Re: [Boost-users] [Shared_ptrs] As I doing this right

From: Windflower Waters (WWaters_at_[hidden])
Date: 2007-05-01 10:08:55

This is what I would do:

Main()
{
  shared_ptr<B> poly(new B);   // Start with a pointer that matches the
type

  MyFunction( dynamic_pointer_cast<A>(poly) );	// Cast it in the
argument list

}

-- Windflower

-----Original Message-----
From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Hughes, James
Sent: Tuesday, May 01, 2007 4:19 AM
To: Boost-users_at_[hidden]
Subject: [Boost-users] [Shared_ptrs] As I doing this right

Although I have been using shared_ptr's for a while, I still get a bit
confused with polymorphic stuff ...no change there.

An pseudocode example:

Class A
{
};

Class B : public A
{
 DoStuff1();
 DoStuff2();
 DoStuff3();
 DoStuff4()
};

// MyFunction handles A and classes derived from A
MyFunction(shared_ptr<A>)
{
};

Main()
{
  shared_ptr<A> poly<new B);   // OK so far

  MyFunction(poly);		 // Still OK - 

  // OK, done this, but now if I want to call a load of stuff in an
instance of B
  // I either have to make a load of pointer casts (because poly is an
A)

  dynamic_pointer_cast<B>(poly)->DoStuff1();
  dynamic_pointer_cast<B>(poly)->DoStuff2();
  dynamic_pointer_cast<B>(poly)->DoStuff3();
  dynamic_pointer_cast<B>(poly)->DoStuff4();  

  // Or this?

  shared_ptr<B> tmp = dynamic_pointer_cast<B>(poly);  // is this right? 
        
// I have to create a new pointer?

  tmp->DoStuff1();
  tmp->DoStuff2();
  tmp->DoStuff3();
  tmp->DoStuff4();  
  

}

So, the quesiton is one of efficiency - if I was using pointers, I would
just new a B and assign to a B* variable, then pass in to the function
which would use it as an A - no temp required. With above a new tmp
pointer seems to be required - am I right or have I missed something
blindingly obvious.

James

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