Re: [boost] proposal for metafunction mpl::eval

Hi, On 09/11/13 22:26, klaus triendl wrote:

Nevertheless I'd like to propose a new metafunction that simply evaluates its argument:

template<typename F> struct eval: F::type {};

I've found it somehow useful because it avoids syntactical sugar when specifying a condition to be evaluated for eval_if:

eval_if< eval>, F1, F2 >;

instead of writing:

eval_if< typename apply::type, F1, F2 >;

Any comments on this?

"apply" is an expression in template metaprogramming. By writing "apply::type" you evaluate it. Adding an extra call to the "eval<...>" metafunction will not evaluate it for you unless "eval<T>" is an alias of T::type. You could write a version of eval_if that evaluates its condition for you, so you don't need to do it yourself: template struct lazy_eval_if : eval_if {}; Then you could use it in a nice way in your metaprograms: lazy_eval_if, F1, F2> You can find this lazy_eval_if function in Metamonad (it is called metamonad::if_): http://abel.web.elte.hu/mpllibs/metamonad/if_.html Another way of tackling the problem is taking the metaprogram eval_if, F1, F2> and evaluating it in a lazy way: metamonad::lazy< eval_if< apply, metamonad::lazy_argument<F1>, metamonad::lazy_argument<F2>

::type

The "lazy<...>" template takes its argument (a metaprogram) and evaluates every argument of every metafunction call in it. Since eval_if is a special metafunction (its second and third arguments are lazy), it needs the extra lazy_argument templates as well. You can read about it and laziness in metaprograms in the Metamonad user manual (http://abel.web.elte.hu/mpllibs/metamonad/manual.html#lazy-metafunctions) I gave a talk at C++Now this year about Metamonad and the first part of the talk covered laziness in metaprograms. You can watch it here: http://www.youtube.com/watch?v=aIj034VCUD8 Regards, Ábel