[C/C++标准库]_[初级]_[函数对象functor的使用]

Functor

std::set

场景:

1. functor其实就是一个类对象,因为它重载了operator()操作符,所以可以把实例当作函数名那样操作,在C里也只有函数可以这么做.比如语句 f(1,2);在cpp

必须通过上下文判断f是对象还是函数.

2.在使用stl的容器类或算法函数时，经常需要自己传入一个functo作为参数或模板.

#include <string.h>

#include <iostream>
#include <fstream>
#include <string>
#include <utility>
#include <set>

using namespace std;

class Person
{
public:
	Person(int code,const string& name,const string& grade,
		const string& address):code_(code)
	{
		name_ = name;
		grade_ = grade;
		address_ = address;
	}
	~Person(){}
	int code_;// 学号.
	string name_;// name.
	string grade_; //grade.
	string address_;

	void operator()()
	{
		cout << " code_: " << code_ 
		     << " name_: " << name_ 
		     << " grade_: " << grade_ 
		     << " address_: " << address_
			 << endl;
	}
};

class PersonCompare
{
public:
	bool operator() (const Person* p1, const Person* p2) const 
	{
		return p1->code_ < p2->code_;
	}
	/* data */
};

class test_functor
{
public:
	void operator()(int x,int y)
	{
		cout << "x+y: " << x+y << endl;
	}

	/* data */
};

int main(int argc, char const *argv[])
{
	//1.simple functor
	test_functor tf;
	tf(1,2);

	//2.sort functor
	typedef set<Person*,PersonCompare> PersonSet;
	PersonSet::iterator it;
	PersonSet ps;
	ps.insert(new Person(3,"张三","高三(1)班","北京朝阳区"));
	ps.insert(new Person(2,"李四","高三(2)班","北京海淀区"));
	ps.insert(new Person(1,"龙五","高三(3)班","北京王府井"));
	// ps.insert(new Person(1,"龙四","高三(3)班","北京王府井"));

	for (it = ps.begin(); it != ps.end(); ++it)
	{
		(*(*it))();
	}

	cout << endl;
	pair<PersonSet::iterator,bool> res = ps.insert(new Person(1,"张四","高三(3)班","北京王府井"));
	if (!res.second)
	{
		cout << "insert fail,exist name: " << (*(res.first))->name_ << endl;
	}
	for (it = ps.begin(); it != ps.end(); ++it)
	{
		(*(*it))();
	}

	return 0;
}

输出:

x+y: 3
 code_: 1 name_: 龙五 grade_: 高三(3)班 address_: 北京王府井
 code_: 2 name_: 李四 grade_: 高三(2)班 address_: 北京海淀区
 code_: 3 name_: 张三 grade_: 高三(1)班 address_: 北京朝阳区

insert fail,exist name: 龙五
 code_: 1 name_: 龙五 grade_: 高三(3)班 address_: 北京王府井
 code_: 2 name_: 李四 grade_: 高三(2)班 address_: 北京海淀区
 code_: 3 name_: 张三 grade_: 高三(1)班 address_: 北京朝阳区
[Finished in 0.1s]

参考:

1.《C++ Standard Library, The: A Tutorial and Reference》