Re: A very stupid bug...

From:

cpp4ever <n2xssvv.g02gfr12930@ntlworld.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 05 May 2010 03:54:10 +0100

Message-ID:

<n55En.13$U65.11@newsfe23.ams2>

On 05/05/2010 12:43 AM, Alf P. Steinbach wrote:

On 05.05.2010 00:59, * cpp4ever:

On 05/03/2010 03:50 PM, Alf P. Steinbach wrote:

At one point I became convinced that the compiler generated incorrect
destruction code for multiple inheritance.

Oh well.

I finally found it:

   virtual ~RawPtrHolder()
   {
       std::auto_ptr< Type>( myPtr ); // Destroy via std::auto_ptr
for
access.
   }

Uh oh.

Can you see what I did wrogn?

Tried to free invalid memory because myPtr had not been
reset/initialised to NULL? That would be nasty.

Using std::auto_ptr in a constructor to simplify the throwing
of exceptions, maybe, but I'm not sure I'd ever use std::auto_ptr
like that.

Eric J. Holtman had it mostly right (else-thread).

The problem is that the statement is a declaration, of a variable
'myPtr', instead of constructing a temporary std::auto_ptr instance;
it's known as "the most vexed parse of C++", that anything that
syntactically can be treated as a declaration is treated as a
declaration...

My fix was to write

std::auto_ptr<Type>( myPtr+0 )

:-)

Cheers,

- Alf

DOH!!! Of course that's the problem. Serves you right for trying to be
too clever by half. I'd have probably declared a temporary variable for
the std::auto_ptr, if only because it's easier to read. Doesn't A.L.F
stand for alien life form?

JB