c++ Is a class derived from a specific class?

Question

Is it possible what I'm trying to achieve. This is the problem:

class Content{};

class Content1 : public Content{};
class Content1_1 : public Content1{};
class Content1_2 : public Content1{};

class Content2 : public Content{};
class Content2_1 : public Content2{};
class Content2_2 : public Content2{};

class Container{
public:
    Content* c;
};

Is it possible to determine if the pointer of the Container class points to an object of Content1 or an object of a class derived from Content1?

Thanks.

EDIT:

If I look up the type of the pointer in the container class, it is treated as a content type, though it might actually be a content1 or content2 type, so dynamic_cast doesn't work.

class content{};
class content1 : public content{};
class content2 : public content{};

class container{
public:
    content* c;
};

int main(void)
{
    container* x = new container;
    x->c = new content2;

    if( dynamic_cast<content1*>((content1*)x->c) == NULL){
        //This doesn't fail, eventhough 'content1' and 'content2' shouldn't be compatible.
        //This means that every class, derived from 'content' will be treated as 'content1' when using dynamic_cast.
    }

    return 0;
}

Answer 1

If your types are polymorphic (i.e. if they have at least one virtual function), you can use dynamic_cast<>. The result of dynamic_cast<> will be a null pointer (i.e. nullptr for C++11, NULL for C++03) in case the pointed object is not actually an instance of the specified class, or of a derived class thereof:

Container cont;
Content1* pContent1 = dynamic_cast<Content1*>(cont.c);
if (pContent1 != nullptr)
{
     // cont.c points to an instance of Content1 
     // or a class derived from Content1
}

A common practice is to make the destructor of your base class virtual, so that objects of a class derived from your base class can be deleted through a base class pointer (if the destructor is not virtual, attempting to do so results in undefined behavior):

class Content //Abstract.
{
public:
    virtual ~Content() { }
//  ^^^^^^^
};

Answer 2

Yes, use dynamic_cast - http://www.cplusplus.com/doc/tutorial/typecasting/

It will give you RTTI - Runtime Type Information.

Content1 * p = dynamic_cast<Content1 *>(c);

if p is non-NULL, then c points to an object which is Content1 or derived from Content1. However, your class heirarchy needs to be polymorphic. Since your class Content is Abstract (I am assuming you have a pure virtual function in Content), your whole heirarchy automatically is polymorphic.

Answer 3

If you have to know the exact runtime type of a polymorphic class, you should rethink your design. Usually it is better to either keep objects of different types in different containers (additionally or, instead of one central container) or add a virtual function to each of the derived classes which performs the action where you would have to discriminate between the types.

There is an entry in the C++ FAQ which might help you: I have a heterogeneous list of objects, and my code needs to do class-specific things to the objects. Seems like this ought to use dynamic binding but can't figure it out. What should I do?

Answer 4

If you want to see if Content* points to an object of a class derived from Content2 class just cast it with dynamic_cast and see.

This works:

#include <iostream>
#include <typeinfo>

using namespace std;

class Content{public: virtual ~Content(){}};

class Content1 : public Content{public: virtual ~Content1(){}};
class Content1_1 : public Content1{public: virtual ~Content1_1(){}};
class Content1_2 : public Content1{public: virtual ~Content1_2(){}};

class Content2 : public Content{public: virtual ~Content2(){}};
class Content2_1 : public Content2{public: virtual ~Content2_1(){}};
class Content2_2 : public Content2{public: virtual ~Content2_2(){}};

class Container{
public:
    Content* c;
};

int main()
{
    Content* cnt=new Content2_1;
    if(dynamic_cast<Content2_1*>(cnt))
        cout << "True" << endl;
    return 0;
}