Question

I need to initialize a boost::shared_ptr based on a condition. Sample code is below that depicts the scenario i ma looking for.

class A{};
class B:public A{};
class C:public A();

void some_func(int opt)
{
   boost::shared_ptr<A> abstract_ptr;
   if(opt==RELEVENT_TO_B)
   {
      abstract_ptr(new B);
   }
   else
   {
      abstract_ptr(new C);
   }
}

I know, above code sample is wrong. boost::shared_ptr<A> Abstract_ptr in second line of some_func() will create a shared_ptr object with the shared_ptr constrcutor that takes no arguments and since there is no assignment operator defined and no shared_ptr takes raw pointer after initialization so code within if-else is wrong.

My question is, how i can achieve this (conditional initialization as shown in some_func) modestly with boost::shared_ptr.

Answer 1

You could use .reset() to put a raw pointer to the shared pointer.

   boost::shared_ptr<A> abstract_ptr;
   if (opt == RELEVENT_TO_B)
      abstract_ptr.reset(new B);
   else
      abstract_ptr.reset(new C);

You could also directly assign a shared ptr to it:

   boost::shared_ptr<A> abstract_ptr;
   if (opt == RELEVENT_TO_B)
      abstract_ptr = boost::make_shared<B>();
   else
      abstract_ptr = boost::make_shared<C>();