C++ 仿函数

一. 仿函数（Functor）

仿函数（functor）又称为函数对象（function object）是一个能行使函数功能的类。仿函数的语法几乎和我们普通的函数调用一样，不过作为仿函数的类，都必须重载operator()运算符，举个例子：

class Func{
    public:
        void operator() (const string& str) const {
            cout<<str<<endl;
        }
};

 用仿函数实现迭代增量：

// C++ program to demonstrate working of 
// functors. 
#include <bits/stdc++.h> 
using namespace std; 
  
// A Functor 
class increment 
{ 
private: 
    int num; 
public: 
    increment(int n) : num(n) {  } 
  
    // This operator overloading enables calling 
    // operator function () on objects of increment 
    int operator () (int arr_num) const { 
        return num + arr_num; 
    } 
}; 
  
// Driver code 
int main() 
{ 
    int arr[] = {1, 2, 3, 4, 5}; 
    int n = sizeof(arr)/sizeof(arr[0]); 
    int to_add = 5; 
  
    transform(arr, arr+n, arr, increment(to_add)); 
  
    for (int i=0; i<n; i++) 
        cout << arr[i] << " "; 
} 

其中，std::transform : 

std::transform在指定的范围内应用于给定的操作，并将结果存储在指定的另一个范围内:

template<class InputIt, class OutputIt, class UnaryOperation>
OutputIt transform(InputIt first1, InputIt last1, OutputIt d_first, 
                   UnaryOperation unary_op)
{
    while (first1 != last1) {
        *d_first++ = unary_op(*first1++);
    }
    return d_first;
}

template<class InputIt1, class InputIt2, 
         class OutputIt, class BinaryOperation>
OutputIt transform(InputIt1 first1, InputIt1 last1, InputIt2 first2, 
                   OutputIt d_first, BinaryOperation binary_op)
{
    while (first1 != last1) {
        *d_first++ = binary_op(*first1++, *first2++);
    }
    return d_first;
}

 

 

二. STL中的典型仿函数

std::less ，std::equal_to和 std::greater 可用于比较，典型使用示例：

// greater example
#include <iostream>     // std::cout
#include <functional>   // std::greater
#include <algorithm>    // std::sort

int main () {
  int numbers[]={20,40,50,10,30};
  std::sort (numbers, numbers+5, std::greater<int>());
  for (int i=0; i<5; i++)
    std::cout << numbers[i] << ' ';
  std::cout << '\n';
  return 0;
}

 

三. std::bind1st和std::bind2nd

std::bind1st和std::bind2nd函数用于将一个二元算子转换成一元算子。

示例：

#include <iostream>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <cmath>
#include <cstddef>
#include <vector>
 
int main()
{
    std::vector<double> a = {0, 30, 45, 60, 90, 180};
    std::vector<double> r(a.size());
    const double pi = std::acos(-1); // since C++20 use std::numbers::pi
 
    std::transform(a.begin(), a.end(), r.begin(),
        std::bind1st(std::multiplies<double>(), pi / 180.));
//  an equivalent lambda is: [pi](double a){ return a*pi / 180.; });
 
    for(std::size_t n = 0; n < a.size(); ++n)
        std::cout << std::setw(3) << a[n] << "° = " << std::fixed << r[n]
                  << " rad\n" << std::defaultfloat;
}