6.C++的构造函数与析构函数

C++的构造函数与析构函数

1.构造函数

构造函数的函数名与类名相同，通过构造函数的条件可以确定创建对象的条件。

当类中有定义构造函数时，默认的无参构造函数会失效。

示例：（求学生的平均分）

Student.h

#include <iostream>
using namespace std;

class Student{
	private:
		string stuId;
		string stuName;
		int score;
		static int stuCount,totalScore;
	public:
		Student(string stuId,string stuName);//构造函数，需要传入学号与姓名才能创建
	 	void setScore(int sc);
	 	string getStuId_Name();
	 	int getScore();
	 	void print();
	 	static int getStuCount();
	 	static float getAvg();
};
int Student::stuCount = 0; 
int Student::totalScore = 0; 

Student::Student(string stuId,string stuName){
	this->stuId = stuId;
	this->stuName = stuName;
	this->score = 0;
	stuCount++;
}
void Student::setScore(int sc){
	totalScore = totalScore - this->score + sc;
	this->score = sc;

}

string Student::getStuId_Name(){
	return this->stuId + " " + this->stuName;
}

int Student::getScore(){
	return this->score;
}

void Student::print(){
	cout << this->stuId + " " + this->stuName + " 成绩：" 
		<< this->score  
		<< " 总分为：" << totalScore
		<< endl;
}
int Student::getStuCount(){
	return stuCount;
}
float Student::getAvg(){
	return totalScore*1.0/stuCount;
}

main.cpp

#include <iostream>
#include "Student.h"

using namespace std;

int main(){
	Student stu("1","张三");
	Student stu2("2","李四");
	Student stu3("3","李华");
	stu.setScore(98);
	stu.print();
	stu2.setScore(60);
	stu2.print();
	stu3.setScore(45);
	stu3.print();
	
//	avg = (stu.getScore()+stu2.getScore()+stu3.getScore())/3.0;
//	cout << "平均分为：" << avg <<endl; 
	cout << "学生人数为：" << Student::getStuCount()
		<< " 平均分为：" << Student::getAvg() <<endl;
		
	stu3.setScore(60);
	stu3.print();
	cout << "学生人数为：" << stu3.getStuCount()
	<< " 平均分为：" << stu3.getAvg() <<endl;		
	return 0;
}

2.析构函数

析构函数声明为名称与类名相同，无参，类名前使用~定义，析构函数是当创建的对象被删除时调用。

实例：（商品的进出货）

Item.h

#include <iostream>

using namespace std;

class Item{
	private:
		string ID,name;
		static int count;
	public:
		Item(string pID,string pName);
		~Item();
		static int getCount();
		string getID_Name();
}; 
int Item::count = 0;
Item::Item(string pID,string pName){
	ID = pID;
	name = pName;
	count++;
	cout << pName << "商品进货成功!" <<endl;	
}
Item::~Item(){
	cout << "商品出货成功!" <<endl;	
	count--;
}
int Item::getCount(){
	return count;
}
string Item::getID_Name(){
	return ID+" "+name;;
}

main.cpp

#include "Item.h"

int main(){
	// 分配静态内存  Item item("1","衣服"); 
	//分配动态内存，才能被delete删除 
	Item *item  = new Item("1","衣服");
	Item *item2 = new Item("2","裤子");
	Item *item3 = new Item("3","鞋子");
	Item *item4 = new Item("4","袜子");
	
	cout << "当前商品总数：" <<  item->getCount() << endl;
	//free(item3);
	//cout << "当前商品总数：" <<  item->getCount() << endl;
	cout << "出售了" << item4->getID_Name() << endl;
	//delete(item4);
	delete item4;
	cout << "当前商品总数：" <<  item->getCount() << endl;
	return 0;
}