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std::reference_wrapper(3) | C++ Standard Libary | std::reference_wrapper(3) |
NAME¶
std::reference_wrapper - std::reference_wrapper
Synopsis¶
Defined in header <functional>
template< class T > (since C++11)
class reference_wrapper;
std::reference_wrapper is a class template that wraps a reference in a
copyable,
assignable object. It is frequently used as a mechanism to store references
inside
standard containers (like std::vector) which cannot normally hold
references.
Specifically, std::reference_wrapper is a CopyConstructible and
CopyAssignable
wrapper around a reference to object or reference to function of type T.
Instances
of std::reference_wrapper are objects (they can be copied or stored in
containers)
but they are implicitly convertible to T&, so that they can be used as
arguments
with the functions that take the underlying type by reference.
If the stored reference is Callable, std::reference_wrapper is callable with
the
same arguments.
Helper functions std::ref and std::cref are often used to generate
std::reference_wrapper objects.
std::reference_wrapper is also used to pass objects by reference to
std::bind, the
constructor of std::thread, or the helper functions std::make_pair and
std::make_tuple.
std::reference_wrapper is guaranteed to be TriviallyCopyable. (since
C++17)
T may be an incomplete type. (since C++20)
Member types¶
type definition
type T
result_type(deprecated in C++17)(removed The return type of T if T is a
function.
in C++20) Otherwise, not defined
1) if T is a function or pointer to
function that takes one argument of type
A1, then argument_type is A1.
argument_type(deprecated in C++17)(removed 2) if T is a pointer to member
function
in C++20) of class T0 that takes no arguments, then
argument_type is T0*, possibly
cv-qualified
3) if T is a class type with a member
type T::argument_type, then argument_type
is an alias of that
1) if T is a function or pointer to
function that takes two arguments of
types A1 and A2, then first_argument_type
is A1.
first_argument_type(deprecated in 2) if T is a pointer to member function
C++17)(removed in C++20) of class T0 that takes one argument, then
first_argument_type is T0*, possibly
cv-qualified
3) if T is a class type with a member
type T::first_argument_type, then
first_argument_type is an alias of that
1) if T is a function or pointer to
function that takes two arguments of type
s A1 and A2, then second_argument_type is
A2.
second_argument_type(deprecated in 2) if T is a pointer to member function
C++17)(removed in C++20) of class T0 that takes one argument A1,
then second_argument_type is A1, possibly
cv-qualified
3) if T is a class type with a member
type T::second_argument_type, then
second_argument_type is an alias of that
Member functions¶
constructor stores a reference in a new std::reference_wrapper
object
(public member function)
operator= rebinds a std::reference_wrapper
(public member function)
get accesses the stored reference
operator T& (public member function)
operator() calls the stored function
(public member function)
Deduction guides(since C++17)
Possible implementation¶
namespace detail { template <class T> constexpr T& FUN(T& t) noexcept { return t; } template <class T> void FUN(T&&) = delete; }
template <class T> class reference_wrapper { public:
// types
typedef T type;
// construct/copy/destroy
template <class U, class = decltype(
detail::FUN<T>(std::declval<U>()),
std::enable_if_t<!std::is_same_v<reference_wrapper,
std::remove_cvref_t<U>>>()
)>
constexpr reference_wrapper(U&& u)
noexcept(noexcept(detail::FUN<T>(std::forward<U>(u))))
: _ptr(std::addressof(detail::FUN<T>(std::forward<U>(u)))) {}
reference_wrapper(const reference_wrapper&) noexcept = default;
// assignment
reference_wrapper& operator=(const reference_wrapper& x) noexcept =
default;
// access
constexpr operator T& () const noexcept { return *_ptr; }
constexpr T& get() const noexcept { return *_ptr; }
template< class... ArgTypes >
constexpr std::invoke_result_t<T&, ArgTypes...>
operator() ( ArgTypes&&... args ) const {
return std::invoke(get(), std::forward<ArgTypes>(args)...);
}
private:
T* _ptr; };
// deduction guides template<class T> reference_wrapper(T&) -> reference_wrapper<T>;
Example¶
Demonstrates the use of std::reference_wrapper as a container of
references, which
makes it possible to access the same container using multiple indexes.
// Run this code
#include <algorithm>
#include <list>
#include <vector>
#include <iostream>
#include <numeric>
#include <random>
#include <functional>
void print(auto const rem, std::ranges::range auto const& v) {
for (std::cout << rem; auto const& e : v)
std::cout << e << ' ';
std::cout << '\n';
}
int main()
{
std::list<int> l(10);
std::iota(l.begin(), l.end(), -4);
// can't use shuffle on a list (requires random access), but can use it on a
vector
std::vector<std::reference_wrapper<int>> v(l.begin(),
l.end());
std::ranges::shuffle(v, std::mt19937{std::random_device{}()});
print("Contents of the list: ", l);
print("Contents of the list, as seen through a shuffled vector: ",
v);
std::cout << "Doubling the values in the initial list...\n";
std::ranges::for_each(l, [](int& i) { i *= 2; });
print("Contents of the list, as seen through a shuffled vector: ",
v);
}
Possible output:¶
Contents of the list: -4 -3 -2 -1 0 1 2 3 4 5
Contents of the list, as seen through a shuffled vector: -1 2 -2 1 5 0 3 -3
-4 4
Doubling the values in the initial list...
Contents of the list, as seen through a shuffled vector: -2 4 -4 2 10 0 6 -6
-8 8
See also¶
ref creates a std::reference_wrapper with a type deduced from its
cref argument
(C++11) (function template)
(C++11)
bind binds one or more arguments to a function object
(C++11) (function template)
unwrap_reference
unwrap_ref_decay get the reference type wrapped in std::reference_wrapper
(C++20) (class template)
(C++20)
2022.07.31 | http://cppreference.com |