Re: Null Pointer Considerations

"Matthias Hofmann" <>
5 Aug 2006 15:00:09 -0400
"Jesper Schmidt" <> schrieb im Newsbeitrag

Thanks for making my point much clearer. Now that you understand my
problem, I am curious to know whether you have experience the *problem*
yourself. Typical places to look for are heap managers, serialization
libraries, communication protocols and so on with a decomposed design.
Note that the problem is often easy to avoid in a monolithic design
because of the singular design context. That why the first example
looked so silly.

I cannot remember having any problem related directly to the null pointer
value, but it is common knowledge that defensive programming (i.e. doing a
lot of error checking) can slow down your program. In your specific example,
the check for nullness is done repeatedly because the function containing it
is called repeatedly. Therefore, I would simple move the check to the
outside of it:

int* upcast(int* ptr)
{ return ptr - 1; }

int* downcast(int* ptr)
{ return ptr + 1; }

int* f(int* p)
   if ( p != 0 )
     return upcast(downcast(upcast(downcast(p))));
   return 0;

Actually, this is just what you are asking the compiler to do, the
difference being that your compiler now can be sure that the unnecessary
checks can be omitted - because *you* omitted them. Whether this is a
practicle solution depends on whether there is a danger of client code
calling upcast() or downcast() on a null pointer. It works best when these
two functions are private inside a class.

As we have learned in this discussion, the null pointer value is not a
pointer to memory address zero (despite me saying something different in a
previous post, but apparently I was wrong). For that reason, I don't think
that the introduction of a keyword like 'nullptr' will allow any better
optimization than the null pointer, as it is semantically identical (except
that the former has its own type, which comes in handy for overload

Matthias Hofmann
Anvil-Soft, CEO - The Creators of Toilet Tycoon - Die Macher des Klomanagers

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