C ++多态性，不完整的向下转换

Question

I have an array which holds references to a bland base type, let's call it Object .

I have derived Class1 from Object and Class2 from Class1 .

#include <vector>

class Object {};
class Class1 : public Object {
public:
virtual std::string ToString() {return "it is 1";}
};
class Class2 : public Class1 {
public:
    virtual std::string ToString() {return "it is 2";}
};

int _tmain(int argc, _TCHAR* argv[])
{
    std::vector<Object *> myvec;
    myvec.push_back(new Class2());
    printf("%s", (static_cast<Class1*>(myvec[0]))->ToString().c_str());
    return 0;
}

I call ToString() by casting from Object * to Class1 * like so

    printf("%s", (static_cast<Class1*>(myvec[0]))->ToString().c_str());

My question is, will this output 1 or 2 when the object is in fact of Class2 but not down cast to that specifically? Is the virtual keyword having the intended effect?

Answer 1

It should call Class2 's virtual function, since the object is of that type.

Remember: the purpose of a virtual function is to call the actual class's function, not the function for the class the pointer/reference currently appears to be.

Answer 2

I slightly misread your question when I first answered, regarding demons and your nostrils.

The virtual keyword will have it's intended effect. That is, You will see 2. Although you have casted to Class1 , the vtable will work it's magic.

Answer 3

First, this is not a safe cast. But assuming that myvec[i] is a pointer the an object you are casting to, then it will call the appropriate method for that class.