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

[Dahlia Trimble]

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.
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> >
> >
> > --
> > - Campbell
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