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

patrick boelens p_boelens at msn.com
Mon Oct 15 02:45:03 CEST 2012 

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
> >>>>> >> _______________________________________________
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> >>>>> >> 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
> >>>>> _______________________________________________
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> >>>>> http://lists.blender.org/mailman/listinfo/bf-committers
> >>>>>
> >>>> _______________________________________________
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> >>
> >>
> >>
> >> --
> >> - Campbell
> >
> >
> >
> > --
> > - Campbell
> > _______________________________________________
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