Re: class *a = new ?;

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 28 Sep 2009 04:30:08 -0700 (PDT)

Message-ID:

<827ac404-a096-4ddc-a138-ed74ca1fa9e7@p23g2000vbl.googlegroups.com>

On 28 Sep., 13:14, feribiro <ferib...@index.hu> wrote:

Hi,

could you tell me please how it is possible to pass the type of a
class to a function as a parameter, and then initiate a new class of
that type?

class A {...}
class B : public A {...}
class C : public A {...}

void Main()
{
Create(B);
Create(C);
}

void Create(??? param)
{
A *a = new ??param??;
delete a;
}

Thank you very much

You cannot do this dynamically like in other languages which keep
enough meta data about classes in memory for "reflection" (I'm
thinking of Java in this case) -- at least not without getting your
hands dirty (building registries, registering types, etc by yourself).

But you can do it easily at compile-time. In your case ("Create(B);")
you already know the type at compile-time (B)...

   class A
   {
     public:
       virtual ~A() {}
     //...
   };

   class B : public class A {...};
   class C : public class A {...};

   template<typename T>
   void Create()
   {
     A* a = new T;
     delete a;
   }

   int main()
   {
     Create<B>();
     Create<C>();
   }

Note: The class A must have a virtual destructor because you invoke
delete with a pointer of type A* that points to a possibly derived
object. Inheritance must be public in your case. The main function's
name is all lower case and its return type is int.

Cheers,
SG