【C++学习】std::function

0.引入

引用阅读1

引用阅读2

引用阅读3

https://en.cppreference.com/w/cpp/header/functional

https://en.cppreference.com/w/cpp/utility/functional/function

https://blog.csdn.net/nirendao/article/details/51586888--CopyConstructible, MoveConstructible, CopyAssignable, MoveAssignable

1.手册中如何描述它

Class template std::function is a general-purpose polymorphic function wrapper. Instances 
of std::function can store, copy, and invoke any CopyConstructible Callable target -- 
functions, lambda expressions, bind expressions, or other function objects, as well as 
pointers to member functions and pointers to data members.

The stored callable object is called the target of std::function. If a std::function 
contains no target, it is called empty. Invoking the target of an empty std::function 
results in std::bad_function_call exception being thrown.

std::function satisfies the requirements of CopyConstructible and CopyAssignable.




类模板std::function是一个通用的多态函数包装器.函数的实例可以存储、复制和调用任何
CopyConstructableCallable目标函数、lambda表达式、绑定表达式或其他函数对象，以及指向成员函数的指针
和指向数据成员的指针.
存储的可调用对象称为std::function.如果std::function不包含目标,则称之为空,调用空std::function的目
标会导致抛出std::bad_function_call异常.
函数满足CopyConstructible和CopyAssignable的要求。

2.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);
    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);
}