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

Thu Oct 11 06:03:54 CEST 2012

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

-- 
- Campbell