首页 > 代码库 > C++语言笔记系列之十五——派生类、基类、子对象的构造和析构函数调用关系

C++语言笔记系列之十五——派生类、基类、子对象的构造和析构函数调用关系

2024-07-10 11:35:31   222人阅读

例子
example 1
注：若一个基类同时派生出两个派生类，即两个派生类从同一个基类继承，那么系统将为每一个简历副本，每个派生类独立地使用自己的基类副本（比如基类中有属于自己类的静态变量等）。
#include <iostream.h>

class Person
{
public:
    person() {cout<<"Construction of person."<<endl;}
    ~person() {cout<<"Destruction of person."<<endl;}
};
class Student:public person
{
public:
    student() {cout<<"Construction of student."<<endl;}
    ~student() {cout<<"Destruction of student."<<endl;}
};
class Teacher:public Person
{
public:
    Teacher() {cout<<"Construction of teacher."<<endl;}
    ~Teacher() {cout<<"Destruction of teacher."<<endl;}
};

int main()
{
    Student s;
    Teacher t;
}
程序输出：
Construction of person.
Construction of student.
Construction of person.
Construction of teacher.
Destruction of teacher.
Destruction of person.
Destruction of student.
Destruction of person.
结论：若一个程序中有多个对象，每个对象必须按照顺序创建，在创建派生类对象时，调用构造函数的顺序为：基类构造-->子对象构造-->派生类构造。析构函数的调用顺序严格与构造函数相反。在派生类的构造函数中，当基类的构造函数缺省参数时，在派生类构造函数中可以缺省对基类构造函数的调用。

example 2

#include <iostream.h>
class Data
{
public:
    Data(int x) {Data::x = x; cout<<"Data construction."<<endl;}
    ~Data() {cout<<"Data destruction."<<endl;}
private:
    int x;
};
class Base
{
public:
    Base(int x):d1(x) {cout<<"Base construction."<<endl;}
    ~Base() {cout<<"Base destruction."<<endl;}
private:
    Data d1;
};
class Device:public Base
{
public:
    Device(int x):Base(x), d2(x) //参数共用
    {cout<<"Device construction."<<endl;}
    ~Device() {cout<<"Device destruction."<<endl;}
private:
    Data d2;
};

int main()
{
    Device obj(5);
}
程序输出：
Data construction.
Base construction.
Data construction.
Device construction.
Device destruction.
Data destruction.
Base destruction.
Data destruction.
构造顺序分析：调用Base类构造，而Base中含有子对象d1，所以要先调用d1的构造函数再调用Base的构造函数；Base构造函数调用完成之后再调用d2的构造；最后一步是调用派生类的构造。

example 3

class Base
{
public:
    Base(int xx = 0):x(xx) //注意没有分号，等价于普通的构造函数类型
    {cout<<"Base construction."<<endl;} //这个花括号是不能省略的
    ~Base() {cout<<"Base destrcution."<<endl;}
    void print() {cout<<x<<",";}
    int Getx() {return x;}
private:
    int x;
};
class Device:public Base
{
public:
    Device(itn xx = 0, int yy = 0):Base(xx), y(yy), z(xx+yy)
    {cout<<"Device construction."<<endl;}
    ~Device() {cout<<"Device destruction."<<endl;}
    void print()
    {
        Base::print();
        cout<<y<<","<<z.Getx()<<endl;
    }
private:
    int y;
    Base z;
};
int main()
{
    Device obj1(1), obj2(4, 6);
    obj1.print();
    obj2.print();
}
程序输出：
Base construction.
Base construction.
Device construction.
Base construction.
Base construction.
Device construction.
2,0,2
4,6,10
析构略......


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