Does anyone know why using-declarations don't seem to work for importing type names from dependent base classes? They work for member variables and functions, but at least in GCC 4.3, they seem to be ignored for types.
template <class T>
struct Base
{
typedef T value_type;
};
template <class T>
struct Derived : Base<T>
{
// Version 1: error on conforming compilers
value_type get();
// Version 2: OK, but unwieldy for repeated references
typename Base<T>::value_type get();
// Version 3: OK, but unwieldy for many types or deep inheritance
typedef typename Base<T>::value_type value_type;
value_type get();
// Version 4: why doesn't this work?
using typename Base<T>::value_type;
value_type get(); // GCC: `value_type' is not a type
};
I have a base class with a set of allocator-style typedefs that I'd like to inherit throughout several levels of inheritance. The best solution I've found so far is Version 3 above, but I'm curious why Version 4 doesn't seem to work. GCC accepts the using-declaration, but seems to ignore it.
I've checked the C++ Standard, C++ Prog. Lang. 3rd ed. [Stroustrup], and C++ Templates [Vandevoorde, Josuttis], but none seem to address whether using-declarations can be applied to dependent base class types.
In case it helps to see another example, here is the same question being asked, but not really answered, on the GCC mailing list. The asker indicates that he has seen 'using typename' elsewhere, but that GCC doesn't seem to support it. I don't have another conforming compiler available to test it.
