cppreference.com

Namespaces

Variants

Actions

decay-copy

From cppreference.com

< cpp‎ | standard library

Compiler support

Freestanding and hosted

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

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.

Contents

[edit] Parameters

value - the value to be copied

[edit] Return value

A decayed copy of value as a prvalue.

[edit] 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.

[edit] 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

[edit] See also

constructs new thread object
(public member function of std::thread) [edit]

constructs new jthread object
(public member function of std::jthread) [edit]

(C++11)

runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result
(function template) [edit]

(C++20)

returns an iterator to the beginning of a range
(customization point object)[edit]

(C++20)

returns a sentinel indicating the end of a range
(customization point object)[edit]

(C++20)

returns a reverse iterator to a range
(customization point object)[edit]

(C++20)

returns a reverse end iterator to a range
(customization point object)[edit]

(C++20)

returns an integer equal to the size of a range
(customization point object)[edit]

(C++20)

obtains a pointer to the beginning of a contiguous range
(customization point object)[edit]

views::all_tviews::all

(C++20)

a view that includes all elements of a range
(alias template) (range adaptor object)[edit]

ranges::take_viewviews::take

(C++20)

a view consisting of the first N elements of another view
(class template) (range adaptor object)[edit]

ranges::drop_viewviews::drop

(C++20)

a view consisting of elements of another view, skipping the first N elements
(class template) (range adaptor object)[edit]

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/standard_library/decay-copy&oldid=157132"