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

8 Sep 2014

      On Tue, Sep 2, 2014 at 10:13 AM, Vicente J. Botet Escriba
 wrote:
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Le 02/09/14 17:34, Roland Bock a écrit :
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On 2014-09-02 14:08, Sebastian Redl wrote:
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On 02 Sep 2014, at 8:58, Roland Bock  wrote:
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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>>;
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/concurr....
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 

template< bool B >
typename std::enable_if::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