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

Campbell Barton ideasman42 at gmail.com
Sun Oct 7 16:55:20 CEST 2012

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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