如何使用基类指针引用派生类成员？

Question

class a{};
class b : public class a{
public:
int c;
};
a* var = new b;
var->c=2;  

Last line is not correct. Is it possible to refer to the derived class member?

Answer 1

If the member is not in the base class, you should not be allowed to access it and it should give a compilation error. To access a member of derived class, you can typecast it to the derived class.

class a{};
class b : public class a{
public:
    int c;
};
a* var = new b;
((b*)var)->c=2;

Answer 2

while var is ab object at compile time the compiler does not know that so you have to cast it to ab object or pointer like this.

(*(b*)var).c = 2;
((b*)var)->c = 2;

My computer science teacher explained that treat two cases when doing polymorphism run-time and compile time

Hope this helps.

Answer 3

您可以使用static_cast从Base降到Derived，然后访问该成员。

static_cast<b*>(var)->c = 2;