Metaprogramming in D : Some Real-world Examples

[Bill Baxter]

On Thu, Nov 12, 2009 at 10:46 AM, Tomas Lindquist Olsen
<tomas.l.olsen at gmail.com> wrote:
> On Tue, Nov 10, 2009 at 1:27 AM, Bill Baxter <wbaxter at gmail.com> wrote:
>> On Mon, Nov 9, 2009 at 4:09 PM, Walter Bright
>> <newshound1 at digitalmars.com> wrote:
>>> Looks like Bill Baxter is giving a presentation on D Nov. 18!
>>>
>>> http://www.nwcpp.org/
>>
>> Yep, that's right, and I'd be quite grateful to you smart folks here
>> if you could share your meta-programming favorites with me!   If
>> you've got a real-world example of meta-programming in D that you
>> think is particularly handy, then please send it my way
>>
>> I'm looking for small-but-useful things that are easy to explain, and
>> make something easier than it would be otherwise.  Things like places
>> where static if can save your butt,  or loop unrolling,  and passing
>> code snippets to functions like in std.algorithm.
>>
>> Things like a compile-time raytracer or regexp parser (though quite
>> cool!) are not what I'm after.  Too involved for a short talk.
>>
>> --bb
>>
>
> I think tuples are a good example of something that makes your life
> easier, lately I've been very fond of this little snippet:
>
>>>>>>>>>>>>>>>>>>>>>>>
>
> void delegate() Bind(Args...)(void delegate(Args) dg, Args args)
> {
>    struct Closure
>    {
>        Args arguments;
>        void delegate(Args) callee;
>        void call()
>        {
>            callee(arguments);
>        }
>    }
>
>    auto c = new Closure;
>
>    // foreach not strictly necessary, but ldc currently chokes on
> just an assignment... I should fix that..
>    foreach(i,a;args)
>        c.arguments[i] = a;
>    c.callee = dg;
>
>    return &c.call;
> }
>
> class C
> {
>    void foo(int,float) {}
> }
>
> void main()
> {
>    auto c = new C;
>    auto dg = Bind(&c.foo, 1, 2.0f);
>    // register delegate somewhere
> }
>
> <<<<<<<<<<<<<<<<<<<<<<<<
>
> Not sure if this gets easier in C++0x , haven't read up on that...

I think it does.  C++0x has variadic templates.  And some kind of
lambdas/closure thing.  So probably it can do something similar.
Anyway, the meta- aspect of Bind seems kinda weak.  Certainly a nice
use of variadic templates and closures, though.

--bb