How to do Generic Programming in D?

[Andrei Alexandrescu]

On 2/22/11 1:04 PM, Nick wrote:
> Coming from Andrei's work in C++ "Modern C++ Programming" I wonder how
> to implement many of those patterns in D?
>
> In C++ I would work with type lists and use lots of multiple inheritance
> and templates to get the magic I need.
>
> D lacks MI, classes and delegates seem heavy (allocated on heap, with
> one extra pointer and new for each delegate) and has instead template
> mixins which do not create types and string mixing which create...
> anything.
>
> So I am a bit lost.
>
> I guess I am looking for some code&examples to read to "get" GP in D. A
> bit like "Modern C++ Programming".
>
> Of course, the basic question being: is D2 sufficient to support
> everything that is possible in C++?
>
> Thanks!

Welcome to the group.

Using D for the kind of generic designs featured in MC++D may be a bit 
disconcerting because a lot of the C++ implementation is forced to use 
unintuitive mechanisms to get things done. The expectation is that the 
same mechanisms need to be used for the D implementation, too, which 
shouldn't be the case. In fact accomplishing any of MC++D's tasks is 
considerably easier in D.

For example, indeed in C++ a policy-based design would need to use 
multiple inheritance. But this is not motivated by a need for multiple 
subtyping - MI is just the best proxy C++ has for injecting 
parameterized implementation. In D for that goal you'd use mixin 
templates, or for ultimate flexibility string mixins. If you do need 
multiple subtyping, you'd use classes in conjunction with e.g. 
parameterized interfaces and/or "alias this".

Typelists are an elaborate C++ construct that is essentially built in 
into D.

For the Command pattern, delegates would be a great start. Added value 
for e.g. currying and binding is trivial to implement, to the extent 
that we decided to deprecate a dedicated module (std.bind).

There would be more to say. One way or another, policy-based design is 
difficult and often unintuitive. Therefore, it requires mastery of both 
the design topic at hand and of the implementation language. I think 
trying to get a solid PBD component off the ground as a D beginner would 
be pushing it. Nevertheless, it may be worth a try. I suggest starting 
with a simple, well-defined component such as Object Factory.


Andrei