Question

I am using a C++ third party library that places all of its classes in a versioned namespace, let’s call it tplib_v44. They also define a generic namespace alias:

namespace tplib = tplib_v44;

If a forward-declare a member of the library in my own .h file using the generic namespace…

namespace tplib { class SomeClassInTpLib; }

… I get compiler errors on the header in the third-party library (which is being included later in my .cpp implementation file):

error C2386: 'tplib' : a symbol with this name already exists in the current scope

If I use the version-specific namespace, then everything works fine, but then … what’s the point? What’s the best way to deal with this?

[EDIT] FYI for future viewers, this was the ICU library. A solution (at least in my situation) is in the comments to the accepted answer.

Answer 1

It looks like there is an ugly workaround for this, but no good solution.

For ACE (with a decent explanation) and Xerces (with a snarky “this is how c++ works” comment), they define macros that you can use to do this “generically”.

ACE_BEGIN_VERSIONED_NAMESPACE_DECL
class ACE_Reactor;
ACE_END_VERSIONED_NAMESPACE_DECL

XERCES_CPP_NAMESPACE_BEGIN
class DOMDocument;
class DOMElement;
XERCES_CPP_NAMESPACE_END

It looks like an unfortunate c++ artifact, try searching around in your tplib for these macros.

The standard treats namespaces and namespace aliases as different things. You’re declaring tplib as a namespace, so when the compiler tries to assign an alias later, it cannot be both, so the compiler complains.