Re: [boost] Workaround for compiler bugs

Subject: Re: [boost] Workaround for compiler bugs
From: Edward Diener (eldiener_at_[hidden])
Date: 2010-12-08 00:10:30

On 12/6/2010 7:43 PM, Stephan T. Lavavej wrote:
> [Edward Diener]
>> You missed the fact in the attachment that the T passed in must be the
>> full signature of a pointer to data member, as in 'Gipf Dvonn::*' in
>> your first example below.
>
> I wasn't sure if mentioning the data member's type was a desired feature of your code. Here's how to achieve that:
>
> C:\Temp>type meow.cpp
> #include<type_traits>
>
> #define DEFINE_HAS_MEMBER_OF_TYPE(NAME)                                     \
> template<typename T>  class has_member_ ## NAME {                           \
> private:                                                                    \
>      template<typename U>  static std::true_type helper(decltype(&U::NAME)); \

I quickly played around with this a little seeing if I could remove 
'decltype'. Evidently the VC++ compiler can not handle:

"template<typename U>  static std::true_type helper(&U::NAME);"

spitting out the message of

'error C2998: 'std::tr1::true_type helper' : cannot be a template 
definition'

instead. While 'decltype' solves that problem for expressions of the 
type '&T::someVariablename' in template code in VC10, VC9 has the same 
problem and there, as I mentioned, there is no solution which I have 
been able to discover since 'decltype' does not exist in VC++ prior to 
the latest release. So as I mentioned I can use 'decltype' in VC10 for a 
solution but I am still without a solution for earlier versions of VC++. 
BTW, without the 'decltype' in your example code I do not see why the 
code is not valid. Care to explain why VC++ thinks it is necessary ? I 
can not imagine not being able to understand '&T::someVariablename' by a 
C++ compiler when T is known to be a type.

I will continue to look for a workaround for VC9 and earlier, as I think 
my library should be usable by VC9, VC8, and perhaps VC7.1. Thanks for 
your neat solution for VC10 !

>      template<typename U>  static std::false_type helper(...);               \
> public:                                                                     \
>      typedef decltype(helper<T>(nullptr)) type;                              \
>      static const bool value = type::value;                                  \
> };                                                                          \
>                                                                              \
> template<typename T, typename X, bool B>                                    \
>      struct has_member_ ## NAME ## _of_type_helper                           \
>      : public std::false_type { };                                           \
>                                                                              \
> template<typename T, typename X>                                            \
>      struct has_member_ ## NAME ## _of_type_helper<T, X, true>                \
>      : public std::is_same<decltype(&T::NAME), X T::*>  { };                  \
>                                                                              \
> template<typename T, typename X>                                            \
>      struct has_member_ ## NAME ## _of_type                                  \
>      : public has_member_ ## NAME ## _of_type_helper<                         \
>          T, X, has_member_ ## NAME<T>::value>  { };
>
> DEFINE_HAS_MEMBER_OF_TYPE(cMem)
>
> struct Gipf { };
>
> struct Dvonn {
>      Gipf cMem;
> };
>
> struct Tzaar {
>      int cMem;
> };
>
> struct Yinsh {
>      int Zertz;
> };
>
> int main() {
>      static_assert( has_member_cMem_of_type<Dvonn, Gipf>::value, "one");
>      static_assert(!has_member_cMem_of_type<Dvonn,  int>::value, "two");
>
>      static_assert(!has_member_cMem_of_type<Tzaar, Gipf>::value, "three");
>      static_assert( has_member_cMem_of_type<Tzaar,  int>::value, "four");
>
>      static_assert(!has_member_cMem_of_type<Yinsh, Gipf>::value, "five");
>      static_assert(!has_member_cMem_of_type<Yinsh,  int>::value, "six");
> }