🚀 go-pugleaf

In article <4k4noe$igl@jabba.lehman.com>, ajay@lehman.com (Ajay Kamdar) wrote:

> ...
> It is not clear at all that the copy semantics of auto_ptr
> are essential for exception-safe transfer of resources.
> The same example coded as follows does not use
> the copy semantics of auto_ptr:
> 
> 
>     extern X* get_X();   // returns a resource acquired
>                          // by the callee, to be deleted
>                          // by the caller.
> 
>     void f() {
>         auto_ptr<X> ptr = get_X();
>                         // resource allocated by get_X()
>         ...
>    }
> 
> And get_X() is not unnecessarily complicated either:
> 
>    X* get_X()
>    {
>       auto_ptr<X> p = new X;
> 
>       // ... stuff that could throw an exception
> 
>       // We got here. Means normal return.
>       return p.release();
>    }
> 
> What's wrong with this? It doesn't require copy semantics
> for auto_ptr. Yet both the caller and the callee
> are exception safe and there is no loss of clarity.

The problem is that it requires handling the raw pointer.
Any time you transfer ownership by using release() to extract
the raw pointer and then later use the raw pointer to
construct another auto_ptr, there is a window where there is
no exception safety.

Of course you can carefully craft the code to make sure that
no exceptions can be propagated during the window.  But there
are two problems:

  (1) The maintainers of the code may not be as careful about
      exception safety.  When everything is handled by
      auto_ptr the risk of bugs creeping in is smaller.  Such
      bugs are very difficult to find by testing.

  (2) The interface of get_X does not implicitly document
      that the returned pointer refers to an object which
      should be automatically deleted on an exception.

-- 
Bill Gibbons
bill@gibbons.org
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