# declaration

[Swift](https://lochiwei.gitbook.io/ios/swift) ⟩ [concept](https://lochiwei.gitbook.io/ios/master/term) ⟩ declaration

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defines <mark style="color:yellow;">**new variables or types**</mark>.
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{% tab title="🔴 主題" %}

* [func](https://lochiwei.gitbook.io/ios/swift/type/category/basic/func "mention")
* [class](https://lochiwei.gitbook.io/ios/swift/type/category/basic/class "mention")
* [enum](https://lochiwei.gitbook.io/ios/swift/type/category/basic/enum "mention")
* struct 🚧
* [protocol](https://lochiwei.gitbook.io/ios/swift/type/category/protocol "mention")
* ... 🚧
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{% tab title="⭐️ 重點" %}
{% hint style="success" %} <mark style="color:orange;">**use cases**</mark>&#x20;

* define [func](https://lochiwei.gitbook.io/ios/swift/type/category/basic/func "mention")s and **methods** 🚧
* define **variables** and **constants**
* define [enum](https://lochiwei.gitbook.io/ios/swift/type/category/basic/enum "mention"), <mark style="color:blue;">**struct**</mark>  🚧, [class](https://lochiwei.gitbook.io/ios/swift/type/category/basic/class "mention"), [protocol](https://lochiwei.gitbook.io/ios/swift/type/category/protocol "mention")
* <mark style="color:yellow;">**extend**</mark> the behavior of an <mark style="color:yellow;">**existing**</mark>**&#x20;**<mark style="color:orange;">**named**</mark>**&#x20;**<mark style="color:yellow;">**type**</mark>
* <mark style="color:yellow;">**import**</mark> symbols into your program that are declared elsewhere.
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GRAMMAR OF A DECLARATION

* declaration → [import-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_import-declaration)
* declaration → [constant-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_constant-declaration)
* declaration → [variable-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_variable-declaration)
* declaration → [typealias-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_typealias-declaration)
* declaration → [function-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_function-declaration)
* declaration → [enum-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_enum-declaration)
* declaration → [struct-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_struct-declaration)
* declaration → [class-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_class-declaration)
* declaration → [actor-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_actor-declaration)
* declaration → [protocol-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_protocol-declaration)
* declaration → [initializer-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_initializer-declaration)
* declaration → [deinitializer-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_deinitializer-declaration)
* declaration → [extension-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_extension-declaration)
* declaration → [subscript-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_subscript-declaration)
* declaration → [operator-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_operator-declaration)
* declaration → [precedence-group-declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_precedence-group-declaration)
* declarations → [declaration](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_declaration) [declarations](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html#grammar_declarations) opt
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In Swift, most <mark style="color:purple;">**declarations**</mark> are also <mark style="color:yellow;">**definitions**</mark> in the sense that they’re implemented or <mark style="color:yellow;">**initialized**</mark> **at the&#x20;**<mark style="color:yellow;">**same time**</mark>**&#x20;they’re&#x20;**<mark style="color:yellow;">**declared**</mark>, .... the distinction <mark style="color:red;">**isn’t**</mark>**&#x20;**<mark style="color:yellow;">**that important**</mark> in Swift, the term <mark style="color:purple;">**declaration**</mark> covers both <mark style="color:orange;">declarations</mark> and <mark style="color:orange;">definitions</mark>.
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{% tab title="👥 相關" %}

* <mark style="color:purple;">**declaration**</mark> is a [](https://lochiwei.gitbook.io/ios/swift/statement/simple "mention").
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{% tab title="📘 手冊" %}

* [ ] [Declarations](https://docs.swift.org/swift-book/ReferenceManual/Declarations.html)
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