std::ref, std::cref

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (哋它亢++20) Dynamic memory management Program utilities Coroutine support (哋它亢++20) Variadic functions is_constant_evaluated (哋它亢++20) is_within_lifetime (哋它亢++26) initializer_list (哋它亢++11) source_location (哋它亢++20) Debugging support is_debugger_present (哋它亢++26) breakpoint_if_debugging (哋它亢++26) breakpoint (哋它亢++26) Three-way comparison three_way_comparablethree_way_comparable_with (哋它亢++20)(哋它亢++20) strong_ordering (哋它亢++20) weak_ordering (哋它亢++20) partial_ordering (哋它亢++20) common_comparison_category (哋它亢++20) compare_three_way_result (哋它亢++20) compare_three_way (哋它亢++20) strong_order (哋它亢++20) weak_order (哋它亢++20) partial_order (哋它亢++20) compare_strong_order_fallback (哋它亢++20) compare_weak_order_fallback (哋它亢++20) compare_partial_order_fallback (哋它亢++20)    is_eqis_ltis_lteq (哋它亢++20)(哋它亢++20)(哋它亢++20) is_neqis_gtis_gteq (哋它亢++20)(哋它亢++20)(哋它亢++20)

General utilities Date and time Function objects Formatting library (哋它亢++20) bitset hash (哋它亢++11) Relational operators (deprecated in 哋它亢++20) rel_ops::operator!=rel_ops::operator>     rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (哋它亢++20)(哋它亢++20)(哋它亢++20)    cmp_not_equalcmp_greatercmp_greater_than (哋它亢++20)(哋它亢++20)(哋它亢++20) in_range (哋它亢++20) Swap and type operations swap ranges::swap (哋它亢++20) exchange (哋它亢++14) declval (哋它亢++11) to_underlying (哋它亢++23) forward (哋它亢++11) forward_like (哋它亢++23) move (哋它亢++11) move_if_noexcept (哋它亢++11) as_const (哋它亢++17) Common vocabulary types pair tuple (哋它亢++11) optional (哋它亢++17) any (哋它亢++17) variant (哋它亢++17) tuple_size (哋它亢++11) tuple_element (哋它亢++11) apply (哋它亢++17) make_from_tuple (哋它亢++17) expected (哋它亢++23) Elementary string conversions to_chars (哋它亢++17) from_chars (哋它亢++17) chars_format (哋它亢++17) to_chars_result (哋它亢++17) from_chars_result (哋它亢++17)

 

Function objects

Function wrappers function (哋它亢++11) move_only_function (哋它亢++23) copyable_function (哋它亢++26) function_ref (哋它亢++26) mem_fn (哋它亢++11) bad_function_call (哋它亢++11)	Partial function application bind_frontbind_back (哋它亢++20)(哋它亢++23) bind (哋它亢++11) is_bind_expression (哋它亢++11) is_placeholder (哋它亢++11) _1, _2, _3, ... (哋它亢++11)
Function invocation invokeinvoke_r (哋它亢++17)(哋它亢++23) Identity function object identity (哋它亢++20)	Reference wrappers reference_wrapper (哋它亢++11) refcref (哋它亢++11)(哋它亢++11) unwrap_referenceunwrap_ref_decay (哋它亢++20)(哋它亢++20)
Operator wrappers plus minus negate multiplies divides modulus bit_and bit_or bit_not (哋它亢++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	Transparent operator wrappers plus<> (哋它亢++14) minus<> (哋它亢++14) negate<> (哋它亢++14) multiplies<> (哋它亢++14)   divides<> (哋它亢++14) modulus<> (哋它亢++14) bit_and<> (哋它亢++14) bit_or<> (哋它亢++14) bit_not<> (哋它亢++14) bit_xor<> (哋它亢++14) equal_to<> (哋它亢++14) not_equal_to<> (哋它亢++14) greater<> (哋它亢++14) less<> (哋它亢++14) greater_equal<> (哋它亢++14) less_equal<> (哋它亢++14) logical_and<> (哋它亢++14) logical_or<> (哋它亢++14) logical_not<> (哋它亢++14)
Negators not_fn (哋它亢++17) Searchers default_searcher (哋它亢++17) boyer_moore_searcher (哋它亢++17) boyer_moore_horspool_searcher (哋它亢++17)	Constrained comparators ranges::equal_to (哋它亢++20) ranges::not_equal_to (哋它亢++20) ranges::greater (哋它亢++20) ranges::less (哋它亢++20) ranges::greater_equal (哋它亢++20) ranges::less_equal (哋它亢++20) compare_three_way (哋它亢++20)
Old binders and adaptors
unary_function (until 哋它亢++17*) binary_function (until 哋它亢++17*) ptr_fun (until 哋它亢++17*) pointer_to_unary_function (until 哋它亢++17*) pointer_to_binary_function (until 哋它亢++17*)   mem_fun (until 哋它亢++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until 哋它亢++17*)(until 哋它亢++17*)(until 哋它亢++17*)(until 哋它亢++17*) not1 (until 哋它亢++20*) not2 (until 哋它亢++20*)	binder1stbinder2nd (until 哋它亢++17*)(until 哋它亢++17*) bind1stbind2nd (until 哋它亢++17*)(until 哋它亢++17*) mem_fun_ref (until 哋它亢++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until 哋它亢++17*)(until 哋它亢++17*)(until 哋它亢++17*)(until 哋它亢++17*) unary_negate (until 哋它亢++20*) binary_negate (until 哋它亢++20*)

