Fact: Templates are static!
But templates are still normal functioning code and can actually
work dynamically. We can see this by including a D compiler
inside an app and have the app re-compile the templates for
specific dynamic types. It doesn't need templates to do of
course, but it shows that "Templates are static" is not quite as
simple as it sounds.
But could D actually create some sort of internal expression of D
templates as dynamic objects that can be used in run-time?
If have a template T that only uses 3 types, then one can simple
do something like(pseudo):
t(object o)
{
// use dynamic reflection or whatever to get the type of the
object o
if (o.type == 'int')
return T!(int)(o.value, o.unit);
if (o.type == 'float')
return T!(float)(o.value, o.unit);
return T!(unknown)(o.value, o.unit);
}
etc...
t then is a "dynamic representation" of the static functionality
of T.
But my guess is that the compiler could essentially do this for
us and allow one to use templates in a dynamic way. Most of the
time it may not be of much use but, say, if one is interacting
with console and T converts a string to a number. T could convert
a number with unit such as "3mm" in to some base representation
such as 0.003(m). float's are handled differently than
ints(rounding depending on some criteria).
Maybe we use the T internally and its lightning fast. But for the
console case, we have write a dynamic version of the template or
create something similar to the code above.
But the compiler already knows, at least, some of the times that
will be used and it should be able to instantiate the template
for each of those types.
e.g., if you use T!(float) in the code, it will know to create
the "dynamic wrapper" to include that type in the list. If you
don't every use T!(float) explicitly then you can simply create a
useless statement like {auto x = T!(float)("3.04mm");} which will
be optimized but the compiler will then include float as a type
used in the dynamic version of T. I may not be making much sense
here, but it's the best I'll do.
The compiler then can do something like assemble the dynamic
version of the template into a new function which can be used at
compile time on any object.
Using a simple special character could inform the compiler to use
the dynamic template version.
object u = userInput();
&T(u); // (instead of using t(u)).
Here the compiler does all the work for us. If u has type float
then this gets effectively dispatched to T!(float)(cast(float)u).
If it's an int, then T!(int)(cast(int)u).
What it mainly does for us is simplify having to do the cast our
self along with the string of if checking for all the types or
having to code that works dynamically
Or is it impossible for the D compiler to accomplish such a task
automatically for us?
