A question regarding template instantiation

[LarryChen]

Suppose I have a template class defined as,

template<class T>
class A
{
public:
   void foo()
   {
         T* p = new T;
         p->bar();
   }
};

When I instantiate A, I either use A<B> or use A<C>. Here B and C is just two different types. B and C both define member function bar. My question is that in run time, is there any way I can detect whether p is of B or C? Thanks.

[Paul McKenzie]

My question is that in run time, is there any way I can detect whether p is of B or C? Thanks.

The question I would like to ask you is why you need to know whether p is a B or a C at that point. If you need to know this, you should revisit your design. So let's say you could do this, what would you be doing with the code?

template<class T>
class A
{
public:
   void foo()
   {
         T* p = new T;
         if (it's a B)
           p->bar();
         else
         if (it's a C)
           do_something_else();
   }
};

So you're drifting back into old-style 'C' type coding, defeating the whole purpose of a template. What happens if its a D, E, F object, all with bar() functions?

If you really are going to do something based on a different type in the template, then you need to specialize the template on that type. That is not a runtime concept -- template specialization occurs at compile time,

Regards,

Paul McKenzie

[Ejaz]

When I instantiate A, I either use A<B> or use A<C>.

You can use A<B>, A<C> or even A<int>, A<WhatEver>. However, in order to get your code compile, the "T" should have bar().

My question is that in run time, is there any way I can detect whether p is of B or C?

Depends, upon what you want to do (as Paul stated). If you provide more details, it can be helpful. Check boost type traits is_same for further details.