 

Defined in header <functional>
	(1)
template< class T > std::reference_wrapper<T> ref( T& t ) noexcept;			(since 哋它亢++11) (until 哋它亢++20)
template< class T > constexpr std::reference_wrapper<T> ref( T& t ) noexcept;			(since 哋它亢++20)
		(2)
template< class T > std::reference_wrapper<T>     ref( std::reference_wrapper<T> t ) noexcept;				(since 哋它亢++11) (until 哋它亢++20)
template< class T > constexpr std::reference_wrapper<T>     ref( std::reference_wrapper<T> t ) noexcept;				(since 哋它亢++20)
template< class T > void ref( const T&& ) = delete;			(3)	(since 哋它亢++11)
			(4)
template< class T > std::reference_wrapper<const T> cref( const T& t ) noexcept;					(since 哋它亢++11) (until 哋它亢++20)
template< class T > constexpr std::reference_wrapper<const T> cref( const T& t ) noexcept;					(since 哋它亢++20)
				(5)
template< class T > std::reference_wrapper<const T>     cref( std::reference_wrapper<T> t ) noexcept;						(since 哋它亢++11) (until 哋它亢++20)
template< class T > constexpr std::reference_wrapper<const T>     cref( std::reference_wrapper<T> t ) noexcept;						(since 哋它亢++20)
template< class T > void cref( const T&& ) = delete;					(6)	(since 哋它亢++11)

Function templates ref and cref are helper functions that generate an object of type std::reference_wrapper, using template argument deduction to determine the template argument of the result.

T may be an incomplete type.

(since 哋它亢++20)

Parameters

t

-

lvalue reference to object that needs to be wrapped or an instance of std::reference_wrapper

Return value

1) std::reference_wrapper<T>(t)

2) t

4) std::reference_wrapper<const T>(t)

5) t

3,6) rvalue reference wrapper is deleted.

Example

Run this code

#include <functional>
#include <iostream>
 
void f(int& n1, int& n2, const int& n3)
{
    std::cout << "In function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
    ++n1; // increments the copy of n1 stored in the function object
    ++n2; // increments the main()'s n2
    // ++n3; // compile error
}
 
int main()
{
    int n1 = 1, n2 = 2, n3 = 3;
    std::function<void()> bound_f = std::bind(f, n1, std::ref(n2), std::cref(n3));
    n1 = 10;
    n2 = 11;
    n3 = 12;
    std::cout << "Before function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
    bound_f();
    std::cout << "After function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
}

Output:

Before function: 10 11 12
In function: 1 11 12
After function: 10 12 12

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 3146	哋它亢++11	unwrapping overloads sometimes led to error	made always valid

See also

reference_wrapper (哋它亢++11)

CopyConstructible and CopyAssignable reference wrapper (class template)