D vs C++

[Don]

Mariusz Gliwiński wrote:
> Monday 27 December 2010 @ 17:18:17 Daniel Gibson:
>> Am 27.12.2010 17:01, schrieb Caligo:
>>> On Mon, Dec 27, 2010 at 7:20 AM, Jonathan M Davis <jmdavisProg at gmx.com
>>>     <mailto:newshound2 at digitalmars.com>> wrote:
>>>      > > 11. generative programming
>>>      > 
>>>      > Does someone have a pointer to any kind of doc about this? (in D)
>>>     
>>>     Anything on templates, template mixins, and string mixins. All of
>>>     them generate
>>>     code. And some people have done some pretty crazy stuff with them
>>>     (especially
>>>     string mixins).
>>>     
>>>     - Jonathan M Davis
>>>
>>> So is it like template metaprogramming in C++?  a small D example would
>>> be helpful.  There doesn't seem to be anything about it in TDPL.
>> http://www.digitalmars.com/d/2.0/templates-revisited.html
> 
> Firstly,
> I admit I'm still new in programming so treat me like that but...
> On my peasant-like brain, if You can't store compilation-time variable, to 
> read it later... from other template, even module with normal language rules 
> but in compile time, it's not *fully* generative programming, is it? You can't 
> make many things without that.

You can store all compile-time results in local variables. (When a 
couple of implementation bugs get fixed, you'll be able to store it in 
heap-allocated variables as well).
So yes, with C++ style template metaprogramming, there's not so much you 
can do. D CTFE metaprogramming is far more powerful, and it's also 
simple to understand.

> As I said, I'm new in programming so maybe that's why, but D was my ideal 
> language (so i could express everything i imagined). But this little thing 
> makes templates only small spice to what I've seen before, instead of big step 
> forward. I understand it might be hard to implement with clear rules of usage, 
> but I abstracted it out.
> 
> Ps. I want compile-time raytracer downloadable again, please :)

BTW -- I don't recommend doing anything complicated with template 
metaprogramming. It becomes incomprehensible very quickly. CTFE, on the 
other hand, scales very nicely.