Is There a way to use dynamic_cast When Casting to a Child?

Question

Say that I that I have these classes:

struct Parent {};
struct Child : public Parent {
    void func() {}
};

Now say that I wanted to create a function like this:

void foo(Parent* arg) {
    auto child = dynamic_cast<Child*>(arg);

    if(child != nullptr) child->func();
}

But obviously that will obviously give me the error:

dynamic_cast : Parent is not a polymorphic type

So I can't do the dynamic_cast step, is there a way that I can validate that arg is in fact a Child* at run-time?

Answer 1

Give your Parent class a virtual function. Destructor comes into mind, for several reasons (like deleting child via base ptr, etc.).

Answer 2

No, you can't. The information you can get about an object at runtime is achieved using the RTTI ( Run Time Type Information ). The RTTI of an object is stored within the virtual table of it's class.

Each object of a polymorphic class (ie a class which has one or more virtual functions, or a class which is derived from a polymorphic class) contains a vptr , which is a pointer to the virtual table of the corresponding class. An object of a non-polymorphic type has no vptr and there is not a virtual table for its class. Therefore you cannot use dynamic_cast on non-polymorphic types.

So, as @lorro said, you should add a virtual function to Parent , and the simplest option is adding a virtual destructor (which you should do anyway):

struct Parent {
    virtual ~Parent() {} 
};