Checking the object type in C++11

Question

I have class B that inherits from A.

class A
{
};

class B : public A
{
};

And I have three objects.

A* a = new A();
A* a2          


        

Answer 1

Some classes are polymorphic, some are non-polymorphic.

A polymorphic class has one or more virtual functions (possibly inherited), a non-polymorphic class has zero virtual functions.

Your A and B are non-polymorphic.

A polymorphic version of A and B will exhibit the behaviour you want:

#include 
#include 

using namespace std;

struct A
{
    virtual ~A() {}; // add virtual function
};

class B : public A
{
};

A* a = new A();
A* a2 = new B();
B* b = new B();

int main()
{
    cout << (typeid(*a) == typeid(A)) << endl; // -> 1
    cout << (typeid(*a2) == typeid(A)) << endl; // -> 0 <-- CHANGED
    cout << (typeid(*b) == typeid(A)) << endl; // -> 0

    cout << (typeid(*a) == typeid(B)) << endl; // -> 0
    cout << (typeid(*a2) == typeid(B)) << endl; // -> 1 <-- CHANGED
    cout << (typeid(*b) == typeid(B)) << endl; // -> 1
}

Instances of a polymorphic class store the dynamic type of their most derived object at runtime.

(In your example a2 is of type pointer-to-A, and is pointing at an object of type A, however this object is only a base class subobject of the most dervived object of type B. What you want to get is the type of this most derived object B when querying a2. For this you need a polymorphic class.)

That is how polymorphic classes support dynamic_cast and typeid of the most derived object (as well as virtual function dispatch).

Non-polymorphic classes do not have this information, so they can only report the static type known at compile-time. Non-polymorphic classes are more compact and efficient then polymorphic classes. That is why not all C++ classes are polymorphic. The language leaves it up to the programmer to chose the tradeoff between performance and functionality. For example:

struct X { int x; };
struct Y : X {};
struct Z : Y {};

On my system non-polymorphic Z is sizeof(Z) == 4 bytes, same as an int.

struct X { int x; virtual ~X() {}; };
struct Y : X {};
struct Z : Y {};

Now after making Z polymorphic, sizeof(Z) == 16 bytes. So an array of Z is now 300% larger, because each Z instance has to store its type information at runtime.