仿函数那些事---算数运算与关系运算

1.仿函数在STL中的定义

要使用STL内建的仿函数，必须包含functional头文件。

1.Arithmetic operations

1. 加：plus
2. 减：minus
3. 乘：multiplies
4. 除：divides
5. 求余数：modulus
6. 否定：negate

源代码

#include <iostream>
#include <numeric>
#include <vector> 
#include <functional> 
using namespace std;

int main()
{
    int ia[]={1,2,3,4,5};
    vector<int> iv(ia,ia+5);
    cout<<accumulate(iv.begin(),iv.end(),1,multiplies<int>())<<endl;  //求取5*4*3*2，记住它的调用方式

    cout<<multiplies<int>()(3,5)<<endl;

    modulus<int>  modulusObj;
    cout<<modulusObj(3,5)<<endl; // 计算3%5的余数
    return 0; 
} 

运行结果

• accumulate_cplusplus

---

关于里面accumlate的例子

// accumulate example
#include <iostream>     // std::cout
#include <functional>   // std::minus
#include <numeric>      // std::accumulate

int myfunction (int x, int y) {return x+2*y;}
struct myclass {
    int operator()(int x, int y) {return x+3*y;}
} myobject;

int main () {
  int init = 100;
  int numbers[] = {10,20,30};

  std::cout << "using default accumulate: ";
  std::cout << std::accumulate(numbers,numbers+3,init);
  std::cout << '\n';

  std::cout << "using functional's minus: ";
  std::cout << std::accumulate (numbers, numbers+3, init, std::minus<int>());
  std::cout << '\n';

  std::cout << "using custom function: ";
  std::cout << std::accumulate (numbers, numbers+3, init, myfunction);  //①
  std::cout << '\n';

  std::cout << "using custom object: ";
  std::cout << std::accumulate (numbers, numbers+3, init, myobject); //②
  std::cout << '\n';

  return 0;
}

---

• 对于①，myfunction函数的两个参数(x,y),x=init;y=*number。每一次init=myfunction的返回值，同时y=*(number+1);

  

• 对于②，同理于①

---

运行结果

---

modulus在cplusplus中的定义

2.关系运算类仿函数

等于：equal_to
不等于：not_equal_to
大于：greater
大于等于：greater_equal
小于：less
小于等于：less_equal
从大到小排序：

#include <iostream>
#include <algorithm>
#include <vector>
#include<functional> 

using namespace std;

template <class T> 
class display
{
public:
    void operator()(const T &x)
    {
        cout<<x<<" "; 
    } 
};

int main()
{
    int ia[]={1,5,4,3,2};
    vector<int> iv(ia,ia+5);
    sort(iv.begin(),iv.end(),greater<int>());
    for_each(iv.begin(),iv.end(),display<int>()); 
    return 0; 
}