Re: A scoped_ptr class that does not allow NULL ?

=?ISO-8859-1?Q?=D6=F6_Tiib?= <>
Tue, 1 Jun 2010 08:03:11 -0700 (PDT)
On Jun 1, 4:49 pm, Marcel M=FCller <> wrote:

=D6=F6 Tiib wrote:

On Jun 1, 2:58 pm, Marcel M=FCller <> wrote=


However, I still do not catch what you want to document with the
non-NULL pointer.

But ... his templates name documents that class has a smartpointer
member that can not be NULL because that name tells it in English?

Well this was not my question. I meant with respect to his program. Why
he wants to emphasize the not NULL property?

If you get the pointer from some legacy factory function that may
return NULL that you may forget to check. However such factories
usually have deleters too and you can not give deleter to scoped_ptr,
so i am also somewhat puzzled.

Whoever is familiar with the C++ language will see that any dereferenced
pointer must not be NULL.

Unfortunately surprizingly few people use such defensive style that
first half or more of each function is error checking and asserting.

I personally dislike too much goodies like NeverNullScopedPtr because
they make third party code more difficult to service. You never know how
the class NeverNullScopedPtr exactly behaves unless you have analyzed
it. Furthermore the runtime exceptions are not very helpfull since they
require a very good code coverage during testing.

I would just check for NULL in constructor anyway. Trouble for me is
that i must have member initializer for such member and so it is again
that geeky try-catch-constructor to catch it ASAP and rethrow
something that makes sense.

 Bar::Bar( ParsePosition dirtyData )
    : noNullThing_( thingFactory( dirtyData ) )
 catch ( NullPointerError const& )
    throw ParseError( "Thing factory failed to parse Thing of Bar.",
dirtyData );

I really dislike it if to compare with:

 Bar::Bar( ParsePosition dirtyData )
    : usualScopedThing_( thingFactory( dirtyData ) )
    if ( !usualScopedThing_ )
       throw ParseError( "Thing factory failed to parse Thing of
Bar.", dirtyData );

So I would assent to Markus's hint to use references. This has the
additional advantage that if polymorphism is used the compiler will drop
the unnecessary NULL check when transforming pointers in up or down casts=


References are no silver bullet since most compilers let you create
invalid references without problems. In our situation:

 Bar::Bar( ParsePosition dirtyData )
    : refToThing_( *thingFactory( dirtyData ) )
    if ( !&refToThing_ ) // XXX: UB
       throw ParseError( "Thing factory failed to parse Thing of
Bar.", dirtyData );

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