Cannot add const to references?

**Elysia** · 07-14-2008

I'm having problems, and I'm wondering if this is normal or nor and if there's some workaround.
Consider:

Code:

namespace mpl = Stuff::mpl;

template<typename T>
struct foo { typedef const T type; };

int main()
{
	A a;
	a.x = 0;
	cout << mpl::type_name<foo<A&>::type>::name << endl;
}

mpl::type_name is a meta-template struct helper that helps in printing out names of the types since Microsoft's typeid.name() ignores const and reference identifiers on the types.
Regardless of that, this does not work as expected. You would expect it to return (or I would) const A&, but it doesn't. It's still the same old A&.

Visual Studio warns me:
Warning 1 warning C4181: qualifier applied to reference type; ignored g:\w00t\visual studio 2008\projects\temp\temp2.cpp 35
Which essentially means I cannot append const to a reference type.

So is there a way to add const to a reference type?
The idea is to do:

void foo(typename mpl::ArgumentHelper<T>::const_type Arg)

And const_type will be T if it's a native type and const T& if it's a class or struct.

**grumpy** · 07-14-2008

Yes, what you're experiencing is "normal", or at least ... expected.

I may not have fully understood your intent, but my guess is that you're looking for something like this;

Code:

template<typename T> class MyTraitsClass
{
     typedef const T & const_type;
};

template<> class MyTraitsClass<short>
{
    typedef short const_type;
};

// other specialisations for the other basic types.

template<typename T>  class TypeStripper
{
      typedef typename T actual_type;
};

// and a series of partial specialisations for reference types

template<typename T &>  class TypeStripper
{
      typedef typename T actual_type;
};

template<typename const T &>  class TypeStripper
{
      typedef typename T actual_type;
};

template<typename T> class ArgumentHelper
{
    typedef typename MyTraitsClass<TypeStripper<T>::actual_type>::const_type const_type; 
       // may need typename keyword more than once in here:  check that
};

**Elysia** · 07-14-2008

Strip the reference, add const and re-add the reference!
Works like a charm! Thanks!

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