Scope of variables in if statements

by

“Do variables declared in a conditional go out of scope at the end of the conditional?”

Yes – the scope of a local variable only falls within enclosing brackets:

{
   int x; //scope begins

   //...
}//scope ends
//x is not available here

In your case, say you have class A.

If you’re not dealing with pointers:

A a( condition ? 1 : 2 );

or if you’re using a different constructor prototype:

A a = condition ? A(1) : A(2,3);

If you’re creating the instance on the heap:

A* instance = NULL;
if ( condition )
{
   instance = new A(1);
}
else
{
   instance = new A(2);
}

or you could use the ternary operator:

//if condition is true, call A(1), otherwise A(2)
A* instance = new A( condition ? 1 : 2 );

EDIT:

Yes you could:

A* x = NULL; //pointer to abstract class - it works
if ( condition )
   x = new B();
else
   x = new C();

EDIT:

It seems what you’re looking for is the factory pattern (look it up):

 class A; //abstract
 class B : public A;
 class C : public A;

 class AFactory
 {
 public:
    A* create(int x)
    {
       if ( x == 0 )
          return new B;
       if ( x == 1 )
          return new C;
       return NULL;
    }
 };

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