<span class="syntax-def-name">
variable-declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-declaration-head" href="#//apple_ref/swift/grammar/variable-declaration-head">variable-declaration-head</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/pattern-initializer-list" href="#//apple_ref/swift/grammar/pattern-initializer-list">pattern-initializer-list</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_520"></a>
<span class="syntax-def-name">
variable-declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-declaration-head" href="#//apple_ref/swift/grammar/variable-declaration-head">variable-declaration-head</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-name" href="#//apple_ref/swift/grammar/variable-name">variable-name</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/type-annotation" href="Types.html#//apple_ref/swift/grammar/type-annotation">type-annotation</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/code-block" href="#//apple_ref/swift/grammar/code-block">code-block</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_521"></a>
<span class="syntax-def-name">
variable-declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-declaration-head" href="#//apple_ref/swift/grammar/variable-declaration-head">variable-declaration-head</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-name" href="#//apple_ref/swift/grammar/variable-name">variable-name</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/type-annotation" href="Types.html#//apple_ref/swift/grammar/type-annotation">type-annotation</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/getter-setter-block" href="#//apple_ref/swift/grammar/getter-setter-block">getter-setter-block</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_522"></a>
<span class="syntax-def-name">
variable-declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-declaration-head" href="#//apple_ref/swift/grammar/variable-declaration-head">variable-declaration-head</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-name" href="#//apple_ref/swift/grammar/variable-name">variable-name</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/type-annotation" href="Types.html#//apple_ref/swift/grammar/type-annotation">type-annotation</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/getter-setter-keyword-block" href="#//apple_ref/swift/grammar/getter-setter-keyword-block">getter-setter-keyword-block</a></span>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_523"></a>
<span class="syntax-def-name">
variable-declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-declaration-head" href="#//apple_ref/swift/grammar/variable-declaration-head">variable-declaration-head</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/variable-name" href="#//apple_ref/swift/grammar/variable-name">variable-name</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/type-annotation" href="Types.html#//apple_ref/swift/grammar/type-annotation">type-annotation</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/initializer" href="#//apple_ref/swift/grammar/initializer">initializer</a></span><sub class="subscript">opt</sub></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/willSet-didSet-block" href="#//apple_ref/swift/grammar/willSet-didSet-block">willSet-didSet-block</a></span>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/variable-declaration-head"></a>
<span class="syntax-def-name">
variable-declaration-head
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/declaration-specifiers" href="#//apple_ref/swift/grammar/declaration-specifiers">declaration-specifiers</a></span><sub class="subscript">opt</sub></span><code class="literal">var</code>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/variable-name"></a>
<span class="syntax-def-name">
variable-name
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/identifier" href="Lexical Structure.html#//apple_ref/swift/grammar/identifier">identifier</a></span>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/getter-setter-block"></a>
<span class="syntax-def-name">
getter-setter-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/getter-clause" href="#//apple_ref/swift/grammar/getter-clause">getter-clause</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-clause" href="#//apple_ref/swift/grammar/setter-clause">setter-clause</a></span><sub class="subscript">opt</sub></span><code class="literal">}</code>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_527"></a>
<span class="syntax-def-name">
getter-setter-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-clause" href="#//apple_ref/swift/grammar/setter-clause">setter-clause</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/getter-clause" href="#//apple_ref/swift/grammar/getter-clause">getter-clause</a></span><code class="literal">}</code>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/getter-clause"></a>
<span class="syntax-def-name">
getter-clause
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">get</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/code-block" href="#//apple_ref/swift/grammar/code-block">code-block</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/setter-clause"></a>
<span class="syntax-def-name">
setter-clause
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">set</code><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-name" href="#//apple_ref/swift/grammar/setter-name">setter-name</a></span><sub class="subscript">opt</sub></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/code-block" href="#//apple_ref/swift/grammar/code-block">code-block</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/setter-name"></a>
<span class="syntax-def-name">
setter-name
</span>
<span class="arrow">
→
</span><code class="literal">(</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/identifier" href="Lexical Structure.html#//apple_ref/swift/grammar/identifier">identifier</a></span><code class="literal">)</code>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/getter-setter-keyword-block"></a>
