On Fri, 04 Mar 2011 09:33:04 +0100, spir wrote:
> On 03/04/2011 09:15 AM, Lars T. Kyllingstad wrote:
>> On Fri, 04 Mar 2011 13:53:33 +1030, Graham St Jack wrote:
>>
>>> On 04/03/11 12:34, Bekenn wrote:
>>>> On 3/3/11 3:30 PM, Graham St Jack wrote:
>>>>> My first instinct would be to use non-templated functions that take
>>>>> const
>>>>> char[].
>>>>>
>>>>>
>>>> Please don't ever restrict encodings like that. As much as possible,
>>>> libraries should seek to be encoding agnostic (though I'm all for
>>>> const-qualifying parameters). This is one area where I feel the
>>>> standard library severely lacks at present.
>>>>
>>>> As a Windows developer, I prefer to use wchar strings by default and
>>>> use only the W versions of the Windows API functions, because the A
>>>> versions severely limit functionality. Only the W versions have full
>>>> support for Unicode; the A versions are entirely dependent on the
>>>> current (8-bit) code page. This means no support for UNC paths or
>>>> paths longer than 260 characters, and also means that international
>>>> characters commonly end up completely garbled. Good practice in
>>>> Windows is to consider the A versions deprecated and avoid them like
>>>> the plague.
>>>
>>> Ok, I don't mind supporting wchar and dchar in addition to char,
>>> especially if Windows insists on using them.
>>>
>>> My main issue here is with the constness of the parameters. I think
>>> the correct parameter to pass is const C[]. This has the advantages
>>> of: * Accepting both mutable and immutable data. * Declares that the
>>> function won't mutate the data. * Declares that the function doesn't
>>> expect the data to be immutable.
>>
>> The problem is that the functions return slices of their input
>> argument, which means that the constancy of the input argument gets
>> transferred to the return value. Here's an example to illustrate:
>>
>> C[] first(C)(const C[] s) { return s[0 .. 1]; }
>>
>> char[] a = "hello".dup;
>> auto b = first(a);
>>
>> Try to compile this, and you get the error message
>>
>> Error: cannot implicitly convert expression (s[0u..1u]) of type
>> const(char[]) to char[]
>
> IIUC, this means const should never be used on input parameters. Instead
> of meaning what the func will (not) do with its param(s), it imposes
> undue requirements on the outside world. Or do I miss something? From my
> point of view, qualifiers inside a function's interface should only
> describe the function behaviour.
It should not be used if the function's return value is an alias of an
input parameter. That's what inout is for. In all other cases, const is
fine.
http://www.digitalmars.com/d/2.0/function.html#inout-functions
-Lars
