decay-copy

 

C++

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Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Standard library

Customization point object (C++20)

Exposition-only entities

decay-copy (C++11)

synth-three-waysynth-three-way-result (C++20)(C++20)

 

template< class T > typename std::decay<T>::type decay-copy( T&& value );		(since C++11) (until C++20) (exposition only*)
template< class T >     requires std::convertible_to<T, std::decay_t<T>> constexpr std::decay_t<T> decay-copy( T&& value )     noexcept(std::is_nothrow_convertible_v<T, std::decay_t<T>>);		(since C++20) (exposition only*)

Returns std::forward<T>(value) (implicitly converted to the decayed type), a decayed prvalue copy of value.

Parameters

value

-

the value to be copied

Return value

A decayed copy of value as a prvalue.

Notes

decay-copy was introduced by the resolution of LWG issue 929. It is initially used in the concurrency support library to ensure that arguments are decayed when passing-by-value, and is later used in the ranges library.

The language feature auto(x) introduced in C++23 also allows decayed copies to be created as prvalues. The only difference is that decay-copy always materializes value and produces a copy, while auto(expr) is a no-op if expr is a prvalue.

All usages of decay-copy in the standard library (see below) except views::all, ranges::take_view and ranges::drop_view are replaced with auto(x) since C++23.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3724	C++20	decay-copy was not constrained	constrained

See also

(constructor)	constructs new thread object (public member function of std::thread)
(constructor)	constructs new jthread object (public member function of std::jthread)
async (C++11)	runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)
ranges::begin (C++20)	returns an iterator to the beginning of a range (customization point object)
ranges::end (C++20)	returns a sentinel indicating the end of a range (customization point object)
ranges::rbegin (C++20)	returns a reverse iterator to a range (customization point object)
ranges::rend (C++20)	returns a reverse end iterator to a range (customization point object)
ranges::size (C++20)	returns an integer equal to the size of a range (customization point object)
ranges::data (C++20)	obtains a pointer to the beginning of a contiguous range (customization point object)
views::all_tviews::all (C++20)	a view that includes all elements of a range (alias template) (range adaptor object)
ranges::take_viewviews::take (C++20)	a view consisting of the first N elements of another view (class template) (range adaptor object)
ranges::drop_viewviews::drop (C++20)	a view consisting of elements of another view, skipping the first N elements (class template) (range adaptor object)