The class is not actually defined in the platformstl namespace. Rather, it resolves to the appropriate type for the given platform, relying on Intersecting Conformance of the resolved platform-specific types.
When compiling on UNIX platforms, the platformstl::performance_counter type resolves to the unixstl::performance_counter class. On Windows platforms it resolves to the winstl::performance_counter class. It is not defined for other platforms.