c++ 实现一个function object

版权声明：瞎几把写 https://blog.csdn.net/weixin_43184615/article/details/82961489

标准库预先定义了许多function object，所谓function object是某种class的实例对象。
functional头文件里自带的六个关系运算：less, less_equal, greater, greater_equal, equal_to, not_equal_to.
搭配bind2nd()函数使用,bind2nd(less(),val),将val绑定到less() function object的第二个参数，作为比较的基准数。

这边是自定义一个function object，lessthan()。

#include<iostream>
#include<vector>
#include<iterator>    
#include<algorithm>  //用于find_if
using namespace std;
class LessThan
{
private:
	int _val;
public:
	LessThan(int val) : _val (val) {}  //用成员初始化列表的方式申明并定义构造函数
	int comp_val() const   { return _val; }
	void comp_val(int nval){_val = nval; }

	bool operator()(int _value) const;
	
};

inline bool LessThan:: operator()(int _value) const { return _value < _val; }

//计算数组中小于给定数的数字个数
int count_less_than(const vector<int> &vec, int comp)
{
	LessThan lt(comp);

	int count = 0;
	for (int i = 0; i < vec.size(); ++i)
	{
		if (lt(vec[i]))
			++count;
	}
	return count;
}

//打印这些数，用find_if函数
void print_less_than(const vector<int> &vec, int comp)
{
	LessThan lt(comp);

	vector<int>::const_iterator  iter = vec.begin();
	vector<int>::const_iterator  it_end = vec.end();

	cout << "The elements less than " << lt.comp_val() << endl;
	while ((iter = find_if(iter, it_end, lt)) != it_end)
	{
		cout << *iter << ' ';
		++iter;
	}
}

int main()
{
	//int ia[16] = { 17, 12, 44, 3, 23, 9, 12, 10, 22, 11,
		//90, 24, 65, 3, 4, 8 };
	vector<int> vec;
	cout << "please enter the number to make up the vector: "<<endl;
	int num;
	while (cin >> num)
		vec.push_back(num);

	int comp_val = 20;
	cout << "Number of elements less than " << comp_val << " are " << count_less_than(vec, comp_val) << endl;

	print_less_than(vec, comp_val);

	system("pause");
}