std::function<R(Args...)>::operator bool

 

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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)
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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)

 

explicit operator bool() const noexcept;		(C++11 起)

檢查 *this 是否儲存了一個可呼叫函式目標，即不為空。

引數

(無)

返回值

如果 *this 儲存了一個可呼叫函式目標，則為 true，否則為 false。

示例

執行此程式碼

#include <functional>
#include <iostream>
 
void sampleFunction()
{
    std::cout << "This is the sample function!\n";
}
 
void checkFunc(std::function<void()> const& func)
{
    // Use operator bool to determine if callable target is available.
    if (func)  
    {
        std::cout << "Function is not empty! Calling function.\n";
        func();
    }
    else
        std::cout << "Function is empty. Nothing to do.\n";
}
 
int main()
{
    std::function<void()> f1;
    std::function<void()> f2(sampleFunction);
 
    std::cout << "f1: ";
    checkFunc(f1);
 
    std::cout << "f2: ";
    checkFunc(f2);
}

輸出

f1: Function is empty. Nothing to do.
f2: Function is not empty! Calling function.
This is the sample function!

參閱

operator bool

檢查 std::move_only_function 是否有目標 (std::move_only_function 的公有成員函式)