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

[Matthew Knepley]

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
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