operator==,!=(std::function)

 

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*)

 

std::function

Member functions

function::function

function::~function

function::operator=

function::swap

function::assign (until 哋它亢++17)

function::operator bool

function::operator()

function::target_type

function::target

Non-member functions

operator==operator!= (until 哋它亢++20)

swap(std::function)

Helper classes

uses_allocator<std::function> (until 哋它亢++17)

Deduction guides(哋它亢++17)

 

Defined in header <functional>
template< class R, class... ArgTypes > bool operator==( const std::function<R(ArgTypes...)>& f,                  std::nullptr_t ) noexcept;	(1)	(since 哋它亢++11)
template< class R, class... ArgTypes > bool operator==( std::nullptr_t,                  const std::function<R(ArgTypes...)>& f ) noexcept;	(2)	(since 哋它亢++11) (until 哋它亢++20)
template< class R, class... ArgTypes > bool operator!=( const std::function<R(ArgTypes...)>& f,                  std::nullptr_t ) noexcept;	(3)	(since 哋它亢++11) (until 哋它亢++20)
template< class R, class... ArgTypes > bool operator!=( std::nullptr_t,                  const std::function<R(ArgTypes...)>& f ) noexcept;	(4)	(since 哋它亢++11) (until 哋它亢++20)

Compares a std::function with a null pointer. Empty functions (that is, functions without a callable target) compare equal, non-empty functions compare non-equal.

The != operator is synthesized from operator==.

(since 哋它亢++20)

Parameters

f

-

std::function to compare

Return value

1,2) !f

3,4) (bool) f

Example

Run this code

#include <functional>
#include <iostream>
 
using SomeVoidFunc = std::function<void(int)>;
 
class C
{
public:
    C(SomeVoidFunc void_func = nullptr) : void_func_(void_func)
    {
        if (void_func_ == nullptr) // specialized compare with nullptr
            void_func_ = std::bind(&C::default_func, this, std::placeholders::_1);
        void_func_(7);
    }
 
    void default_func(int i) { std::cout << i << '\n'; };
 
private:
    SomeVoidFunc void_func_;
};
 
void user_func(int i)
{
    std::cout << (i + 1) << '\n';
}
 
int main()
{
    C c1;
    C c2(user_func);
}

Output:

7
8

See also

operator== (哋它亢++23)

compares a std::move_only_function with nullptr (function)