当前位置：首页 > news >正文

C++ 中的仿函数 functor

news 来源：原创 2024/5/10 20:35:14

一仿函数的概念

1. 定义

仿函数（functor）是一种使用上像函数的类，其本质是一个实现了 operato() 函数的类，这种类就有了类似于函数一样的使用行为，这就是仿函数的类。

仿函数在 C++ STL标准库中被大量使用。

2. 特点

1. 仿函数是一个类，不是一个函数

2. 仿函数的类需要重载 operator() 函数，以此拥有函数的行为

二 STL 中常见的仿函数介绍

1. 基类

1.1 unary_function

 /***  This is one of the @link functors functor base classes@endlink.*/template<typename _Arg, typename _Result>struct unary_function{/// @c argument_type is the type of the argumenttypedef _Arg 	argument_type;   /// @c result_type is the return typetypedef _Result 	result_type;  };

作为拥有一个输入参数的仿函数常用基类，该类主要回答了两个问题：

1. 该仿函数类的输入参数是什么类型：argument_type

2. 该仿函数类的返回参数是什么类型: result_type

1.2 binary_function

 /***  This is one of the @link functors functor base classes@endlink.*/template<typename _Arg1, typename _Arg2, typename _Result>struct binary_function{/// @c first_argument_type is the type of the first argumenttypedef _Arg1 	first_argument_type; /// @c second_argument_type is the type of the second argumenttypedef _Arg2 	second_argument_type;/// @c result_type is the return typetypedef _Result 	result_type;};

作为拥有两个输入参数的仿函数常用基类，该类主要回答了两个问题：

1. 该仿函数类的输入参数是什么类型：first_argument_type 与 second_argument_type

2. 该仿函数类的返回参数是什么类型: result_type

2. STL 中常见仿函数

2.1 plus

 /// One of the @link arithmetic_functors math functors@endlink.template<typename _Tp>struct plus : public binary_function<_Tp, _Tp, _Tp>{_Tpoperator()(const _Tp& __x, const _Tp& __y) const{ return __x + __y; }};

2.2 minus

/// One of the @link arithmetic_functors math functors@endlink.template<typename _Tp>struct minus : public binary_function<_Tp, _Tp, _Tp>{_Tpoperator()(const _Tp& __x, const _Tp& __y) const{ return __x - __y; }};

2.3 multiplies

  /// One of the @link arithmetic_functors math functors@endlink.template<typename _Tp>struct multiplies : public binary_function<_Tp, _Tp, _Tp>{_Tpoperator()(const _Tp& __x, const _Tp& __y) const{ return __x * __y; }};

2.4 divides

template<typename _Tp>struct divides : public binary_function<_Tp, _Tp, _Tp>{_Tpoperator()(const _Tp& __x, const _Tp& __y) const{ return __x / __y; }};

2.5 equal_to

 /// One of the @link comparison_functors comparison functors@endlink.template<typename _Tp>struct equal_to : public binary_function<_Tp, _Tp, bool>{booloperator()(const _Tp& __x, const _Tp& __y) const{ return __x == __y; }};

2.6 less

 /// One of the @link comparison_functors comparison functors@endlink.template<typename _Tp>struct less : public binary_function<_Tp, _Tp, bool>{booloperator()(const _Tp& __x, const _Tp& __y) const{ return __x < __y; }};

2.7 greater

/// One of the @link comparison_functors comparison functors@endlink.template<typename _Tp>struct greater : public binary_function<_Tp, _Tp, bool>{booloperator()(const _Tp& __x, const _Tp& __y) const{ return __x > __y; }};

2.8 _Select1st

template<typename _Pair>struct _Select1st: public unary_function<_Pair, typename _Pair::first_type>{typename _Pair::first_type&operator()(_Pair& __x) const{ return __x.first; }const typename _Pair::first_type&operator()(const _Pair& __x) const{ return __x.first; }};

2.9 _Select2nd

template<typename _Pair>struct _Select2nd: public unary_function<_Pair, typename _Pair::second_type>{typename _Pair::second_type&operator()(_Pair& __x) const{ return __x.second; }const typename _Pair::second_type&operator()(const _Pair& __x) const{ return __x.second; }};

3. 使用例子

#include<algorithm>
#include<iostream>int main()
{// 1. plusstd::cout << "------ plus ------" << std::endl;std::vector<int>  vec = {1, 3, 5};std::plus<int> pl;int init = 0;int res1 = std::accumulate(vec.begin(), vec.end(), init, pl);std::cout << res1 << std::endl;std::cout << "------ plus ------" << std::endl; // 9// 2. minusstd::cout << "------ minus ------" << std::endl;init = 10;std::minus<int> mis;int res2 =  std::accumulate(vec.begin(), vec.end(), init, mis);std::cout << res2 << std::endl; // 1std::cout << "------ minus ------" << std::endl;// 3. multiesstd::cout << "------ multies ------" << std::endl;init = 1;std::multiplies<int> multiply;int res3 = std::accumulate(vec.begin(), vec.end(), init, multiply);std::cout << res3 << std::endl; // 15std::cout << "------ multies ------" << std::endl;// 4. dividesstd::cout << "------ divides ------" << std::endl;init = 90;std::divides<int> divid;int res4 = std::accumulate(vec.begin(), vec.end(), init, divid);std::cout << res4 << std::endl; // 6std::cout << "------ divides ------" << std::endl;// 5. equal_tostd::cout << "------ equal_to ------" << std::endl;std::pair<int, std::string> pair1 = std::make_pair(1, "abc");std::pair<int, std::string> pair2 = std::make_pair(2, "abc");std::equal_to<std::string> equal;std::_Select2nd<std::pair<int, std::string>> second_argu;std::cout << equal(second_argu(pair1), second_argu(pair2)) << std::endl;// 1std::cout << "------ equal_to ------" << std::endl;// 6. lessstd::cout << "------ less ------" << std::endl;std::less<int> less;std::cout << less(3, 6) << std::endl; // 1std::cout << "------ less ------" << std::endl;// 7. greaterstd::cout << "------ greater ------" << std::endl;std::greater<int> greater;std::cout << greater(3, 6) << std::endl; // 0std::cout << "------ greater ------" << std::endl;// 8. _Select1ststd::cout << "------ _Select1st ------" << std::endl;std::pair<int, std::string> pair3 = std::make_pair(1, "abc");std::_Select1st<std::pair<int, std::string>> select1st;std::cout << select1st(pair3) << std::endl; // 1std::cout << "------ _Select1st ------" << std::endl;// 9. _Select2ndstd::cout << "------ _Select2nd ------" << std::endl;std::pair<int, std::string> pair4 = std::make_pair(1, "abc");std::_Select2nd<std::pair<int, std::string>> select2nd;std::cout << select2nd(pair3) << std::endl; // abcstd::cout << "------ _Select2nd ------" << std::endl;return 0;
}

输出：