[petsc-dev] Preprocessor hell: #define VecType

Jed Brown jedbrown at mcs.anl.gov
Sun Sep 30 09:40:03 CDT 2012

High five, Karl.
On Sep 30, 2012 6:46 AM, "Karl Rupp" <rupp at mcs.anl.gov> wrote:

> Hi,
> >>> ^^^ Note that this const is superfluous.
>> Why is it superfluous?  Isn't the second argument type const char* const
>>> this way?
>>> It's superfluous for the same reason we don't "set" by passing "const
>>> PetscInt". The const is irrelevant to the caller. All it means is that the
>>> implementation doesn't change the *its* copy (pass by value) and even that
>>> isn't type checked with respect to the public declaration. It's just
>>> clutter and suggests that the person who wrote it doesn't understand types.
>>    Which clearly I don't :-(
>>     So do we just go with typedef const char* VecType   and then all
>> signatures are  VecType?
> yes, Jed is absolutely right on that. The important thing to keep in mind
> here is that typedef is atomic, i.e. it is not a 'string replacement' as
> opposed to what the C preprocessor does.
> Example:
>   #define VecType char*
>   int Set(const VecType)
> expands to
>   int Set(const char*)
> However, with
>   typedef const char* VecType
>   int Set(const VecType)
> there is no immediate string replacement, i.e. it is NOT the same as
>   int Set(const const char*)
> with the compiler ignoring one of the const keywords. Instead, denoting
> the precedence using brackets, the type passed to the Set() is
>   const {const char*}
> which is a const copy to a const char*. Now, as 'const char*' is copied
> when passed to the function anyways, the first 'const' is superfluous as
> Jed pointed out. As a consequence, we can simply use
>   Set(VecType);
>   Get(VecType*);
> which is pretty much the standard prototype for any pair of Getter/Setter
> and also the way arguments of type PetscInt are handled.
> In conclusion, we have not only eliminated the preprocessor magic, we are
> also able to provide a cleaned-up interface for XYZSetType() and
> XYZGetType(). Thanks, Barry, for making the decision in favor of a typedef
> :-)
> Best regards,
> Karli
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