C++继承机制下的构造函数

派生类同样有构造函数。当我们创建一个派生类对象的时候，基类构造函数将会被自动调用，用于初始化派生类从基类中继承过来的成员变量。而派生类中新增的成员变量则需要重新定义构造函数用于初始化了。

#include<iostream>
using namespace std;

class book
{
public:
    book();
    book(char* a, double p = 5.0);
    void setprice(double a);
    double getprice()const;
    void settitle(char* a);
    char * gettitle()const;
    void display();
private:
    double price;
    char * title;
};

class book_derived :public book
{
public:
	book_derived(){cout<<"book_derived 构造函数"<<endl;}
    void display();
};

book::book(char* a, double p) 
{
    title = a;
    price = p;
}

book::book()
{
	cout<<"book 构造函数"<<endl;
    title = "NoTitle";
    price = 0.0;
}

void book::setprice(double a)
{
    price = a;
}

double book::getprice()const
{
    return price;
}

void book::settitle(char* a)
{
    title = a;
}

char * book::gettitle()const
{
    return title;
}

void book::display()
{
    cout<<"book The price of "<<title<<" is $"<<price<<endl;
}

void book_derived::display()
{
    cout<<"book_derived The price of "<<gettitle()<<" is $"<<getprice()<<endl;
}

int main()
{
    book_derived b;
    b.display();
    return 0;
}

运行结果：

book 构造函数
book_derived 构造函数
book_derived The price of NoTitle is $0

创建对象时先是执行基类的构造函数，然后再是执行派生类构造函数。构造函数执行顺序是按照继承顺序自顶向下执行。