polymorphic struct declaration

Question

I am sure that this question has been asked and answered before, but I am having trouble searching without the proper terminology.

I have two unique structs A & B (not classes). My code uses functional overloading to treat the structs differently. I don't understand how to handle the declaration polymorphism. I apologize for a pseudocode example.

if( flag ==1 ){
   declare stuct A scoped to main
}
else{
   declare stuct B scoped to main
}

The pseudo code above will not work since the declarations are scoped within the if statements. My "real" code doesn't compile because of the scoping issues I am trying to describe.

EDIT

Clarification:

Depending on a specified flag (arg) I want to declare one of two different structs. Once the proper struct is declared all polymorphism is already handled by operation overloading.

Thanks

Answer 1

This question fundamentally has nothing to do with structs, or classes, or polymorphism, (though the solution does require the latter).

It's about trying to declare objects of different types, conditionally, without running afoul of block scoping. The simple answer is that you can't.

The usual approach is to use a late-initialised smart pointer instead:

#include <memory>
#include <cstdlib>

struct A { virtual ~A() {} };
struct B : A {};
struct C : A {};

void f(A&);

int main()
{
   srand(time(0));
   const int x = rand();

#if 0
   /** Can't do this: **/
   if (x > 500)
      B obj;
   else
      C obj;

   f(obj);
#endif

   /** Can do this: **/
   std::unique_ptr<A> ptr;
   if (x > 500)
      ptr.reset(new B);
   else
      ptr.reset(new C);

   f(*ptr);

#if 0
   /** Some older examples may propose this: **/
   A* ptr = NULL;
   if (x > 500)
      ptr = new B;
   else
      ptr = new C;

   f(*ptr);

   delete ptr;
   ptr = NULL;
#endif
}

Since f accepts a reference, polymorphic is maintained even though you're feeding it a A.

Typically you'd actually invoke a member function through ptr, using virtual dispatch to ensure that the correct code is executed:

   std::unique_ptr<A> ptr;
   if (x > 500)
      ptr.reset(new B);
   else
      ptr.reset(new C);

   ptr->someFunction();

No function overloads required.