template< class T > typename std::decay<T>::type decay-copy( T&& value ); // exposition only | (since C++11) (until C++20) | |
template< class T >
requires std::convertible_to<T, std::decay_t<T>>
constexpr std::decay_t<T> decay-copy( T&& value ) // exposition only
noexcept(std::is_nothrow_convertible_v<T, std::decay_t<T>>);
| (since C++20) |
Returns std::forward<T>(value) (implicitly converted to the decayed type), a decayed prvalue copy of value.
| value | - | the value to be copied |
A decayed copy of value as a prvalue.
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.
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 |
constructs new thread object (public member function of std::thread) |
|
constructs new jthread object (public member function of std::jthread) |
|
|
(C++11) | runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template) |
|
(C++20) | returns an iterator to the beginning of a range (customization point object) |
|
(C++20) | returns a sentinel indicating the end of a range (customization point object) |
|
(C++20) | returns a reverse iterator to a range (customization point object) |
|
(C++20) | returns a reverse end iterator to a range (customization point object) |
|
(C++20) | returns an integer equal to the size of a range (customization point object) |
|
(C++20) | obtains a pointer to the beginning of a contiguous range (customization point object) |
|
(C++20) | a view that includes all elements of a range (alias template) (range adaptor object) |
|
(C++20) | a view consisting of the first N elements of another view (class template) (range adaptor object) |
|
(C++20) | a view consisting of elements of another view, skipping the first N elements (class template) (range adaptor object) |
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