Feature: util hash

src/content/docs/cpp/language/named_req/_meta.yml

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+label: Name Requirements

src/content/docs/cpp/language/named_req/copy-asssignable.mdx

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+---
+title: "C++ named requirements: CopyAssignable"
+cppdoc:
+  revision:
+    since: C++11
+---
+
+import { DR, DRList } from "@components/defect-report";
+import { Desc, DescList } from "@components/desc-list";
+import Missing from "@components/Missing.astro";
+import { Revision, RevisionBlock } from "@components/revision";
+
+Specifies that an instance of the type can be copy-assigned from an lvalue expression.
+
+
+## Requirements
+
+The type T satisfies _CopyAssignable_ if
+
+- The type T satisfies _MoveAssignable_, and
+
+Given
+
+- `t`, a modifiable lvalue expression of type `T`,
+- `v`, an lvalue expression of type `T` or `const T` or an rvalue expression of type `const T`.
+
+The following expressions must be valid and have their specified effects.
+
+|Expression| Return type | Return value |Post-conditions|
+|---|---|---|---|
+|`t=v`| `T&` | `t` |The value of t is equivalent to the value of v.<br/> The value of v is unchanged.  |
+
+## See also
+
+<DescList>
+  <Desc kind="class template">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_copy_assignable` </Missing>
+      </RevisionBlock>
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_trivially_copy_assignable` </Missing>
+      </RevisionBlock>
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_nothrow_copy_assignable` </Missing>
+      </RevisionBlock>
+    </Fragment>
+    checks if a type has a copy assignment operator
+  </Desc>
+  <Desc kind="concept">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++20">
+        <Missing> `assignable_from` </Missing>
+      </RevisionBlock>
+    </Fragment>
+    specifies that an object of a type can be default constructed
+  </Desc>
+</DescList>
+

src/content/docs/cpp/language/named_req/default-constructible.mdx

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+---
+title: "C++ named requirements: DefaultConstructible"
+cppdoc:
+  revision:
+    since: C++11
+---
+
+import { DR, DRList } from "@components/defect-report";
+import { Desc, DescList } from "@components/desc-list";
+import Missing from "@components/Missing.astro";
+import { Revision, RevisionBlock } from "@components/revision";
+
+Specifies that an instance of the type can be default constructed.
+
+
+## Requirements
+The type `T` satisfies _DefaultConstructible_ if all following statements and expressions are valid and have their specified effects:
+
+Given
+
+- `u`, an expression of type `T`.
+
+- `u`, an lvalue expression of type `Key`.
+
+|Expression/Statement| Postcondition|
+|---|---|
+|`T u;`| The object `u` is default-initialized.|
+|`T u{};`| The object `u` is value-initialized or aggregate-initialized.|
+|`T()`| All resources owned by `u` are reclaimed, no exceptions are thrown.|
+|`T{}`| A temporary object of type `T` is value-initialized or aggregate-initialized.|
+
+## Notes
+
+For objects of non-aggregate class type, a public default constructor must be defined (either user-defined or implicitly defined) to satisfy _DefaultConstructible_.
+
+Non-const objects of non-class object type are always _DefaultConstructible_.
+
+Const non-class types are not _DefaultConstructible_.
+
+Const aggregate types are not _DefaultConstructible_ if any of their members is an object of non-class type.
+
+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_.
+
+# Defect reports
+
+The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
+
+<DRList>
+  <DR kind="lwg" id={724} std="C++98">
+    <Fragment slot="behavior-published">
+      the requirements of _DefaultConstructible_ were missing
+    </Fragment>
+    <Fragment slot="correct-behavior">
+      added
+    </Fragment>
+  </DR>
+  <DR kind="lwg" id={2170} std="C++98">
+    <Fragment slot="behavior-published">
+      initialzing an object of a _DefaultConstructible_ type with an
+empty initializer could only result in value-initialization
+    </Fragment>
+    <Fragment slot="correct-behavior">
+      can also lead to aggregate-initialization
+    </Fragment>
+  </DR>
+</DRList>
+## See also
+
+<DescList>
+  <Desc kind="class template">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_default_constructible` </Missing>
+      </RevisionBlock>
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_trivially_default_constructible` </Missing>
+      </RevisionBlock>
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_nothrow_default_constructible` </Missing>
+      </RevisionBlock>
+    </Fragment>
+    checks if a type has a default constructor
+  </Desc>
+  <Desc kind="concept">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++20">
+        <Missing> `default_initializable` </Missing>
+      </RevisionBlock>
+    </Fragment>
+    specifies that an object of a type can be default constructed
+  </Desc>
+</DescList>
+

