How do I check if an object's type is a particular subclass in C++?

Question

I was thinking along the lines of using typeid() but I don't know how to ask if that type is a subclass of another class (which, by the way, is abstract)

Answer 1

 

class Base {   public: virtual ~Base() {} };  class D1: public Base {};  class D2: public Base {};  int main(int argc,char* argv[]); {   D1   d1;   D2   d2;    Base*  x = (argc > 2)?&d1:&d2;    if (dynamic_cast<D2*>(x) == nullptr)   {     std::cout << "NOT A D2" << std::endl;   }   if (dynamic_cast<D1*>(x) == nullptr)   {     std::cout << "NOT A D1" << std::endl;   } } 

Answer 2

You really shouldn't. If your program needs to know what class an object is, that usually indicates a design flaw. See if you can get the behavior you want using virtual functions. Also, more information about what you are trying to do would help.

I am assuming you have a situation like this:

class Base; class A : public Base {...}; class B : public Base {...};  void foo(Base *p) {   if(/* p is A */) /* do X */   else /* do Y */ } 

If this is what you have, then try to do something like this:

class Base {   virtual void bar() = 0; };  class A : public Base {   void bar() {/* do X */} };  class B : public Base {   void bar() {/* do Y */} };  void foo(Base *p) {   p->bar(); } 

Edit: Since the debate about this answer still goes on after so many years, I thought I should throw in some references. If you have a pointer or reference to a base class, and your code needs to know the derived class of the object, then it violates Liskov substitution principle. Uncle Bob calls this an "anathema to Object Oriented Design".