构造函数

1. 类中没有创建默认构造函数，但有带参数的构造函数

编译不通过，除非参数有默认参数值。

如：

class Base

{

public:

      Base( int i )

{

}

};

添加默认参数，例如：

 Base( int i  = 9 )

{

}

 

2.

（1）

class Base

{

public:

    Base()

    {

        cout<<"Base::Base()"<<endl;

    }

  };

 

class Drived: public Base

{

public:

    Drived()

    {

        cout<<"Drived::Drived()"<<endl;

    }

   };

当调用Drived d;时，会先调用Base(), 再调用Drived().

（2）

class Base
{
public:
        Base(int i = 9 )
    {
        cout<<"Base::Base("<<i<<"）"<<endl;
    }
};
 
class Drived: public Base
{
public:
    Drived()
    {
        cout<<"Drived::Drived()"<<endl;
    }
    Drived(int i):Base(i)
    {
        cout<<"Drived::Base("<<i<<")"<<endl;
    }
};
当调用Drived d;时， 会先调用Base(9), 再调用Drived().

（3）
class Base
{
public:
Base( )
    {
        cout<<"Base::Base("<<i<<"）"<<endl;
    }

        Base(int i  )
    {
        cout<<"Base::Base("<<i<<"）"<<endl;
    }
};
 
class Drived: public Base
{
public:
    Drived()
    {
        cout<<"Drived::Drived()"<<endl;
    }
    Drived(int i):Base(i)
    {
        cout<<"Drived::Base("<<i<<")"<<endl;
    }
};
当调用Drived d(4);时， 会先调用Base(int i), 再调用Drived(int i).