任何定义了函数调用操作符的对象都是函数对象。C++ 支持创建、操作新的函数对象，同时也提供了许多内置的函数对象。

旧式绑定器与适配器

早期提供功能支持的几个工具在 C++11 中弃用，并于 C++17 中移除（旧否定器于 C++17 中弃用并于 C++20 中移除）：

函数适配器

从成员函数指针创建包装器，可以一个对象指针调用

std::mem_fun

template< class Res, class T > std::mem_fun_t<Res,T> mem_fun( Res (T::*f)() );	(1)	(C++11 中弃用) (C++17 中移除)
template< class Res, class T > std::const_mem_fun_t<Res,T> mem_fun( Res (T::*f)() const );	(1)	(C++11 中弃用) (C++17 中移除)
template< class Res, class T, class Arg > std::mem_fun1_t<Res,T,Arg> mem_fun( Res (T::*f)(Arg) );	(2)	(C++11 中弃用) (C++17 中移除)
template< class Res, class T, class Arg > std::const_mem_fun1_t<Res,T,Arg> mem_fun( Res (T::*f)(Arg) const );	(2)	(C++11 中弃用) (C++17 中移除)

创建成员函数包装对象，从模板实参推导类型。包装对象期待到指向 T 类型对象的指针作为其 operator() 的首个参数。

1) 等效地调用 std::mem_fun_t<S,T>(f) 或 std::const_mem_fun_t<S,T>(f) 。

2) 等效地调用 std::mem_fun1_t<S,T>(f) 或 std::const_mem_fun1_t<S,T>(f) 。

此函数与相关类型于 C++11 弃用并于 C++17 移除，为了让位给更通用的 std::mem_fn 与 std::bind ，它们都从成员函数创建可调用类型的兼容适配器的函数对象。

参数

f

-

指向要创建包装的成员函数的指针

返回值

包装 f 的函数对象。

异常

可能会抛出由实现定义的异常。

 注解

std::mem_fun 与 std::mem_fun_ref 的区别是前者产生的函数包装期待指向对象指针，而后者——期待引用。

指向零元或一元成员函数指针的包装器，可以一个对象指针调用

std::mem_fun_t, 
std::mem_fun1_t, 
std::const_mem_fun_t, 
std::const_mem_fun1_t

template< class S, class T > class mem_fun_t : public unary_function<T*,S> { public:     explicit mem_fun_t(S (T::*p)());     S operator()(T* p) const; };	(1)	(C++11 中弃用) (C++17 中移除)
template< class S, class T > class const_mem_fun_t : public unary_function<const T*,S> { public:     explicit const_mem_fun_t(S (T::*p)() const);     S operator()(const T* p) const; };	(2)	(C++11 中弃用) (C++17 中移除)
template< class S, class T, class Arg > class mem_fun1_t : public binary_function<T*,A,S> { public:     explicit mem_fun1_t(S (T::*p)(A));     S operator()(T* p, A x) const; };	(3)	(C++11 中弃用) (C++17 中移除)
template< class S, class T, class A > class const_mem_fun1_t : public binary_function<const T*,A,S> { public:     explicit const_mem_fun1_t(S (T::*p)(A) const);     S operator()(const T* p, A x) const; };	(4)	(C++11 中弃用) (C++17 中移除)

