Declaring namespace as macro - C++

Probably not a best practice as it can be difficult to read compared to a vanilla namespace declaration. That said, remember rules don't always apply universally, and I'm sure there is some scenario where a macro might clean things up considerably.

"But I couldn't find the STL files including this. If it is not included, how it can be used?".

All files that use this macro include yvals.h somehow. For example <vector> includes <memory>, which includes <iterator>, which includes <xutility>, which includes <climits>, which includes <yvals.h>. The chain may be deep, but it does include it it some point.

And I want to clarify, this only applies to this particular implementation of the standard library; this is in no way standardized.

1. In general No. The macros were probably used at the time when namespaces were not implemented by some compilers, or for compatibity with specific platforms.
2. No idea. The file would probably be included by some other file that was included into the STL file.

One approach that I saw in a library that I recently used was:

BEGIN_NAMESPACE_XXX()

where XXX is the number of namespace levels for example:

BEGIN_NAMESPACE_3(ns1, ns1, ns3)

would take three arguments and expand to

namespace ns1 {
    namespace ns2 {
        namespace ns2 {

and a matching END_NAMESPACE_3 would expand to

        }
    }
}

(I have added the newlines and indentation for clarity's sake only)