cc6574bc
From: dave@boostpro.com
on Sun Nov 20 2011, pfultz2 wrote:
> Sometimes, it would be nice to have polymorphic lambdas in C++, not
> because I want use the lambdas polymorphic, but I just want the
> compiler to deduce the type for me.
OK so far. The examples, though, took me forever to begin to
understand. Maybe that's because of the naming...
> Well I thought maybe there could be a way for the compiler to do this
> if the function made known how the lambda should be called. Then the
> function could be called with a special marco like this:
>
> CALL(test, x) { printf("Number:%i\n";};
>
> which would result in this:
>
> test & [&] (decltype(test)::lambda_param<0>::type x) { printf("Number:
> %i\n";};
>
> This test class could be defined like this:
>
> struct test_t
> {
> template
> struct lambda_param;
>
> template
> struct lambda_param<0, Dummy>
> {
> typedef int type;
> };
> template
> void operator&(Yield yield)
> {
> yield(5);
> }
>
> } test;
> $(for_each(some_range), x)
> {
> printf("%i\n", x);
> };
> auto it = $$(find_if(some_range), x)(x < 3);
Now if I understand correctly, "test" at the beginning of your posting
is supposed to stand for some example "algorithm object" of the same ilk
as "for_each" in the snippet above. So you're suggesting encoding the
computation of the lambda's parameter and return types into the
algorithm object itself? Makes sense...
> Now, of course, it seems like it would be nice for range adaptors to
> be lamba aware too, but I cant seem to quite make it work. I would
> like to be able to call these range adaptors like this:
>
> range | $$(filtered, x)(x < 3);
>
> This wont work since filtered doesnt know what range its using(and
> thus what type x should be), because the way the macro is structured.
> This macro essentially gets tranformed into this:
>
> range | filtered & [&] (decltype(filtered)::lambda_param<0>::type x)
> {return x < 3;};
>
> Now I can make it work if I write like this:
>
> $$(range | filtered, x)(x < 3);
>
> But I hate to have to nest things like this. I was wondering if there
> was some other approach I could do to improve this, or maybe this is
> just as about as far as the rabbit hole goes.
>
> Hopefully, all this made sense.
>
> What do you think?
I think it might be a whole lot easier to have real polymorphic lambdas
-)
--
Dave Abrahams
BoostPro Computing
http://www.boostpro.com
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