One of the lesser-known features of C++ templates.

Providing Explicit Specializations for Non-Template Members of Class Template

by Pavel Frolov

From the article:

C++ is full of surprises (albeit not always good ones . It is a well known fact that you can provide explicit specializations for function templates and class templates. But it was a total surprise to me, that you can provide explicit specializations for non-template members of class template without specializing the class template itself!