[Bf-committers] Using 'const' on primitive function arguments passed by value (Please don't do this)

[Sergey Sharybin]

In areas where you're maintainer you could stick to rules you want
(only generally defined rules are [1]). In other areas you'll just stick to
style used in that areas. If there're already mixed style in some files,
stick to style f functions you're working and talk to module maintainer to
update stylistic there. Can not see the issue here.

[1] http://wiki.blender.org/index.php/Dev:Doc/CodeStyle

On Tue, Oct 16, 2012 at 10:03 PM, Jason Wilkins
<jason.a.wilkins at gmail.com>wrote:

> My main problem was the inconsistency and the fact that my intuition
> led me astray because what I was seeing was non-idiomatic.  If the
> inconsistencies could be ironed out then highly verbose declarations
> could/should probably be tolerated (I guess *grudge*).  I realize
> these kinds of inscrutable declarations actually happen all the time
> in library code now that I think about it more.  Documentation may be
> the only place I can realistically expect somebody to keep things
> simple.  Unfortunately, the only documentation most of this stuff has
> is the code itself (hopefully automatic documentation generation could
> scrape all the extra crap off, and consider it for "the compiler's
> eyes only").
>
> Why does C still require separate header files?... It isn't 1973 anymore
> ;-)
>
> On Sun, Oct 14, 2012 at 9:34 PM, Campbell Barton <ideasman42 at gmail.com>
> wrote:
> > @patrick, this isn't really a good option IMHO,
> > the reason this is useful is that you can be sure the value wont
> > change under normal conditions when assignment isnt so obvious (in a
> > macro for eg), which you wont get with a naming convention,
> > Possibly if we had all developers full-time and have them agree on
> > conventions - but with merging GSOC branches and having patches
> > accepted from less involved developers, its hard to make sure they
> > follow loose conventions like this.
> > If someone sets a 'int c_blah' in a function, it could slip through in
> > a merge and its likely nobody would notice.
> >
> > I don't really see the big problem with having const in function
> declarations,
> > if you start to get picky with this stuff in C - there are many other
> > things you could complain about that are very cumbersome. like having
> > attributes for nonull or unused result.
> >
> > eg.
> >
> > void *MEM_callocN(size_t len, const char *str)
> > #if MEM_GNU_ATTRIBUTES
> > __attribute__((warn_unused_result))
> > __attribute__((nonnull(2)))
> > __attribute__((alloc_size(1)))
> > #endif
> >
> > having compilers (gcc and clang in this case) know this info is very
> > useful and can help track real errors in code, but its not pretty.
> > in these cases I'm happy to choose verbose&safe declarations over brief
> ones.
> >
> > On Mon, Oct 15, 2012 at 11:45 AM, patrick boelens <p_boelens at msn.com>
> wrote:
> >>
> >> It's been a while since I've read the initial mails on this, but if I
> remember correctly the main reason for using foo(const int bar) was as a
> reminder that bar shouldn't change. In that case, maybe a nice middle
> ground would be to use a var naming convention instead (i.e.: foo(int
> c_bar) or foo(int const_bar))? That way we can forego the discussion on
> whether or not such uses of const are warranted, warnings for inconsistent
> use would be prevented, and the indiciation is still there. I have no idea
> if this is considered ugly(er), but just thought I'd throw it out there.
> >>
> >> -Patrick
> >>
> >>> Date: Sun, 14 Oct 2012 18:37:12 -0500
> >>> From: jason.a.wilkins at gmail.com
> >>> To: bf-committers at blender.org
> >>> Subject: Re: [Bf-committers] Using 'const' on primitive function
> arguments passed by value (Please don't do this)
> >>>
> >>> Well that's unfortunate because that would have been acceptable to me.
> >>>
> >>> On Wed, Oct 10, 2012 at 11:03 PM, Campbell Barton <
> ideasman42 at gmail.com> wrote:
> >>> > Checked, and that would work in MSVC since its the same warning that
> >>> > checks a function matches its declaration in the header,
> >>> > so turning that off isn't an option (was thinking it may have been a
> >>> > different warning ID).
