Re: [Boost-users] eval_if fails - problem for C++ guru

Subject: Re: [Boost-users] eval_if fails - problem for C++ guru
From: Robert Ramey (ramey_at_[hidden])
Date: 2015-06-07 11:15:01

On 6/6/15 8:45 PM, Lee Clagett wrote:

> The problem are the immediate instantations of `typename
> get_promotion_policy<T>::type` which cause the static assert to be
> instantiated too. You need to delay instantating ::type in these objects
> until std::is_same needs to be evaluated. I don't know know of a way to
> do this with standard MPL (see
> https://abel.web.elte.hu/mpllibs/metamonad/lazy.html ), but one easy way
> with C++11 (if metaparse is not an option):
>
>      template<template<typename...> class F, typename... A>
>      struct lazy_eval
>      {
>          using type = typename F<typename A::type...>::type;
>      };
>
>      // if both types are safe, the policies have to be the same!
>      static_assert(
>          boost::mpl::eval_if<
>              boost::mpl::and_<
>                  is_safe<T>,
>                  is_safe<U>
>              >,
>              lazy_eval<
>                  std::is_same,
>                  get_promotion_policy<T>,
>                  get_promotion_policy<U>
>              >,
>              boost::mpl::identity<boost::mpl::true_>
>          >::type::value,
>          "if both types are safe, the policies have to be the same!"
>      );
>
> which will work with any metafunction F that accepts 1 or more
> arguments. There might be some way to re-write what you are trying to
> avoid this, but I haven't put much thought into that.

Thanks for responding.

I also believe that the problem is that is_same is getting invoked "too 
soon".  It is for this reason I used eval_if which is intended to avoid 
instantiation of it's arguments but just return the type of the selected 
one. Clearly I've got something wrong.  I tried your code exactly as 
you've written it but I get the same error.  MPL has similar 
functionality in the metafunction "apply" - though I hadn't tried that.

So I'm still stumped - but, again, thanks for taking the time to look at 
this.

Robert Ramey