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

加：plus

减：minus

乘：multiplies

除：divides

求余数：modulus

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;
}