[Boost-users] Please help with enable_if problem

Subject: [Boost-users] Please help with enable_if problem
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-04-13 19:18:22

I'm getting tons of this from Microsoft's compiler:

        error C2893: Failed to specialize function template 'enable_if<boo...

Reading the compiler docs, C2893 gives an example

        occurs because f's template parameter T is deduced to be 
        std::map<int,int>, but std::map<int,int> has no member data_type 
        (T::data_type can not be instantiated with T = std::map<int,int>.)

but isn't that the whole point of using enable_if?  I'm using it already in my project and understand the idea behind it.

Here is the function in question:

        template <typename T, typename D>
        typename enable_if<
           boost::mpl::and_<
              std::tr1::is_base_of<generic_linear_time_ntbase, T>, 	   
              boost::mpl::or_<
                 is_dimensional_time<D>, 
                 std::tr1::is_same<duration,D>,
                 std::tr1::is_same<linear_duration,D> 
	         > 
	      >,
           T>::type&
        operator+= (T& left, const D& right)
         {
         left.addop_ (right);  // real implementation is in the generic base class
         return *this;  // but I need this template to return the correct derived (actual) class.
         }

Basically, I want to implement operator+= in a base class, but actual use is through derived classes of the common base.  Rather than repeating a half dozen forms in each class that calls the common base and downcasts the return value, I made the real implementation in a named function, and this stand-alone function that calls it.

enable_if is used to make this += present only for left arguments that are derived from the class of interest, and a right argument that is two specific classes or otherwise noted with the predicate.  I've successfully used the predicate (derives from true_type or false_type) in other uses of enable_if.

I've even mixed boost::mpl::or_ and and_ with the compiler-supplied TR1 before, with something quite similar.  Here's one that does work:

        template <typename T, typename D>
        typename enable_if<
           boost::mpl::and_<is_Universal_time_API<T>, 	boost::mpl::or_<is_Universal_duration_API<D>,is_dimensional_time<D> > >,
           T>::type operator+ (const T& t1, const D& d1)
         {
         T temp (t1);
         temp += d1;
         return temp;
         }

My predicates are based on the native TR1's true_type/false_type.

Thanks,
--John







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