 围绕成员函数指针的包装器。将要调用其成员函数的类实例作为指针传递给 operator() 。

1) 包装无参数的非 const 成员函数。

2) 包装无参数的 const 成员函数。

3) 包装有单参数的非 const 成员函数。

4) 包装有单参数的 const 成员函数。

调用示例

#include <iostream>
#include <functional>
#include <algorithm>
#include <cmath>
#include <iterator>struct Cell
{int x;int y;Cell() = default;Cell(int a, int b): x(a), y(b) {}Cell(const Cell &cell){x = cell.x;y = cell.y;}Cell &operator+(const Cell &cell){x += cell.x;y += cell.y;return *this;}Cell &operator+=(const Cell &cell){x += cell.x;y += cell.y;return *this;}Cell &operator*=(int n){x *= n;y *= n;return *this;}Cell &operator++(){x += 1;y += 1;return *this;}friend Cell operator +(const Cell &cell1, const Cell &cell2){Cell cell = cell1;cell += cell2;return cell;}friend Cell operator *(const Cell &cell1, const Cell &cell2){Cell cell = {cell1.x * cell2.x, cell1.y * cell2.y};return cell;}friend Cell operator /(const Cell &cell1, const Cell &cell2){Cell cell = {cell1.x / cell2.x, cell1.y / cell2.y};return cell;}friend Cell operator %(const Cell &cell1, const Cell &cell2){Cell cell = {cell1.x % cell2.x, cell1.y % cell2.y};return cell;}friend bool operator ==(const Cell &cell1, const Cell &cell2){return cell1.x == cell2.x && cell1.y == cell2.y;}friend bool operator !=(const Cell &cell1, const Cell &cell2){return cell1.x != cell2.x && cell1.y != cell2.y;}friend bool operator <(const Cell &cell1, const Cell &cell2){if (cell1.x == cell2.x){return cell1.y < cell2.y;}else{return cell1.x < cell2.x;}}friend bool operator >(const Cell &cell1, const Cell &cell2){if (cell1.x == cell2.x){return cell1.y > cell2.y;}else{return cell1.x > cell2.x;}}friend bool operator &&(const Cell &cell1, const Cell &cell2){return cell1.x && cell2.x && cell1.y && cell2.y;}friend bool operator ||(const Cell &cell1, const Cell &cell2){return cell1.x || cell2.x || cell1.y || cell2.y;}friend bool operator !(const Cell &cell){return !(cell.x && cell.x);}friend Cell operator &(const Cell &cell1, const Cell &cell2){Cell cell = {cell1.x & cell2.x, cell1.y & cell2.y};return cell;}friend Cell operator |(const Cell &cell1, const Cell &cell2){Cell cell = {cell1.x | cell2.x, cell1.y | cell2.y};return cell;}friend Cell operator ^(const Cell &cell1, const Cell &cell2){Cell cell = {cell1.x ^ cell2.x, cell1.y ^ cell2.y};return cell;}Cell copy_mem_fun(){return *this;}Cell copy_const_mem_fun(){return *this;}void print() const{std::cout << "{" << x << "," << y << "} ";}
};std::ostream &operator<<(std::ostream &os, const Cell &cell)
{os << "{" << cell.x << "," << cell.y << "}";return os;
}struct mem_fun_t : std::mem_fun_t<Cell, Cell>
{Cell operator()(const Cell cell) const{return cell;}
};struct const_mem_fun_t : std::const_mem_fun_t<Cell, Cell>
{Cell operator()(const Cell cell) const{return cell;}
};struct mem_fun1_t : std::mem_fun1_t<Cell, Cell, Cell>
{Cell operator()(const Cell cell) const{return cell;}
};struct const_mem_fun1_t : std::const_mem_fun1_t<Cell, Cell, Cell>
{Cell operator()(const Cell cell) const{return cell;}
};int main()
{std::vector<Cell*> vector1{new Cell{101, 101}, new Cell{102, 102},new Cell{103, 103}, new Cell{104, 104}, new Cell{105, 105}};std::vector<Cell> result1(vector1.size());std::transform(vector1.begin(), vector1.end(), result1.begin(),std::mem_fun(&Cell::copy_mem_fun));std::cout << "data:     ";std::for_each(vector1.begin(), vector1.end(), std::mem_fun(&Cell::print));std::cout << std::endl;std::cout << "result:   ";std::copy(result1.begin(), result1.end(), std::ostream_iterator<Cell>(std::cout, " "));std::cout << std::endl;std::cout << std::endl;return 0;
}

输出

data:     {101,101} {102,102} {103,103} {104,104} {105,105}
result:   {101,101} {102,102} {103,103} {104,104} {105,105}