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feat(named_req) Hashable, DefaultConstructible, Destructible
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‎src/content/docs/cpp/language/named_req/DefaultConstructible.mdx‎

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---
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title: "C++ named requirements: DefaultConstructible"
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cppdoc:
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revision:
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since: C++11
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---
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import { DR, DRList } from "@components/defect-report";
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import { Desc, DescList } from "@components/desc-list";
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Specifies that an instance of the type can be default constructed.
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## Requirements
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The type `T` satisfies _DefaultConstructible_ if all following statements and expressions are valid and have their specified effects:
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Given
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- `u`, an expression of type `T`.
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- `u`, an lvalue expression of type `Key`.
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|Expression/Statement| Postcondition|
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|---|---|
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|`T u;`| The object `u` is default-initialized.|
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|`T u{};`| The object `u` is value-initialized or aggregate-initialized.|
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|`T()`| All resources owned by `u` are reclaimed, no exceptions are thrown.|
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|`T{}`| A temporary object of type `T` is value-initialized or aggregate-initialized.|
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## Notes
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For objects of non-aggregate class type, a public default constructor must be defined (either user-defined or implicitly defined) to satisfy _DefaultConstructible_.
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Non-const objects of non-class object type are always _DefaultConstructible_.
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Const non-class types are not _DefaultConstructible_.
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Const aggregate types are not _DefaultConstructible_ if any of their members is an object of non-class type.
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Non-object types (function types, reference types, and the (possibly cv-qualified) type void) as well as the const non-object types are never _DefaultConstructible_.
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# Defect reports
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The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
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<DRList>
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<DR kind="lwg 724" id={1} std="C++98">
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<Fragment slot="behavior-published">
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the requirements of _DefaultConstructible_ were missing
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</Fragment>
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<Fragment slot="correct-behavior">
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added
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</Fragment>
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</DR>
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<DR kind="lwg 2170" id={1} std="C++98">
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<Fragment slot="behavior-published">
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initialzing an object of a _DefaultConstructible_ type with an
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empty initializer could only result in value-initialization
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</Fragment>
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<Fragment slot="correct-behavior">
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can also lead to aggregate-initialization
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</Fragment>
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</DR>
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</DRList>
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## See also
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<DescList>
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<Desc kind="class template">
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<Fragment slot="item">
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<RevisionBlock noborder since="C++11" vertical>
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<Missing> `std::is_default_constructible` </Missing>
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</RevisionBlock>
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<RevisionBlock noborder since="C++11" vertical>
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<Missing> `std::is_trivially_default_constructible` </Missing>
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</RevisionBlock>
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<RevisionBlock noborder since="C++11" vertical>
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<Missing> `std::is_nothrow_default_constructible` </Missing>
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</RevisionBlock>
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</Fragment>
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checks if a type has a default constructor
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</Desc>
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<Desc kind="concept">
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<Fragment slot="item">
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<RevisionBlock noborder since="C++20" vertical>
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<Missing> `default_initializable` </Missing>
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</RevisionBlock>
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</Fragment>
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specifies that an object of a type can be default constructed
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</Desc>
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</DescList>
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‎src/content/docs/cpp/language/named_req/Destructible.mdx‎

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---
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title: "C++ named requirements: Destructible"
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cppdoc:
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revision:
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since: C++11
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---
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import { DR, DRList } from "@components/defect-report";
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import { Desc, DescList } from "@components/desc-list";
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Specifies that an instance of the type can be destructed.
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## Requirements
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The type `T` satisfies `Destructible` if
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Given
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- `u`, an expression of type `T`.
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- `u`, an lvalue expression of type `Key`.
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|Expression| Post-Conditions|
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|---|---|
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|`u.~T()`| All resources owned by `u` are reclaimed, no exceptions are thrown.|
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## Notes
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Destructors are called implicitly at the end of object lifetime such as when leaving scope or by the delete-expression. Explicit destructor call as shown in the type requirement table is rare.
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Thanks to pseudo destructor call, all scalar types meet the requirement of Destructible, while array types and reference types do not. Note that `std::is_destructible` allows arrays and reference types.
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## See also
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<DescList>
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<Desc kind="class template">
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<Fragment slot="item">
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<RevisionBlock noborder since="C++11" vertical>
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<Missing> `std::is_destructible` </Missing>
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</RevisionBlock>
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<RevisionBlock noborder since="C++11" vertical>
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<Missing> `std::is_trivially_destructible` </Missing>
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</RevisionBlock>
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<RevisionBlock noborder since="C++11" vertical>
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<Missing> `std::is_nothrow_destructible` </Missing>
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</RevisionBlock>
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</Fragment>
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checks if a type has a non-deleted destructor
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</Desc>
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<Desc kind="concept">
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<Fragment slot="item">
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<RevisionBlock noborder since="C++20" vertical>
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<Missing> `destructible` </Missing>
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</RevisionBlock>
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</Fragment>
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specifies that an object of the type can be destroyed
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</Desc>
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</DescList>
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‎src/content/docs/cpp/language/named_req/Hash.mdx‎

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---
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title: "C++ named requirements: Hash"
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cppdoc:
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revision:
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since: C++11
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---
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import { DR, DRList } from "@components/defect-report";
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import { Desc, DescList } from "@components/desc-list";
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A ***Hash*** is a function object for which the output depends only on the input and has a very low probability of yielding the same output given different input values.
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## Requirements
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The type `T` satisfies `Hash` if
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The type `T` satisfies `FunctionObject`, `CopyConstructible`, `Destructible`, and
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Given
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- `h`, a value of type `T` or const `T`, whose argument type is `Key`,
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- `k`, a value of type convertible to `Key` or `const Key`,
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- `u`, an lvalue expression of type `Key`.
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The following expressions must be valid and have their specified effects.
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|Expression| Return type| Requirements|
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|---|---|---|
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|`h(k)`| `std::size_t`|The returned value depends only on the value of `k` for the duration of the program. <br/> All evaluations of `h(k)` executed within a given execution of a program yield the same result for the same value of `k`.<br/> The probability of `h(a) == h(b)` for `a != b` should approach `1.0 / std::numeric_limits<std::size_t>::max()`.|
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|`h(u)` | `std::size_t` | `u` is not modified.|
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## Standard Library
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<DescList>
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<Desc kind="class template">
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<Fragment slot="item">
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<RevisionBlock noborder since="C++11" vertical>
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<DocLink dest="/cpp/library/utility/hash"> `hash` </DocLink>
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</RevisionBlock>
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</Fragment>
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hash function object
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</Desc>
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</DescList>
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# Defect reports
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The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
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<DRList>
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<DR kind="lwg 2291" id={1} std="C++11">
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<Fragment slot="behavior-published">
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same results for same arguments were required in all cases
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</Fragment>
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<Fragment slot="correct-behavior">
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only required within a single execution
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</Fragment>
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</DR>
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</DRList>
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‎src/content/docs/cpp/language/named_req/_meta.yml‎

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label: Name Requirements

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