深入理解c++中virtual关键字
1.virtual关键字主要是什么作用？
c++中的函数调用默认不适用动态绑定。要触发动态绑定，必须满足两个条件：第一，指定为虚函数；第二，通过基类类型的引用或指针调用。
由此可见，virtual主要主要是实现动态绑定。
2.那些情况下可以使用virtual关键字？
virtual可用来定义类函数和应用到虚继承。
友元函数 构造函数 static静态函数 不能用virtual关键字修饰；
普通成员函数 和析构函数 可以用virtual关键字修饰；
3.virtual函数的效果
class GrandFather
{
public:
GrandFather() {}
virtual void fun()
{
cout << "GrandFather call function!" << endl;
}
};
class Father : public GrandFather
{
public:
Father() {}
void fun()
{
cout << "Father call function!" << endl;
}
};
class Son : public Father
{
public:
Son() {}
void fun()
{
cout << "Son call function!" << endl;
}
};
void print(GrandFather* father)
{
father->fun();
}
int _tmain(int argc, _TCHAR* argv[])
{
Father * pfather = new Son;
pfather->fun();
GrandFather * pgfather = new Father;
print(pgfather);
return 0;
}
输出为 Son call function
Father call function
4.virtual的继承性
只要基函数定义了virtual，继承类的该函数也就具有virtual属性
即 GrandFather Father Son同时定义virtual void fun()与GrandFather一个定义virtual void fun效果是一样的
5.虚析构函数
class GrandFather
{
public:
GrandFather() {}
virtual void fun()
{
cout << "GrandFather call function!" << endl;
}
~GrandFather()
{
cout << "GrandFather destruction!" << endl;
}
};
class Father : public GrandFather
{
public:
Father() {}
void fun()
{
cout << "Father call function!" << endl;
}
~Father()
{
cout << "Father destruction!" << endl;
}
};
class Son : public Father
{
public:
Son() {}
void fun()
{
cout << "Son call function!" << endl;
}
~Son()
{
cout << "Son destruction!" << endl;
}
};
void print(GrandFather* p)
{
p->fun();
}
int _tmain(int argc, _TCHAR* argv[])
{
Father * pfather = new Son;
delete pfather;
return 0;
}
以上代码输出：Father destruction!
GrandFather destruction!
执行了Son的构造函数，没执行Son的析构函数，故把GrandFather的析构函数设置为virtual
则输出: Son destruction!
Father Destruction!
GrandFather destruction!
6. 纯虚函数
纯虚函数定义如下：
class GrandFather
{
public:
GrandFather() {}
virtual void fun() = 0
{
cout << "GrandFather call function!" << endl;
}
virtual ~GrandFather()
{
cout << "GrandFather destruction!" << endl;
}
};
纯虚函数为后代类提供可覆盖的接口，但这个类中的版本决不会调用。
含有（或继续）一个或多个纯虚函数的类是抽象基类，抽象基类不能实例化！
继承类只有重写这个接口才能被实例化
7.虚继承
虚继承主要解决交叉继承带来的问题。这里给出一片参考文章c++虚继承。
给一个例子如下
class GrandFather
{
public:
GrandFather() {}
void fun()
{
cout << "GrandFather call function!" << endl;
}
virtual ~GrandFather()
{
cout << "GrandFather destruction!" << endl;
}
};
class Father1 : public GrandFather
{
public:
Father1() {}
void fun()
{
cout << "Father call function!" << endl;
}
};
class Father2 : public GrandFather
{
public:
Father2() {}
void fun()
{
cout << "Father call function!" << endl;
}
};
class Son : public Father1, public Father2
{
public:
Son() {}
//void fun()
//{
// cout << "Son call function!" << endl;
/
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