are mixin string templates with functions removed?

[H. S. Teoh]

On Thu, Aug 01, 2013 at 07:52:28PM +0200, JS wrote:
> On Thursday, 1 August 2013 at 17:47:00 UTC, H. S. Teoh wrote:
> >On Thu, Aug 01, 2013 at 07:12:51PM +0200, John Colvin wrote:
> >>On Thursday, 1 August 2013 at 17:09:07 UTC, JS wrote:
> >>>If I have a bunch of templates that are only used for code
> >>>generation, are they removed in the binary(since they are not
> >>>needed)?
> >>
> >>templates don't exist in binaries.
> >
> >Templates are like cookie molds, you use them to generate lots of
> >(almost) identical cookies, but you never serve the mold to the
> >customer. ;-)
> >
> >
> >T
> 
> But what about the functions that exist in them?

Like I said, cookie molds. You use the mold to press cookies, but only
the cookies are served, not the mold. The mold may be very complicated,
containing subcookies attached to bigger cookies, but whatever is
pressed (i.e., instantiated) is what's served on the dinner plate. The
mold always remains in the kitchen.


> template A()
> {
>     void A()
>     {
>         B();
>     }
>     void B() { }
> }
> 
> is everything in the template removed 100% or is there junk that the
> compiler doesn't remove?

There is nothing to remove. If you instantiated the template, then you
get a copy of everything in it. The number of copies equals the number
of distinct instantiations. The template itself is just a mold, an
abstract entity that doesn't exist in binary form. What it does is to
serve as a mold (hence, "template") to make code. So if you use to make
10 copies of the code, that's what you'll get in your executable. If
your template contains 5 functions, then each instantiation produces 5
copies of those functions. Simple as that.

Of course, not all code produces binary data -- enum and alias
definitions don't produce any binary code, for example -- they're just
logical entities that only exist at compile time. So if you have an enum
inside a template, it will get copied however many times you instantiate
the template, but none of those copies end up in the executable because
they're just declarations, not actual code or data.


> Oh... and I'm not talking about theoretical... I'm talking about
> what dmd actually does.

See for yourself: run a disassembler on the result (preferably filtered
through ddemangle so the symbols are actually readable) and see what's
included and what's not. On Posix, you can use objdump or nm (if you use
nm, though, keep in mind that not all symbols correspond with actual
code/data, but they do take up a little space in the executable -- at
least enough to store their names).


T

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