c++ - 将 std::function 作为参数传递给 for_each

Question

#include <initializer_list>
#include <iostream>
#include <algorithm>
#include <vector>
#include <functional>

std::function<void(int)> sample_function()
{
    return
        [](int x) -> void
    {
        if (x > 5)
            std::cout << x;
    };
}

int main()
{
    std::vector<int> numbers{ 1, 2, 3, 4, 5, 10, 15, 20, 25, 35, 45, 50 };
    std::for_each(numbers.begin(), numbers.end(), sample_function);
}   

我试图将 sample_function() 传递给 for_each 但我遇到了这个错误

错误 C2197“std::function”:调用的参数过多

Answer 1

我想你想要的是下面的

#include <iostream>
#include <vector>
#include <functional>

std::function<void(int)> sample_function =  [](int x)
{
    if (x > 5)  std::cout << x << ' ';
};


int main()
{
    std::vector<int> numbers{ 1, 2, 3, 4, 5, 10, 15, 20, 25, 35, 45, 50 };
    std::for_each(numbers.begin(), numbers.end(), sample_function);
}

输出是

10 15 20 25 35 45 50

或者，如果您确实想定义一个返回std::function 类型对象的函数，那么您可以这样写

#include <iostream>
#include <vector>
#include <functional>

std::function<void(int)> sample_function()
{
    return  [](int x)
            {
                if (x > 5)  std::cout << x << ' ';
            };
}


int main()
{
    std::vector<int> numbers{ 1, 2, 3, 4, 5, 10, 15, 20, 25, 35, 45, 50 };
    std::for_each(numbers.begin(), numbers.end(), sample_function() );
}

输出将与上图相同。注意通话

    std::for_each(numbers.begin(), numbers.end(), sample_function() );
                                                                ^^^^