Given that C++ is a superset of C I don’t get why the author wouldn’t just cherry pick the features he needs. Nobody is forcing you to use exceptions or the STL in C++ but you can still get templates and namespaces and smart pointers and references and const and better string handling...
C++ is definitely not a subset and its not just from purely linguistic considerations.
Also, given that zeromq is a community-driven project, requiring the contributors to use C++ but avoid 50% of features is weird and non-idiomatic at best.
If you want to C -- C.
