[C/C++标准库]_[初级]_[unary_function 和 binary_function]

一元和二元函数.

场景:

1.C++算法库里经常出现unary_function和binary_function类型，比如sort,count_if,find_if,或者std::map的构造函数等,这是用来对元素进行比较用的类型.

template <class Arg, class Result> struct unary_function;

template <class Arg1, class Arg2, class Result> struct binary_function;

以下是从cplusplus.com摘录的说明:

二元函数转换为一元函数 bind2nd
http://www.cplusplus.com/reference/functional/bind2nd/?kw=bind2nd
template <class Operation, class T>
  binder2nd<Operation> bind2nd (const Operation& op, const T& x);

Return function object with second parameter bound
This function constructs an unary function object from the binary function object op by binding its second parameter to the fixed value x.

To bind the first parameter to a specific value, see bind1st.


一元函数 unary_function
http://www.cplusplus.com/reference/functional/unary_function/
1.有返回值.
2.只有一个参数.
In the case of unary function objects, this operator() member function takes a single parameter.

unary_function is just a base class, from which specific unary function objects are derived. It has no operator() member defined (which derived classes are expected to define) - it simply has two public data members that are typedefs of the template parameters. It is defined as:	

template <class Arg, class Result>
  struct unary_function {
    typedef Arg argument_type;
    typedef Result result_type;
  };

二元函数 binary_function
http://www.cplusplus.com/reference/functional/binary_function/?kw=binary_function
template <class Arg1, class Arg2, class Result>
  struct binary_function {
    typedef Arg1 first_argument_type;
    typedef Arg2 second_argument_type;
    typedef Result result_type;
  };

测试代码:

#include <stdio.h>
#include <stdint.h>
#include <time.h>
#include <algorithm>
#include <iostream>
#include <functional>
#include <vector>

using namespace std;

static void PrintVector(vector<int>& v)
{
	cout << "....PrintVector...." << endl;
	for (int i = 0; i < v.size(); ++i)
	{
		cout << v[i] << endl;
	}

}

int main(int argc, char const *argv[])
{
	
	vector<int>  data;

	srand( time(NULL) );
	cout << "RAND_MAX: " << RAND_MAX << endl;
	for (int i = 0; i < 10; ++i)
	{
		data.push_back(rand());
	}
	//1.std::less<int>
	sort(data.begin(),data.end(),std::less<int>());
	PrintVector(data);

	//2.std::greator<int>
	sort(data.begin(),data.end(),std::greater<int>());
	PrintVector(data);

	//3.std::greater_equal<int>
	//取>=0的整数
	int numbers[]={20,-30,10,10,-40,0};
	//bind2nd 把 binary_function 转换为 unary fucntion.
  	int cx = std::count_if (numbers, numbers+6, std::bind2nd(std::greater_equal<int>(),0));
  	std::cout << "There are " << cx << " non-negative elements.
";
  	vector<int> result;
  	//1.找出所有大于0的整数,貌似没有好用的算法函数.
  	for (int i = 0; i < 6; ++i)
  	{
  		if(numbers[i] > 0)
		{
			cout << numbers[i] << endl;
		}
  	}

  	//4.小于等于10的整数.
  	cx = std::count_if (numbers, numbers+6, std::bind2nd(std::less_equal<int>(),10));
  	std::cout << "There are " << cx << " elements less or equal 10.
";



	return 0;
}

输出:

RAND_MAX: 32767
....PrintVector....
1315
2075
8984
13167
14730
15153
18586
20698
23082
24151
....PrintVector....
24151
23082
20698
18586
15153
14730
13167
8984
2075
1315
There are 4 non-negative elements.
20
10
10
There are 5 elements less or equal 10.