discrimination of constructors with same number of parameters

[Guilherme Vieira]

On Thu, Dec 30, 2010 at 9:24 AM, bearophile <bearophileHUGS at lycos.com>wrote:

> Jonathan M Davis:
>
> > typedef is definitely on the way out, so that's not a solution,
>
> typedef is deprecated (because its semantics is not flexible enough and
> because it doesn't play well with object oriented language features), but I
> have a real need for something like it. Andrei has discussed about a
> Phobos-based typedef replacement (based on structs + alias this), but
> nothing concrete has come out yet. I hope to see something to solve problems
> like spir ones.
>
>
> > and it would be a pretty fragile one IMHO anyway.
>
> Please, explain better.
>
> Bye,
> bearophile
>

As far as I know, typedef was a form of "discriminated alias". I don't know
the reasons for its deprecation. It just occurred to me that D's typedefs +
templates could be quite handy in this case.

Consider:

struct semantic_wrapper(T)
{
    this(T value) { this.value = value; }

    T value;
}

typedef semantic_wrapper!(int) Position;
typedef semantic_wrapper!(size_t) Count;
typedef semantic_wrapper!(string) Filename;
typedef semantic_wrapper!(string) DirPath;

void func(Position pos) { ... }
void func(Count c) { ... }
void func(Filename fname) { ... }
void func(DirPath dir) { ... }

void main()
{
    func(Position(1)); // calls first overload
    func(Count(5)); // calls second
    func(Filename("file.txt")); // third
    func(DirPath("/dev/null")); // fourth

    func(1); // fails
    func("blah"); // fails
}


Requires a little more typing, but sometimes it can be better than creating
a new function name (which can get extra-big, non-telling or both) or than
creating factory methods (which I personally dislike, although it's just a
matter of taste most of the time; sometimes you may want to instantiate from
inside a template and classes needing factories would not work, for example,
but one could argue on the validity of this anytime).

Just giving my 2 cents. Dunno if I missed some detail.

-- 
Atenciosamente / Sincerely,
Guilherme ("n2liquid") Vieira
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