[boost] Re: Formal Review: Circular Buffer

From: Robert Bell (belvis_at_[hidden])
Date: 2004-03-08 22:32:53

Pavel Vozenilek wrote:
> "David Abrahams" <dave_at_[hidden]> wrote
> 
> 
>>I can't understand this claim.  If the counter works properly, then it
>>will stay valid.
>>
> 
> This example was too simplified (and thus not to the point).
> 
> More complex one:
> 
> -----------------------
> struct  my_class
> {
>   void* database_handle;
> 
>   my_class() { database_handle = allocate_handle(); }
>   ~my_class() { destroy_db_handle(database_handle); }
>   my_class& operator=(const my_class& other) {
>       // no copy of database handle, not needed,
>       // not useful here
>   }
>   my_class(const my_class& other) {
>      database_handle = allocate_handle(); // needs unique db connection
>   }
> };
> 
> circular_buffer<my_class> buffer(1);
> 
> {
>   my_class a;
>   buffer.push_back(a);  // 2 handles after function returns
>   ...
> }
> 
> {
>    my_class b;
>    buffer.push_back(b);  // desctructor not called, handle lost
>    ...
> }

I don't see why a handle is lost.

{
// Only one handle in existence at this point, the one
// previously added to buffer by "buffer.push_back(a);".

    my_class b;

// Now two handles exist, one in b, and one in buffer.

    buffer.push_back(b);

// Two handles still exist -- push_back uses assignment, which
// doesn't affect handles at all. No new handles have been
// created by push_back(b).

}

// Now only one handle exists, in buffer, since b has
// gone out of scope and has been destroyed.

What am I missing?

Bob