function对象使用使用详解

模板类function是一个通用的多态函数包装器，它的实例可以存储、拷贝并调用任何可调用对象- 函数、lambda表达式、bind表达式等其他函数对象。还可以是成员函数指针或数据成员的指针。

对空的function实例调用将会抛出std::bad_function_call异常。

#include <iostream>
#include <functional>

int main()
{
    std::function<int()> f = nullptr;
    try {
        f();
    } catch(const std::bad_function_call& e) {
        std::cout << e.what() << ‘\n‘;
    }
}

示例代码：

#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);

    // 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/*调用者*/, _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/*调用者*/, _1 );
    f_add_display3(3);

    // store a call to a function object
    std::function<void(int)> f_display_obj = PrintNum();
    f_display_obj(18);
}