> >>> >
> >>> >
> >>> > On Thu, Oct 11, 2012 at 12:45 AM, Campbell Barton <
> ideasman42 at gmail.com> wrote:
> >>> >> @Jason, wouldn't it work to remove offending const from headers and
> >>> >> disable the warning in MSVC?
> >>> >>
> >>> >> On Wed, Oct 10, 2012 at 9:57 PM, Jason Wilkins
> >>> >> <jason.a.wilkins at gmail.com> wrote:
> >>> >>> C is ALWAYS call by value and C++ is call by value by default
> there is
> >>> >>> no need to mark the parameters const to know it will not change the
> >>> >>> caller's value in that case.  The const on a call by value function
> >>> >>> parameter is an internal detail of the function itself.  The
> function
> >>> >>> is saying that it won't change its copy.  To the caller this is
> like
> >>> >>> function saying what kind of magazines are hidden under its
> mattress.
> >>> >>>
> >>> >>> I do agree that C# is nice in that you have to mark a parameter as
> >>> >>> 'ref' both in the interface and at each call site (same with
> having to
> >>> >>> declare a shadowing variable as 'shadow').  However imagine if you
> had
> >>> >>> to prefix 'val' every time you called the function even though
> that is
> >>> >>> the default.  That would be very ugly and unneeded.  That sort of
> >>> >>> thing would be similar to what I'm arguing against here.
> >>> >>>
> >>> >>> I've made my case though, so if people disagree I'll just have to
> live
> >>> >>> with their code.  I won't belabor the point any more.
> >>> >>>
> >>> >>> On Tue, Oct 9, 2012 at 3:02 PM, Dahlia Trimble <
> dahliatrimble at gmail.com> wrote:
> >>> >>>> I don't poke around much in blender source but I've found in c++
> in general
> >>> >>>> that I kind of like being able to declare a function parameter as
> const,
> >>> >>>> for the simple reason that I don't see well and it helps me easily
> >>> >>>> differentiate foo(const int bar) from foo(int& bar). Where in the
> latter I
> >>> >>>> might not notice it's a reference.
> >>> >>>>
> >>> >>>> I like how C# handles references: if you declare a function you
> need to use
> >>> >>>> the ref keyword both in the declaration and when calling the
> function. This
> >>> >>>> way you cannot call a function that may modify a passed parameter
> without
> >>> >>>> explicitly stating that you are aware it's passed by reference. I
> think c++
> >>> >>>> could use something similar.
> >>> >>>>
> >>> >>>>
> >>> >>>> On Sun, Oct 7, 2012 at 10:17 PM, Jason Wilkins <
> jason.a.wilkins at gmail.com>wrote:
> >>> >>>>
> >>> >>>>> OK, when I get warnings I guess I will fix them by adding the
> const in
> >>> >>>>> the appropriate spot.  At least this kind of const is not viral
> :)
> >>> >>>>>
> >>> >>>>> On Sun, Oct 7, 2012 at 9:55 AM, Campbell Barton <
> ideasman42 at gmail.com>
> >>> >>>>> wrote:
> >>> >>>>> > On Mon, Oct 8, 2012 at 1:26 AM, Jason Wilkins <
> jason.a.wilkins at gmail.com>
> >>> >>>>> wrote:
> >>> >>>>> >> I probably would not notice except that it is done
> inconsistently and
> >>> >>>>> >> I get lots of warnings.  I tend to fix things by removing the
> const
> >>> >>>>> >> though.
> >>> >>>>> >>
> >>> >>>>> >> Another thing I noticed was returning a const pointer from a
> function
> >>> >>>>> >> but then expecting to free it using 'free'.  Dynamically
> allocated
> >>> >>>>> >> memory is not 'const' for the purposes of 'free'.  It would
> probably
> >>> >>>>> >> be better to cast away the const inside a special function
> instead of
> >>> >>>>> >> asking the user to use a raw 'free'.  (or just not use const).
> >>> >>>>> > agree, const can be a bit of a pain like this.
