const references in c++ templates

Question

In a C++ template with the generic type T, I can use

const T &

to get a reference to a constant T. However, if now T itself is a reference

Answer 1

You can always use template specialisation to implement a different version for any kind of reference:

template  struct X {
  void foo(T const&);
};

template  struct X {
  void foo(T const&);
};

Now, X::foo expects an int const& and X::foo expects an int const&, too.

However, it is not entirely clear from your question what you are trying to do exactly.

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Edit: My g++ version (4.6.1) does not complain without template specialisation for the following

int i = 7;
X(i);

While it does for

X(7);

Which is correct IMO, because you try to convert a temporary (7) to a mutable reference (even if that is a reference to a const reference).

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Edit 2: If you want to reduce duplicate code, then do not specialise your original class, but use this:

template  struct R {
  typedef T& Ref;
  typedef T const& ConstRef;
};

template  struct R {
  typedef T& Ref;
  typedef T const& ConstRef;
};

template struct X {
  void foo(typename R::ConstRef x);
};