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cppdraft/func/require.md
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cppdraft/func/require.md
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[func.require]
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# 22 General utilities library [[utilities]](./#utilities)
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## 22.10 Function objects [[function.objects]](function.objects#func.require)
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### 22.10.4 Requirements [func.require]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11501)
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Define *INVOKE*(f, t1, t2, …, tN) as follows:
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- [(1.1)](#1.1)
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(t1.*f)(t2, …, tN) when f is a pointer to a
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member function of a class T andis_same_v<T, remove_cvref_t<decltype(t1)>> ||is_base_of_v<T, remove_cvref_t<decltype(t1)>> is true;
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- [(1.2)](#1.2)
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(t1.get().*f)(t2, …, tN) when f is a pointer to a
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member function of a class T and remove_cvref_t<decltype(t1)> is a specialization of reference_wrapper;
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- [(1.3)](#1.3)
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((*t1).*f)(t2, …, tN) when f is a pointer to a
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member function of a class T and t1 does not satisfy the previous two items;
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- [(1.4)](#1.4)
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t1.*f when N=1 and f is a pointer to
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data member of a class T andis_same_v<T, remove_cvref_t<decltype(t1)>> ||is_base_of_v<T, remove_cvref_t<decltype(t1)>> is true;
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- [(1.5)](#1.5)
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t1.get().*f when N=1 and f is a pointer to
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data member of a class T and remove_cvref_t<decltype(t1)> is a specialization of reference_wrapper;
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- [(1.6)](#1.6)
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(*t1).*f when N=1 and f is a pointer to
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data member of a class T and t1 does not satisfy the previous two items;
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- [(1.7)](#1.7)
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f(t1, t2, …, tN) in all other cases[.](#1.sentence-1)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11537)
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Define *INVOKE*<R>(f, t1, t2, …, tN) asstatic_cast<void>(*INVOKE*(f, t1, t2, …, tN)) if R is cv void, otherwise*INVOKE*(f, t1, t2, …, tN) implicitly converted
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to R[.](#2.sentence-1)
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Ifreference_converts_from_temporary_v<R, decltype(*INVOKE*(f, t1, t2, …, tN))> is true,*INVOKE*<R>(f, t1, t2, …, tN) is ill-formed[.](#2.sentence-2)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11550)
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Every call wrapper ([[func.def]](func.def "22.10.3 Definitions")) meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#3.sentence-1)
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An [*argument forwarding call wrapper*](#def:argument_forwarding_call_wrapper "22.10.4 Requirements [func.require]") is a
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call wrapper that can be called with an arbitrary argument list
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and delivers the arguments to the target object as references[.](#3.sentence-2)
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This forwarding step delivers rvalue arguments as rvalue references
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and lvalue arguments as lvalue references[.](#3.sentence-3)
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[*Note [1](#note-1)*:
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In a typical implementation, argument forwarding call wrappers have
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an overloaded function call operator of the formtemplate<class... UnBoundArgs>constexpr R operator()(UnBoundArgs&&... unbound_args) *cv-qual*;
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â *end note*]
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11570)
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A [*perfect forwarding call wrapper*](#def:call_wrapper,perfect_forwarding "22.10.4 Requirements [func.require]") is
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an argument forwarding call wrapper
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that forwards its state entities to the underlying call expression[.](#4.sentence-1)
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This forwarding step delivers a state entity of type T as cv T& when the call is performed on an lvalue of the call wrapper type and
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as cv T&& otherwise,
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where cv represents the cv-qualifiers of the call wrapper and
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where cv shall be neither volatile nor const volatile[.](#4.sentence-2)
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11582)
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A [*call pattern*](#def:call_pattern "22.10.4 Requirements [func.require]") defines the semantics of invoking
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a perfect forwarding call wrapper[.](#5.sentence-1)
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A postfix call performed on a perfect forwarding call wrapper is
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expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) to
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an expression e determined from its call pattern cp by replacing all occurrences
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of the arguments of the call wrapper and its state entities
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with references as described in the corresponding forwarding steps[.](#5.sentence-2)
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11592)
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A [*simple call wrapper*](#def:simple_call_wrapper "22.10.4 Requirements [func.require]") is a perfect forwarding call wrapper that meets
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the [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements
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and whose copy constructor, move constructor, and assignment operators
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are constexpr functions that do not throw exceptions[.](#6.sentence-1)
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11599)
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The copy/move constructor of an argument forwarding call wrapper has
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the same apparent semantics
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as if memberwise copy/move of its state entities
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were performed ([[class.copy.ctor]](class.copy.ctor "11.4.5.3 Copy/move constructors"))[.](#7.sentence-1)
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[*Note [2](#note-2)*:
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This implies that each of the copy/move constructors has
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the same exception-specification as
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the corresponding implicit definition and is declared as constexpr if the corresponding implicit definition would be considered to be constexpr[.](#7.sentence-2)
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â *end note*]
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L11611)
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Argument forwarding call wrappers returned by
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a given standard library function template have the same type
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if the types of their corresponding state entities are the same[.](#8.sentence-1)
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