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,能以一个对象引用调用
std::mem_fun_ref
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----|-------------------------|
| template< class Res, class T > std::mem_fun_ref_t<Res,T> mem_fun_ref( Res (T::*f)() ); | (1) | (C++11 中弃用) (C++17 中移除) |
| template< class Res, class T > std::const_mem_fun_ref_t<Res,T> mem_fun_ref( 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_ref_t<Res,T,Arg> mem_fun_ref( 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_ref_t<Res,T,Arg> mem_fun_ref( 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 T
类型的引用作为其 operator() 的首个参数。
-
等效地调用 std::mem_fun_ref_t<S,T>(f) 或 std::const_mem_fun_ref_t<S,T>(f) 。
-
等效地调用 std::mem_fun1_ref_t<S,T>(f) 或 std::const_mem_fun1_ref_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。
示例
用 std::mem_fun_ref
绑定 std::string 的成员函数 size() 。
指向零元或一元成员函数指针的包装器,可以一个对象引用调用
std::mem_fun_ref_t,
std::mem_fun1_ref_t,
std::const_mem_fun_ref_t,
std::const_mem_fun1_ref_t
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----|-------------------------|
| template< class S, class T > class mem_fun_ref_t : public unary_function<T,S> { public: explicit mem_fun_ref_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_ref_t : public unary_function<T,S> { public: explicit const_mem_fun_ref_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 A > class mem_fun1_ref_t : public binary_function<T,A,S> { public: explicit mem_fun1_ref_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_ref_t : public binary_function<T,A,S> { public: explicit const_mem_fun1_ref_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引用传递给 operator()
。
-
包装无参数的非 const 成员函数。
-
包装无参数的 const 成员函数。
-
包装有单参数的非 const 成员函数。
-
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_ref()
{
return *this;
}
Cell copy_const_mem_fun_ref()
{
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_ref_t : std::mem_fun_ref_t<Cell, Cell>
{
Cell operator()(const Cell cell) const
{
return cell;
}
};
struct const_mem_fun_ref_t : std::const_mem_fun_ref_t<Cell, Cell>
{
Cell operator()(const Cell cell) const
{
return cell;
}
};
struct mem_fun1_ref_t : std::mem_fun1_ref_t<Cell, Cell, Cell>
{
Cell operator()(const Cell cell) const
{
return cell;
}
};
struct const_mem_fun1_ref_t : std::const_mem_fun1_ref_t<Cell, Cell, Cell>
{
Cell operator()(const Cell cell) const
{
return cell;
}
};
int main()
{
std::vector<Cell> vector1{{101, 101}, {102, 102}, {103, 103}, {104, 104}, {105, 105}};
std::vector<Cell> result1(vector1.size());
std::transform(vector1.begin(), vector1.end(), result1.begin(),
std::mem_fun_ref(&Cell::copy_mem_fun_ref));
std::cout << "data: ";
std::for_each(vector1.begin(), vector1.end(), std::mem_fun_ref(&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}