[petsc-dev] declaring argument as const basic type in PETSc

[Matthew Knepley]

On Wed, Aug 17, 2011 at 10:13 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Aug 17, 2011, at 3:08 PM, Matthew Knepley wrote:
>
> > On Wed, Aug 17, 2011 at 10:05 PM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> >
> >  Because it exposes a completely implementation detail to the public
> interface, (unnecessarily at that) So yes it is for philosophical reasons
> that we must reject this badly designed C standard.
> >
> > So you are saying that, according to the standard (Jed will certainly
> have a reference), that some value parameter
> > declared as const can be assigned to in the function? If that is so, it
> is full of crap. Every compiler I have used bombed
> > on that, and (lacking real authority) during my Google interview they
> thought this was good practice.
>
>    No I am not saying that. Not even close, how could you possibly have
> gotten that from the sentence "Because it exposes a completely
> implementation detail to the public interface, (unnecessarily at that) So
> yes it is for philosophical reasons that we must reject this badly designed
> C standard."


This way. You said it was a "completely implementation detail", from which I
supposed that different compilers were allowed
to interpret it as they see fit (thats what I thought an implementation
detail was). I asked for confirmation on this: "does the
standard allow a 'const' parameter to be assigned to?", since I know every
compiler I have tried does not allow this. I thought
your philosophical reason was that the standard allows a 'const' value
declaration, but enforces nothing. Is this wrong?

   MAtt


>
>   Barry
>
> >
> >    Matt
> >
> >
> >   Barry
> >
> >
> > On Aug 17, 2011, at 2:58 PM, Matthew Knepley wrote:
> >
> > > On Wed, Aug 17, 2011 at 9:08 PM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> > >
> > > On Aug 17, 2011, at 2:04 PM, Chetan Jhurani wrote:
> > >
> > > >> From: Jed Brown
> > > >> Sent: Wednesday, August 17, 2011 12:38 PM
> > > >> To: For users of the development version of PETSc
> > > >> Subject: Re: [petsc-dev] declaring argument as const basic type in
> PETSc
> > > >>
> > > >> On Wed, Aug 17, 2011 at 11:34, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> > > >>> But my question remains, should we const declare function
> arguments?
> > > >>
> > > >> No, the prototype should never use const for stack arguments
> (pointer to const
> > > >> is totally different and should be used anywhere that it is
> correct).
> > > >>
> > > >> I don't know any responsible libraries that const-ify value
> parameters in
> > > >> prototypes. I think it most commonly arises from ignorance of, or
> sloppiness
> > > >> with, the actual semantics of the C language.
> > > >
> > > >
> > >   So by the standard a compiler should never bitch if in the function
> definition it is declared const but in the extern prototype it is not
> declared const? (This is an exception to the usual rule that the extern
> arguments should match the definition arguments?)
> > >
> > > Just so I understand, this declaration and definition:
> > >
> > > void foo(const int a);
> > >
> > > void foo(const int a) {
> > > }
> > >
> > > is legal, and would prevent a developer from accidentally assigning to
> a, but we are outlawing it. I do not
> > > see the rationale.
> > >
> > >     Matt
> > >
> > >    Thanks
> > >
> > >  Barry
> > >
> > > > It is useful to have const pass-by-value parameters in function
> > > > definitions.  Not useful in the function declarations or for the
> > > > caller.
> > > >
> > > > This is so that a developer does not assign to a passed-in int
> > > > unintentionally.  Almost all C and C++ compilers accept it and they
> > > > error out if one does something like the code below.  An exception
> > > > is old IRIX compilers that emit a warning on using const
> pass-by-value
> > > > and said it is meaningless to use them.  I disagree.
> > > >
> > > > void f(const int n, int* j)
> > > > {
> > > >    *j = n;
> > > >    n = 0; // error
> > > > }
> > > >
> > > > Of course it is more useful for preventing bugs in more complex
> > > > functions.
> > > >
> > > > Chetan
> > > >
> > > >
> > >
> > >
> > >
> > >
> > > --
> > > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > > -- Norbert Wiener
> >
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > -- Norbert Wiener
>
>


-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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