Re: singleton question

Rolf Magnus <>
Mon, 16 Jul 2007 06:47:10 +0200
ben chang wrote:

i'm hacking at some code originally written for VC++, trying to port to
linux GCC 4. the linker returns multiple-definition errors with a
singleton object, which i guess is not a problem in visual c++. i'll try
to excerpt the relevant parts without posting all the source files (it's
the maya exporter for the Ogre 3D engine).

there's a pretty standard-looking Singleton template, and in a file called
"materialSet.h" we have a class using it:

namespace OgreMayaExporter
class MaterialSet : public Singleton<MaterialSet>
  (nothing unusual in here, so i won't bore you... )

template<> MaterialSet Singleton<MaterialSet>::ms_Singleton=0;

The above line defines Singleton<MaterialSet>::ms_Singleton. If this is in a
header and that header is included in multiple files, you'll get multiple
definitions of that object.

this declaration is complicated enough that it's a bit hard to wrap my
head around, but isn't this essentially repeatedly declaring a global
(within the namespace) variable, which is what Singletons are supposed to

It's repeatedly _defining_ the very same namespace-level variable, which any
program is supposed to avoid. Put the definition into an implementation
file, not a header.

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