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

[Campbell Barton]

@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