TITLE: double const in templates

(source: comp.std.c++)

PROBLEM: dak <pierreba@poster.cae.ca>

I've just encountered a problem in one of our template design and wondered
what could be done about it. It concerns const reference return value from
a templated member function, for example:

template <class T> class SmartPtr
{
	// ... whatever:
public:
	const T & Dereference ();
};

The problem arise when T is instantiated with a Container of constants:

SmartPtr<const int> a;

Here we have a double const which is not supported by the compiler, nor the
standard, I believe.  Is there a fix or should we abandon all hope of using
const returns in templates ?


RESPONSE: James Kanze <kanze@lts.sel.alcatel.de>, 26 Mar 96

The draft standard forbids two explicit const's.  If the duplicate
Const result from typedef's, etc., it's OK.  (I don't know if the
actual words mention template instantiation types, but if they don't
I'm sure that it is an editorial oversight, and not the intent.)


RESPONSE: Philippe Verdy <100105.3120@compuserve.com>

Declare a temporary template<class T> to use <T> types only, but no <const T>,
then create another template class that will use <const T> as the parameter
of the first template.
To make this class usable for const and non-const references, you can add
another template parameter to the first class : one for the case T should be
const, one for the other case.
Example:

template <class T, class CT>
class SmartPtr2  {
   SmartPtr2(T* p) { mp = const_cast<CT>(p) ; }
   CT & Dereference() {
     return *mp ;
   }
 private :
   CT *mp ;
} ;
template <class T> class ConstSmartPtr : SmartPtr<T, T> {} ;
template <class T> class FreeSmartPtr : SmartPtr<T, const T> {} ;

Now you can safely use:
  FreeSmartPtr<const X> and ConstSmartPtr<X>.

You cannot use:
  ConstSmartPtr<const X>
due to the double const problem;

But you can use:
  FreeSmartPtr<X>
no const at all !