虚函数关键字
考虑到我想调用适合我的对象类型的 fun ，在子类中声明继承的虚函数与“virtual”关键字或不有什么区别。看看评论。
#include <cstdio>
struct A{
int a;
A():a(5){}
virtual int fun(){return a+1;}
};
struct B: public A{
virtual int fun(){return a+5;} //I put virtual here
// int fun(){return a+5;} // Any difference if I put virtual before or not?
};
int main(){
B obj;
printf("%d\n", static_cast<A>(obj).fun()); // A::fun() called. Why?
printf("%d\n", static_cast<A&>(obj).fun()); // B::fun() called. As expected
printf("%d\n", static_cast<A*>(&obj)->fun()); // B::fun() called. As expected
printf("%d\n", static_cast<A>(B()).fun()); // A::fun() again. Why?
// printf("%d\n", static_cast<A&>(B()).fun()); //invalid_cast error. Why?
printf("%d\n", static_cast<A*>(&B())->fun()); //It works! B::fun() call
return 0;
}
Is there any difference between declaring inherited virtual function in a child class with the "virtual" keyword or not, considering I want to call fun appropriate to my objects' type. Look at the comments.
#include <cstdio>
struct A{
int a;
A():a(5){}
virtual int fun(){return a+1;}
};
struct B: public A{
virtual int fun(){return a+5;} //I put virtual here
// int fun(){return a+5;} // Any difference if I put virtual before or not?
};
int main(){
B obj;
printf("%d\n", static_cast<A>(obj).fun()); // A::fun() called. Why?
printf("%d\n", static_cast<A&>(obj).fun()); // B::fun() called. As expected
printf("%d\n", static_cast<A*>(&obj)->fun()); // B::fun() called. As expected
printf("%d\n", static_cast<A>(B()).fun()); // A::fun() again. Why?
// printf("%d\n", static_cast<A&>(B()).fun()); //invalid_cast error. Why?
printf("%d\n", static_cast<A*>(&B())->fun()); //It works! B::fun() call
return 0;
}
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如果基类中的相应函数是虚拟的，则派生类中的重写函数将隐式声明为“虚拟”。只需确保您获得完全相同的签名，否则您可能会无意中隐藏原始函数并声明一个新函数！
在 C++0x 中，可以随意使用
override说明符。你的两个“为什么？”问题是因为切片；您正在创建
A类型的新的复制切片对象。请注意，在B x; 中static_cast(x);转换与A(x)相同。Overriding functions in derived classes are implicitly declared "virtual" if the corresponding function in the base class is virtual. Just make sure you got the exact same signature, or you might inadvertently hide the original function and declare a new one!
In C++0x, feel free to make liberal use of the
overridespecifier.Your two "Why?" questions are because of slicing; you're making new, copy-sliced objects of type
A. Note that inB x; static_cast<A>(x);the cast is the same as sayingA(x).在派生类中重写的成员函数之前保留virtual关键字是可选的。运行时多态性仅适用于指针或引用。
Keeping the virtual key word before the overridden member function in the derived class is optional. Run-time polymorphism works only for pointers or references.