> >>> >>>>> >
> >>> >>>>> >> I'm all for using const, but I really question if making
> primitive
> >>> >>>>> >> arguments const is more trouble than it is worth. I mean, to
> follow
> >>> >>>>> >> through on it would be a huge task.  It only generates a
> warning (no
> >>> >>>>> >> error) when done inconsistently and if you change your mind
> now you
> >>> >>>>> >> have to make a change in two places instead of one.
> >>> >>>>> >
> >>> >>>>> > You dont need to follow it though, if a dev wants to use it,
> they can,
> >>> >>>>> > when done inconsistently it wont give warnings in GCC - but
> this stuff
> >>> >>>>> > is really issue with multi-platform dev, it happens in other
> areas too
> >>> >>>>> > - I often wake up to find blender wont compile because of an
> error in
> >>> >>>>> > some commit from a dev with a different environment (-Werror
> helps
> >>> >>>>> > here too :) ).
> >>> >>>>> > you can correct warnings, or mail some other dev to fix, last I
> >>> >>>>> > compiled on MSVC I didnt see any warnings like this though (a
> few
> >>> >>>>> > weeks back).
> >>> >>>>> >
> >>> >>>>> >> To me this is a C programmers version of those people who
> have to turn
> >>> >>>>> >> a light switch on and off a prime number of times.
> >>> >>>>> >>
> >>> >>>>> >> Reason #5 would be that it is just cluttered and ugly.  It
> decreases
> >>> >>>>> >> readability instead of enhancing it.  It reminds me of when I
> had a
> >>> >>>>> >> phase where I wanted to add 'struct' to everything so that
> people knew
> >>> >>>>> >> that, yes, this is a struct.
> >>> >>>>> >
> >>> >>>>> > think this comes down to personal preference, if you think its
> not
> >>> >>>>> > warranted, don't add it to your code.
> >>> >>>>> >
> >>> >>>>> >> I guess detecting stack corruption does not seem like a plus
> to me
> >>> >>>>> >> because my environment does this very aggressively without
> help
> >>> >>>>> >> (MSVC).
> >>> >>>>> >
> >>> >>>>> > Its no protection against stack corruption,
> >>> >>>>> > It just means you know the var wont change under normal
> conditions, if
> >>> >>>>> > it does change that something exceptional/wrong is happening.
> >>> >>>>> >
> >>> >>>>> >> On Sun, Oct 7, 2012 at 8:22 AM, Campbell Barton <
> ideasman42 at gmail.com>
> >>> >>>>> wrote:
> >>> >>>>> >>> On Sun, Oct 7, 2012 at 8:25 PM, Jason Wilkins <
> >>> >>>>> jason.a.wilkins at gmail.com> wrote:
> >>> >>>>> >>>> If I had a function with the prototype: foo(int bar)
> >>> >>>>> >>>>
> >>> >>>>> >>>> It may be tempting to declare the it as: foo(const int bar)
> >>> >>>>> >>>>
> >>> >>>>> >>>> The reason would be that bar is not modified inside of foo,
> so by
> >>> >>>>> >>>> declaring it this way we prevent ourselves from
> accidentally modifying
> >>> >>>>> >>>> it.
> >>> >>>>> >>>>
> >>> >>>>> >>>> This is not idiomatic C, and for good reasons.
> >>> >>>>> >>>>
> >>> >>>>> >>>> 1) We use 'const' on pointers to indicate that we are not
> going to
> >>> >>>>> >>>> modify what is pointed at, when a programmer sees 'const
> int' it is
> >>> >>>>> >>>> momentarily confusing because we expect 'const int*'
> >>> >>>>> >>>>
> >>> >>>>> >>>> 2) This exposes internal details of the function to the
> outside world.
> >>> >>>>> >>>>  The fact that 'bar' is const in this case is not actually
> a part of
> >>> >>>>> >>>> the interface of that function.
> >>> >>>>> >>>>
> >>> >>>>> >>>> 3) If we change our minds later and actually do want to
> modify the
> >>> >>>>> >>>> copy of 'bar' inside the function then we have to change
> the interface
> >>> >>>>> >>>> again, but as per #2 it actually has nothing to do with the
> user of
> >>> >>>>> >>>> 'foo'
> >>> >>>>> >>>>
> >>> >>>>> >>>> 4) It is just not idiomatic.  Looking at it is like
> listening to a
> >>> >>>>> >>>> foreigner speak your native language in "creative" ways.
