Re: Allowing duplicate template specialisations

From:

Alexander Lamaison <awl03@doc.ic.ac.uk>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 17 Jan 2011 05:50:46 CST

Message-ID:

<105zjp31xlp8f.16a45x1daikmr.dlg@40tude.net>

On Thu, 13 Jan 2011 20:07:20 CST, Martin B. wrote:

On 13.01.2011 10:27, Alexander Lamaison wrote:

Is there a way to allow multiple definitions of the same template
specialisation?
For instance, one way of tagging COM interfaces with their associated UUID
is using templates like this:
template<typename T> struct comtype {};
template<> struct comtype<IUnknown> { static const IID& uuid() { return
IID_IUnknown; } }
template<typename T> inline IID& uuidof(Itf*) { return comtype<T>::uuid();
}
Code can then get at the IID like so:
IUnknown* p = NULL; HRESULT hr = q->QueryInterface(uuidof(p),&p);
The problem is that it is very hard (impossible in the case of a library)
to ensure that there is only one instance of each template specialisation.
Every file that uses a COM interface pointer needs the appropriate
comtype specialisation but files may include two different instances via
seperate include chains. When these comtypes are provided by several
third-parties it becomes a real problem.
As the definitions for any given specialisation should be identical, I
would ideally like any one of them to be chosen by the compiler rather
than an error about multiple definitions. Is this possible?

Enclose these definitions in an anonymous namespace:

- - iunknown_uuid.h - -
   namespace {
     template<typename T> struct comtype {};
     template<> struct comtype<IUnknown> {
       static const IID& uuid() {
         return IID_IUnknown;
       }
     }
     template<typename T> inline IID& uuidof(Itf*) {
       return comtype<T>::uuid();
     }
   };
- -

I'm not sure I understand how that would help. I'm not having link errors
because the implementation appears in two object files. This is a
compilation problem where two definitions of the same specialisation reach
the same compilation unit.

Alex

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