C++中functional包中function的使用

function 定义：wraps callable object of any type with specified function call signature (class template)

类模板std :: function是一个通用的多态函数包装器。 std :: function的实例可以存储，复制和调用任何可调用对象，如函数，lambda表达式，绑定表达式（bind）或其他函数对象，以及指向成员函数和指向数据成员的指针。

存储的可调用对象称为std::unction的target。 如果std :: function不包含target，则将其称为空。 调用空std :: function的target会导致抛出std :: bad_function_call异常。

std :: function满足CopyConstructible和CopyAssignable的要求。

 

常用通项语法： std::function<return-type(args-list)>  renamed_f = init_function_name;

以下列举了常见的几种应用场景

#include <functional>
#include <iostream>
 
struct Foo {
    Foo(int num) : num_(num) {}
    void print_add(int i) const { std::cout << num_+i << '\n'; }
    int num_;
};
 
void print_num(int i)
{
    std::cout << i << '\n';
}
 
struct PrintNum {
    void operator()(int i) const
    {
        std::cout << i << '\n';
    }
};
 
int main()
{
    // store a free function
    std::function<void(int)> f_display = print_num;
    f_display(-9);
 
    // store a lambda
    std::function<void()> f_display_42 = []() { print_num(42); };
    f_display_42();
 
    // store the result of a call to std::bind
    std::function<void()> f_display_31337 = std::bind(print_num, 31337);
    f_display_31337();
 
    // store a call to a member function
    std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
    const Foo foo(314159);
    f_add_display(foo, 1);
    f_add_display(314159, 1);
 
    // store a call to a data member accessor
    std::function<int(Foo const&)> f_num = &Foo::num_;
    std::cout << "num_: " << f_num(foo) << '\n';
 
    // store a call to a member function and object
    using std::placeholders::_1;
    std::function<void(int)> f_add_display2 = std::bind( &Foo::print_add, foo, _1 );
    f_add_display2(2);
 
    // store a call to a member function and object ptr
    std::function<void(int)> f_add_display3 = std::bind( &Foo::print_add, &foo, _1 );
    f_add_display3(3);
 
    // store a call to a function object
    std::function<void(int)> f_display_obj = PrintNum();
    f_display_obj(18);
}

其实，上述的function表达完全可以用auto代替，这样也无需引入functional头文件

补充：

free function：The term free function in C++ simply refers to non-member functions, this is,  it is not associated with any class and hence could be called as free function. Every function that is not a member function is a free function.

struct X {
    void f() {}               // not a free function
};
void g() {}                   // free function
int h(int, int) { return 1; } // also a free function

延伸：function也可以用std::bind代替（引入头文件functional)

         auto bind_f = std::bind(printaa, placeholders::_1);

void print(char first, int other){
...
}
auto new_f = std::bind(print,placeholders::_1,placeholders::_2);

new_f('F', 2019);

参考：

[1] https://en.cppreference.com/w/cpp/language/templates

[2]  https://stackoverflow.com/questions/4861914/what-is-the-meaning-of-the-term-free-function-in-c