std::function<R(Args...)>::target

 

C++

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語言支援 型別支援（基本型別、RTTI） 庫特性測試宏 (C++20) 程式工具 變參函式 initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) 協程支援 (C++20) 契約支援 (C++26) 三路比較 three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

通用工具 函式物件 位操作 (C++20) bitset hash (C++11) 關係運算符 (C++20 中棄用) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= 整數比較函式 cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) 交換與型別操作 swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) 常用詞彙型別 pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

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函式包裝器 function (C++11) move_only_function (C++23) copyable_function (C++26) function_ref (C++26) mem_fn (C++11) bad_function_call (C++11)	部分函式應用 bind_frontbind_back (C++20)(C++23) bind (C++11) is_bind_expression (C++11) is_placeholder (C++11) _1, _2, _3, ... (C++11)
函式呼叫 invokeinvoke_r (C++17)(C++23) 恆等函式物件 identity (C++20)	引用包裝器 reference_wrapper (C++11) refcref (C++11)(C++11) unwrap_referenceunwrap_ref_decay (C++20)(C++20)
運算子包裝器 plus minus negate multiplies divides modulus bit_and bit_or bit_not (C++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	透明運算子包裝器 plus<> (C++14) minus<> (C++14) negate<> (C++14) multiplies<> (C++14)   divides<> (C++14) modulus<> (C++14) bit_and<> (C++14) bit_or<> (C++14) bit_not<> (C++14) bit_xor<> (C++14) equal_to<> (C++14) not_equal_to<> (C++14) greater<> (C++14) less<> (C++14) greater_equal<> (C++14) less_equal<> (C++14) logical_and<> (C++14) logical_or<> (C++14) logical_not<> (C++14)
否定器 not_fn (C++17) 搜尋器 default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	受限比較器 ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
舊繫結器和介面卡
unary_function (直到 C++17*) binary_function (直到 C++17*) ptr_fun (直到 C++17*) pointer_to_unary_function (直到 C++17*) pointer_to_binary_function (直到 C++17*)   mem_fun (直到 C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (直到 C++20*) not2 (直到 C++20*)	binder1stbinder2nd (直到 C++17*)(直到 C++17*) bind1stbind2nd (直到 C++17*)(直到 C++17*) mem_fun_ref (直到 C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (直到 C++20*) binary_negate (直到 C++20*)

 

std::function

成員函式

function::function

function::~function

function::operator=

function::swap

function::assign (C++17 前)

function::operator bool

function::operator()

function::target_type

function::target

非成員函式

operator==operator!= (until C++20)

swap(std::function)

輔助類

uses_allocator<std::function> (C++17 前)

推導指南(C++17)

 

template< class T > T* target() noexcept;	(1)	(C++11 起)
template< class T > const T* target() const noexcept;	(2)	(C++11 起)

返回指向儲存的可呼叫函式目標的指標。

引數

(無)

返回值

如果 target_type() == typeid(T)，則返回指向儲存函式的指標，否則返回空指標。

示例

執行此程式碼

#include <functional>
#include <iostream>
 
int f(int, int) { return 1; }
int g(int, int) { return 2; }
void test(std::function<int(int, int)> const& arg)
{
    std::cout << "test function: ";
    if (arg.target<std::plus<int>>())
        std::cout << "it is plus\n";
    if (arg.target<std::minus<int>>())
        std::cout << "it is minus\n";
 
    int (*const* ptr)(int, int) = arg.target<int(*)(int, int)>();
    if (ptr && *ptr == f)
        std::cout << "it is the function f\n";
    if (ptr && *ptr == g)
        std::cout << "it is the function g\n";
}
 
int main()
{
    test(std::function<int(int, int)>(std::plus<int>()));
    test(std::function<int(int, int)>(std::minus<int>()));
    test(std::function<int(int, int)>(f));
    test(std::function<int(int, int)>(g));
}

輸出

test function: it is plus
test function: it is minus
test function: it is the function f
test function: it is the function g

缺陷報告

下列更改行為的缺陷報告追溯地應用於以前出版的 C++ 標準。

缺陷報告	應用於	釋出時的行為	正確的行為
LWG 2591	C++11	如果 T 不是 Callable，則行為未定義	行為已定義（總是返回 nullptr）

參閱

target_type

獲取儲存目標的 typeid (public member function)