Re: [boost] [static_if] Is there interest in a `static if` emulation library?

Subject: Re: [boost] [static_if] Is there interest in a `static if` emulation library?
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2014-09-08 00:50:56

On Tue, Sep 2, 2014 at 10:13 AM, Vicente J. Botet Escriba
<vicente.botet_at_[hidden]> wrote:
> Le 02/09/14 17:34, Roland Bock a écrit :
>>
>> On 2014-09-02 14:08, Sebastian Redl wrote:
>>>
>>> On 02 Sep 2014, at 8:58, Roland Bock <rbock_at_[hidden]> wrote:
>>>
>>>> On 2014-09-02 03:27, Lorenzo Caminiti wrote:
>>>>
>>>> You could use inheritance. If the condition is true, you inherit from a
>>>> struct that provides the data member, otherwise you inherit from a
>>>> struct that does not. Ugly, I know, but it works if inheritance is an
>>>> option for you.
>>>
>>> That’s what I’m doing, but as you say, it is ugly:
>>>
>>>    template <typename Policy>
>>>    using proxy_base_t = std::conditional_t<
>>>        has(Policy::requirements, features::raw_access),
>>>        proxy_raw_access<proxy_nodot<Policy>>,
>>>        proxy_nodot<Policy>>;
>>>
>>> template <typename Policy>
>>> class proxy : public proxy_detail::proxy_base_t<Policy> {
>>> …
>>> };
>>>
>>> Sebastian
>>>
>> Yep, same here :-)
>
> An here
> https://github.com/boostorg/thread/blob/develop/include/boost/thread/concurrent_queues/queue_base.hpp.

Hello all,

I was thinking about the problem of disabling data member declarations
a bit more. I have a couple of questions on C++14 template variables
(these are not Boost related but given I was trying to solve a problem
raised by this mail thread I hope you all do not mind if I ask them
here instead of say comp.lang.c++).

1. Why is not possible to use enabled_if with template variables?

For example, I'd expect this declaration to be ignored by the compiler
when B is 0 and the use of n<0> to fail at (b). Instead clang 3.4.2
errors at (a) saying I cannot use enable_if there when B is 0.

#include <type_traits>

template< bool B >
typename std::enable_if<B, bool>::type n = B; // (a)

int main ( ) {
    n<1>;
    n<0>; // (b)
    return 0;
}

2. Why is not possible to use template variables for data members?

For example, this does not compile on clang 3.4.2 saying the member is
declared as a template (implying that is illegal).

struct x {
    template< typename T >
    T m;
};

I am sure this is just the way C++14 template variables are supposed
to work by spec, but at first glance I do not understand the reason
for these "limitations".

Thanks a lot!
--Lorenzo