src/content/docs/cpp/language/named_req/destructible.mdx

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+---
+title: "C++ named requirements: Destructible"
+cppdoc:
+  revision:
+    since: C++11
+---
+
+import { DR, DRList } from "@components/defect-report";
+import { Desc, DescList } from "@components/desc-list";
+import Missing from "@components/Missing.astro";
+import { Revision, RevisionBlock } from "@components/revision";
+
+Specifies that an instance of the type can be destructed.
+
+
+## Requirements
+The type `T` satisfies `Destructible` if
+
+Given
+
+- `u`, an expression of type `T`.
+
+- `u`, an lvalue expression of type `Key`.
+
+|Expression| Post-Conditions|
+|---|---|
+|`u.~T()`| All resources owned by `u` are reclaimed, no exceptions are thrown.|
+
+## Notes
+
+
+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.
+
+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.
+
+## See also
+
+<DescList>
+  <Desc kind="class template">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_destructible` </Missing>
+      </RevisionBlock>
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_trivially_destructible` </Missing>
+      </RevisionBlock>
+      <RevisionBlock noborder since="C++11">
+        <Missing> `std::is_nothrow_destructible` </Missing>
+      </RevisionBlock>
+    </Fragment>
+    checks if a type has a non-deleted destructor
+  </Desc>
+  <Desc kind="concept">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++20">
+        <Missing> `destructible` </Missing>
+      </RevisionBlock>
+    </Fragment>
+    specifies that an object of the type can be destroyed
+  </Desc>
+</DescList>
+

src/content/docs/cpp/language/named_req/hash.mdx

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+---
+title: "C++ named requirements: Hash"
+cppdoc:
+  revision:
+    since: C++11
+---
+
+import { DR, DRList } from "@components/defect-report";
+import { Desc, DescList } from "@components/desc-list";
+import DocLink from "@components/DocLink.astro"
+import { Revision, RevisionBlock } from "@components/revision";
+
+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.
+
+
+## Requirements
+The type `T` satisfies `Hash` if
+
+The type `T` satisfies `FunctionObject`, `CopyConstructible`, `Destructible`, and
+Given
+
+- `h`, a value of type `T` or const `T`, whose argument type is `Key`,
+- `k`, a value of type convertible to `Key` or `const Key`,
+- `u`, an lvalue expression of type `Key`.
+
+The following expressions must be valid and have their specified effects.
+
+|Expression| Return type| Requirements|
+|---|---|---|
+|`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()`.|
+|`h(u)` | `std::size_t` | `u` is not modified.|
+
+## Standard Library
+
+<DescList>
+  <Desc kind="class template">
+    <Fragment slot="item">
+      <RevisionBlock noborder since="C++11" vertical>
+        <DocLink dest="/cpp/library/utility/hash"> `hash` </DocLink>
+      </RevisionBlock>
+    </Fragment>
+    hash function object
+  </Desc>
+</DescList>
+
+# Defect reports
+
+The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
+
+<DRList>
+  <DR kind="lwg" id={2291} std="C++11">
+    <Fragment slot="behavior-published">
+      same results for same arguments were required in all cases
+    </Fragment>
+    <Fragment slot="correct-behavior">
+      only required within a single execution
+    </Fragment>
+  </DR>
+</DRList>
+