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

[Dwight Hutto]

On Tue, Oct 9, 2012 at 4:02 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.
>> >>> --
Best Regards,
David Hutto
CEO: http://www.hitwebdevelopment.com