Re: [boost] scoped_ptr rationale

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2002-09-21 16:25:48

On Saturday, September 21, 2002, at 12:06  PM, Peter Dimov wrote:

> A swap-capable scoped_ptr is the best solution. [for implementing 
> handle/body]

I agree in the context of the current language.

If move_ptr comes to be, I think move_ptr might be a better solution.

Rationale:  scoped_ptr::swap is just another name for move assignment 
on scoped_ptr.  It transfers resources.  In the context of move 
semantics, it seems odd that a class does not have move semantics 
(especially move assignment) yet supports a resource transferring swap.

If a handle/body used move_ptr as an implementation, the handle copy 
constructor and copy assignment would be implemented in the same 
fashion as with scoped_ptr.  And the compiler generated versions would 
compile-time like scoped_ptr.  (the move_ptr in the proposal is 
currently lacking a swap function, but that is of course easily 
repaired)

What if the handle/body wanted to implement move semantics?  It might 
look like:

Handle::Handle(Handle&& h)
     : impl_(static_cast<Ptr&&>(h.impl_))
{
}

Handle&
Handle::operator=(Handle&& h)
{
     impl_ = static_cast<Ptr&&>(h.impl_);
     return *this;
}

(Ptr is a typedef for the move_ptr)

With scoped_ptr the move constructor is still doable, but perhaps not 
as intuitive (but that's a judgment call).  You have to take advantage 
of the fact that the only way to transfer scoped_ptr's resource is via 
swap.  That is, you need to default construct the target scoped_ptr 
first, and then swap with the source:

Handle::Handle(Handle&& h)
{
     impl_.swap(h.impl_);
}

And move assignment can also be accomplished with swap.

Handle&
Handle::operator=(Handle&& h)
{
     impl_.swap(h.impl_);
     return *this;
}

I find myself wondering what is the niche that scoped_ptr fills better 
than move_ptr.  And my best guess right now is the role of a local auto 
variable where the resource never needs to be transferred out of the 
scoped_ptr.

-Howard