Minor std.stdio.File.ByLine rant

[Steven Schveighoffer]

On Thu, 27 Feb 2014 12:32:44 -0500, H. S. Teoh <hsteoh at quickfur.ath.cx>  
wrote:

> Actually, now that I think about it, can't we just make ByLine lazily
> constructed? It's already a wrapper around ByLineImpl anyway (since it's
> being refcounted), so why not just make the wrapper create ByLineImpl
> only when you actually attempt to use it? That would solve the problem:
> you can call ByLine but it won't block until ByLineImpl is actually
> created, which is the first time you call ByLine.empty.

I think this isn't any different than making ByLine.empty cache the first  
line.

My solution is basically this:

struct LazyConstructedRange(R)
{
   R r;
   bool isConstructed = false;
   R delegate() _ctor;

   this(R delegate() ctor) {_ctor = ctor;}

   ref R get() {
     if(!isConstructed) { r = _ctor(); isConstructed = true;}
     return r;
   }

   alias get this;
}

Basically, we're not constructing on first call to empty, but first call  
to *anything*. Actually, this kind of a solution would be better that what  
I came up with, because the object itself is a range instead of a delegate  
(satisfies, for instance, isInputRange and isIterable, whereas the  
delegate does not), and you don't need the static if like I wrote. Any  
additional usage of the delegate in my original solution creates a copy of  
the range, but the above would only construct it once.

-Steve