On Thu, 27 Sep 2012 11:46:05 -0400, Namespace <rswhi...@googlemail.com>
wrote:
On Thursday, 27 September 2012 at 15:39:39 UTC, Steven Schveighoffer
wrote:
On Thu, 27 Sep 2012 11:34:15 -0400, Namespace <rswhi...@googlemail.com>
wrote:
Ok. And there is no performance difference? Just for the sake of
completeness. :)
performance where? Template instantiation is done at compile time.
The resulting code should be unaffected runtime-performance wise.
-Steve
I mean: is there any difference by building the template? I don't
understand what "more flexible" exactly mean.
The constraint basically is saying whether to instantiate the template or
not, it has little to do with runtime performance.
Flexible means you have a full boolean logic you can use.
For example, if you had two unrelated objects A and B, how would you
instantiate a template if the parameter derives from A *or* derives from B?
With a template constraint, that's:
template tmp(T) if(is(T : A) || is(T : B))
This is impossible to do with a specialization.
However, there are some limitations. Template constraints don't cascade,
while specializations do.
So while this:
template tmp(T) ...
template tmp(T : A)
works fine, this:
template tmp(T) ...
template tmp(T) if (is(T : A)) ...
does not, because it can instantiate both with an A. Instead you have to
do:
template tmp(T) if(!is(T : A)) ...
template tmp(T) if(is(T : A)) ...
It has been proposed to try and use else (if), but Walter didn't like it.
-Steve