<span class="syntax-def-name">
getter-setter-keyword-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/getter-keyword-clause" href="#//apple_ref/swift/grammar/getter-keyword-clause">getter-keyword-clause</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-keyword-clause" href="#//apple_ref/swift/grammar/setter-keyword-clause">setter-keyword-clause</a></span><sub class="subscript">opt</sub></span><code class="literal">}</code>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_532"></a>
<span class="syntax-def-name">
getter-setter-keyword-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-keyword-clause" href="#//apple_ref/swift/grammar/setter-keyword-clause">setter-keyword-clause</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/getter-keyword-clause" href="#//apple_ref/swift/grammar/getter-keyword-clause">getter-keyword-clause</a></span><code class="literal">}</code>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/getter-keyword-clause"></a>
<span class="syntax-def-name">
getter-keyword-clause
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">get</code>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/setter-keyword-clause"></a>
<span class="syntax-def-name">
setter-keyword-clause
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">set</code>
</p>
</div><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/willSet-didSet-block"></a>
<span class="syntax-def-name">
willSet-didSet-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/willSet-clause" href="#//apple_ref/swift/grammar/willSet-clause">willSet-clause</a></span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/didSet-clause" href="#//apple_ref/swift/grammar/didSet-clause">didSet-clause</a></span><sub class="subscript">opt</sub></span><code class="literal">}</code>
</p><p class="syntax-def">
<a name="TP40014097-CH34-XID_536"></a>
<span class="syntax-def-name">
willSet-didSet-block
</span>
<span class="arrow">
→
</span><code class="literal">{</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/didSet-clause" href="#//apple_ref/swift/grammar/didSet-clause">didSet-clause</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/willSet-clause" href="#//apple_ref/swift/grammar/willSet-clause">willSet-clause</a></span><code class="literal">}</code>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/willSet-clause"></a>
<span class="syntax-def-name">
willSet-clause
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">willSet</code><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-name" href="#//apple_ref/swift/grammar/setter-name">setter-name</a></span><sub class="subscript">opt</sub></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/code-block" href="#//apple_ref/swift/grammar/code-block">code-block</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/didSet-clause"></a>
<span class="syntax-def-name">
didSet-clause
</span>
<span class="arrow">
→
</span><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/attributes" href="Attributes.html#//apple_ref/swift/grammar/attributes">attributes</a></span><sub class="subscript">opt</sub></span><code class="literal">didSet</code><span class="optional"><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/setter-name" href="#//apple_ref/swift/grammar/setter-name">setter-name</a></span><sub class="subscript">opt</sub></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/code-block" href="#//apple_ref/swift/grammar/code-block">code-block</a></span>
</p>
</div>
</div>
</section>
</section>
<section class="section">
<a name="TP40014097-CH34-XID_539"></a>
<h3 tabindex="0" class="section-name">Type Alias Declaration</h3>
<p class="para">
A <em>type alias declaration</em> introduces a named alias of an existing type into your program. Type alias declarations begin with the keyword <code class="code-voice">typealias</code> and have the following form:
</p><span class="caption"></span>
<div class="code-outline">
<ul class="code-outline-lines">
<li><pre class="code-voice"><span class="kt">typealias</span> <em class="variable-text">name</em> = <em class="variable-text">existing type</em></pre></li>
</ul>
</div><p class="para">
After a type alias is declared, the aliased <em>name</em> can be used instead of the <em>existing type</em> everywhere in your program. The <em>existing type</em> can be a named type or a compound type. Type aliases do not create new types; they simply allow a name to refer to an existing type.
</p><p class="para">
See also <span class="x-name"><a data-id="TP40014097-CH34-XID_622" href="#TP40014097-CH34-XID_622">Protocol Associated Type Declaration</a></span>.
</p><div class="syntax-defs">
<p class="syntax-defs-name">
Grammar of a type alias declaration
</p><div class="syntax-defs-group">
<p class="syntax-def">
<a name="//apple_ref/swift/grammar/typealias-declaration"></a>
<span class="syntax-def-name">
typealias-declaration
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/typealias-head" href="#//apple_ref/swift/grammar/typealias-head">typealias-head</a></span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/typealias-assignment" href="#//apple_ref/swift/grammar/typealias-assignment">typealias-assignment</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/typealias-head"></a>
<span class="syntax-def-name">
typealias-head
</span>
<span class="arrow">
→
</span><code class="literal">typealias</code><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/typealias-name" href="#//apple_ref/swift/grammar/typealias-name">typealias-name</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/typealias-name"></a>
<span class="syntax-def-name">
typealias-name
</span>
<span class="arrow">
→
</span><span class="syntactic-cat"><a data-id="//apple_ref/swift/grammar/identifier" href="Lexical Structure.html#//apple_ref/swift/grammar/identifier">identifier</a></span>
</p><p class="syntax-def">
<a name="//apple_ref/swift/grammar/typealias-assignment"></a>