const?? When and why? This is ugly!
Jason House
jason.james.house at gmail.com
Sat Mar 7 18:58:35 PST 2009
Andrei Alexandrescu Wrote:
> Sergey Gromov wrote:
> > Sat, 07 Mar 2009 15:19:50 -0800, Andrei Alexandrescu wrote:
> >
> >> To recap, if an API takes a string and all you have a char[], DO NOT
> >> CAST IT. Call .idup - better safe than sorry. The API may evolve and
> >> store a reference for later. Case in point: the up-and-coming
> >> std.stdio.File constructor initially was:
> >>
> >> this(in char[] filename);
> >>
> >> Later on I decided to save the filename for error message reporting and
> >> the such. Now I had two choices:
> >>
> >> (1) Leave the signature unchanged and issue an idup:
> >>
> >> this.filename = to!string(filename); // issues an idup
> >>
> >> (2) Change the signature to
> >>
> >> this(string filename);
> >>
> >> Now all client code that DID pass a string in the first place (the vast
> >> majority) was safe _and_ efficient. The minority of client code was that
> >> that had a char[] or a const(char)[] at hand. That code did not compile,
> >> so it had to insert a to!string on the caller side.
> >>
> >> As has been copiously shown in other languages, the need for
> >> character-level mutable string is rather rare. So most of the time you
> >> will not traffic in char[], but instead you'll have a immutable(char)[]
> >> to start with. This further erodes the legitimacy of your concern.
> >
> > My file names are constructed most of the time. And most of the time
> > they are simple char[]s.
>
> Ehm. Mine are also constructed, but somehow come in string format, e.g.:
>
> string basename;
> ...
> auto f = File(basename ~ ".txt");
>
> > It is not obvious that File should store the file name. It's not
> > strictly necessary. It's an *implementation detail.* Now you expose
> > this implementation detail through the class interface, and you do this
> > without any good reason. You save a 150 byte allocation per file.
> > Nice.
>
> It's just an example, the point being that there things are always fast
> and safe. In many cases there's much more at stake and you can't rely on
> idioms that allocate memory needlessly.
Your example above does allocate memory. A mutable string could potentially avoid allocating to append ".txt"
> > I can understand when a hash takes an immutable key. It's in the hash's
> > contract. Various lazy functions could take immutable input to
> > guarantee correct lazy execution. But I think that overall use of
> > immutable types should be rare and thoroughly thought-out. They should
> > be used only when it's absolutely, provably necessary. That's why I
> > think aliasing string as immutable is a mistake. It felt wrong when I
> > discovered D a year ago, and it feels wrong now.
>
> That may be because you are writing C in D. Immutable strings should
> allow solid coding without much friction.
>
>
> Andrei
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