> >>> >>>>> >>>>
> >>> >>>>> >>>> I have not figured out who is doing this, but please stop :)
> >>> >>>>> >>>
> >>> >>>>> >>> I've been doing this and Im not convinced its a bad thing,
> in some
> >>> >>>>> >>> functions its a good hint that a var is `fixed` and
> shouldn't be
> >>> >>>>> >>> changed.
> >>> >>>>> >>> If a dev wants to change it they can just remove the `const`
> but it
> >>> >>>>> >>> means they think twice before doing it (as in - maybe there
> is a good
> >>> >>>>> >>> reason it shouldn't be changed).
> >>> >>>>> >>>
> >>> >>>>> >>> The main reason I like to have this sometimes is when
> debugging you
> >>> >>>>> >>> know for sure a var wont change, if it does - its a buffer
> overflow or
> >>> >>>>> >>> something exceptional.
> >>> >>>>> >>> Often its not really an issue - but there are cases it can
> help verify
> >>> >>>>> >>> whats going on when reading the function.
> >>> >>>>> >>>
> >>> >>>>> >>> That the `const` gets in the header is a little
> inconvenience if it
> >>> >>>>> >>> changes often - but IMHO changing those is rare enough that
> its not an
> >>> >>>>> >>> issue.
> >>> >>>>> >>> _______________________________________________
> >>> >>>>> >>> Bf-committers mailing list
> >>> >>>>> >>> Bf-committers at blender.org
> >>> >>>>> >>> http://lists.blender.org/mailman/listinfo/bf-committers
> >>> >>>>> >> _______________________________________________
> >>> >>>>> >> Bf-committers mailing list
> >>> >>>>> >> Bf-committers at blender.org
> >>> >>>>> >> http://lists.blender.org/mailman/listinfo/bf-committers
> >>> >>>>> >
> >>> >>>>> >
> >>> >>>>> >
> >>> >>>>> > --
> >>> >>>>> > - Campbell
> >>> >>>>> > _______________________________________________
> >>> >>>>> > Bf-committers mailing list
> >>> >>>>> > Bf-committers at blender.org
> >>> >>>>> > http://lists.blender.org/mailman/listinfo/bf-committers
> >>> >>>>> _______________________________________________
> >>> >>>>> Bf-committers mailing list
> >>> >>>>> Bf-committers at blender.org
> >>> >>>>> http://lists.blender.org/mailman/listinfo/bf-committers
> >>> >>>>>
> >>> >>>> _______________________________________________
> >>> >>>> Bf-committers mailing list
> >>> >>>> Bf-committers at blender.org
> >>> >>>> http://lists.blender.org/mailman/listinfo/bf-committers
> >>> >>> _______________________________________________
> >>> >>> Bf-committers mailing list
> >>> >>> Bf-committers at blender.org
> >>> >>> http://lists.blender.org/mailman/listinfo/bf-committers
> >>> >>
> >>> >>
> >>> >>
> >>> >> --
> >>> >> - Campbell
> >>> >
> >>> >
> >>> >
> >>> > --
> >>> > - Campbell
> >>> > _______________________________________________
> >>> > Bf-committers mailing list
> >>> > Bf-committers at blender.org
> >>> > http://lists.blender.org/mailman/listinfo/bf-committers
> >>> _______________________________________________
> >>> Bf-committers mailing list
> >>> Bf-committers at blender.org
> >>> http://lists.blender.org/mailman/listinfo/bf-committers
> >>
> >> _______________________________________________
> >> Bf-committers mailing list
> >> Bf-committers at blender.org
> >> http://lists.blender.org/mailman/listinfo/bf-committers
> >
> >
> >
> > --
> > - Campbell
> > _______________________________________________
> > Bf-committers mailing list
> > Bf-committers at blender.org
> > http://lists.blender.org/mailman/listinfo/bf-committers
> _______________________________________________
> Bf-committers mailing list
> Bf-committers at blender.org
> http://lists.blender.org/mailman/listinfo/bf-committers
>



-- 
With best regards, Sergey Sharybin