关于C++类的默认生成函数

 初始化对象数组的方案是，首先使用默认构造函数来创建数组元素，然后花括号中的构造函数将创建临时对象，然后将临时对象的内容拷贝到相应的元素中。因此，要创建类对象数组，则这个类必须有默认构造函数。

 

C++提供的默认拷贝构造函数和默认赋值函数工作的方法是：完成一个成员一个成员的拷贝，如果成员是类对象，则调用其拷贝构造函数或者默认拷贝构造函数。默认拷贝构造函数执行的是浅拷贝(“位拷贝”而非“值拷贝”)。当类成员里边有指针变量时，这两个默认的构造函数肯定会出错。

 

以类对象作为函数参数，调用该函数时将执行拷贝构造函数。

 

employee.h

#ifndef __EMPLOYEE_H#define __EMPLOYEE_Hclass employee{public:employee();employee(const employee& other);employee(const char *nm);~employee();void display();private:char *name;int id;};#endif

employee.cpp

#include "employee.h"#include <iostream>using std::cout;using std::endl;int uid = 0;// 默认构造函数employee::employee(){name = NULL;id = 0;cout << "this is default constructor" << endl;}// 拷贝构造函数employee::employee(const employee& other){name = new char[strlen(other.name) + 1];strcpy(name, other.name);id = other.id;cout << "this is copy constructor" << endl;}// 有参构造函数employee::employee(const char *nm){name = new char[strlen(nm) + 1];strcpy(name, nm);id = ++uid;cout << "this is parameter (const char *nm) constructor" << endl;}employee::~employee(){cout << "this is destructor" << endl;if (name != NULL){cout << "I will delete name" << endl;delete name;name = NULL;}}void employee::display(){cout << "name is " << name << endl << "id = " << id << endl;printf("name addr is %p/n", name);}

main.cpp

#include "employee.h"int main(int argc, char* argv[]){employee em1("zhang"); // 打印"this is parameter (const char *nm) constructor"employee em2 = em1; // 打印"this is copy constructor"employee em3; // 打印"this is default constructor"em3 = em2; // 此处将调用默认的赋值函数[employee& operator=(const employee& other)]em1.display();em2.display();em3.display();//this is parameter (const char *nm) constructor//this is default constructor//this is default constructor//this is destructor//this is destructor//this is destructoremployee ems[3] = {employee("zhao")};return 0;}

 

程序输出

this is parameter (const char *nm) constructorthis is copy constructorthis is default constructorname is zhangid = 1name addr is 00032BC8name is zhangid = 1name addr is 00032C30name is zhangid = 1name addr is 00032C30this is parameter (const char *nm) constructor // ems[0]this is default constructor // ems[1]this is default constructor // ems[2]this is destructor // ems[2]this is destructor // ems[1]this is destructor // ems[0]I will delete namethis is destructor // em3I will delete name // 第一次释放00032C30的地址 this is destructor // em2，该对象的name和em3的name指向同一块内存地址I will delete name // 第二次释放00032C30的地址，g++并没有报异常this is destructor // em1I will delete name