Question

I have the following functions defined:

template <typename T> buffer_t &operator<<(buffer_t &buffer, T data);
template <> buffer_t &operator<<(buffer_t &buffer, const char *data);
template <> buffer_t &operator<<(buffer_t &buffer, const Glib::ustring &data);

When I call:

buffer << Glib::ustring("hello");

The compiler uses the general template definition instead of the specialization with Glib::ustring.

What am I doing wrong here?

Answer 1

You have a function template, and you want to perform template argument deduction. For this purpose, your function call is matched against the templated function argument T data. I believe that 14.8.2.4 applies to the partial ordering for your specialization where P is the template argument and A is the type of the actual argument; emphasis is mine):

Before the partial ordering is done, certain transformations are performed on the types used for partial
ordering:

— If P is a reference type, P is replaced by the type referred to.

— If A is a reference type, A is replaced by the type referred to.

Thus, since the type of your argument is A = Glib::ustring, then this is not as good a match for the specialization const Glib::ustring & as the primary template is, and even if you have an actual const-reference, the reference is stripped off during the partial ordering, and you again end up with a worse match.

The usual way to fix this is to make your primary template a const-reference; this can also bind to temporary objects and should thus be “as good as” a value argument:

template <typename T> buffer_t & operator<<(buffer_t & buffer, T const & data);
//                                                             ^^^^^^^^^