Re: SFINAE help

Tim H wrote:


Tim H wrote:
 > I have a template class 'Property' that has a ValueType typedef in it.
 >
 > I have a method of another class that looks like this:
 >
 > template<class Tkey>
 > const typename Tkey::ValueType &GetProperty() const { ... }
 >
 > I want to support Properties with void ValueType. I added a
 > specialization of Property and that works. what fails is when code
 > calls the above GetProperty<VoidProperty>() method. I get a "no
 > matching function" error (g++).
 >
 > It looks like SFINAE is correctly determining that a 'const void &' is
 > an error. The error message is not very helpful, though.
 >
 > How can I use SFINAE (or other tricks) to make calls to
 > GetProperty<VoidProperty>() blow up in a more obvious manner?

There is no SFINAE involved because you instantiate the function template
_explicitly_.

SFINAE works while the compiler is building a set of viable function overloads.
In doing so it also tries to _implicitly_ instantiate function templates. If
function template instantiation fails when the template arguments are
substituted with particular types, that Substitution Failure Is Not An Error,
the function template is simply disregarded and no compiler error produced.

Your example can be made to work without SFINAE. Something like that:

     template<class T>
     struct MakeRefToConst
     {
         typedef T const& type;
     };

     template<>
     struct MakeRefToConst<void>
     {
         typedef void type; // don't make references to void
     };

     template<class Tkey>
     typename MakeRefToConst<typename Tkey::ValueType>::type GetProperty();

     struct A
     {
         typedef int ValueType;
     };

     struct B
     {
         typedef void ValueType;
     };

     int main()
     {
         GetProperty<A>();
         GetProperty<B>();
     }

Instead of MakeRefToConst<> you can use boost type traits.
http://www.boost.org/doc/libs/1_39_0/libs/type_traits/doc/html/boost_typetraits/category/transform.html

Max