base: Provide stl_helpers::operator<< for more types

This operator can be safely brought in scope when needed with "using

In order to provide a specialization for operator<< with
stl_helpers-enabled types without loosing the hability to use it with
other types, a dual-dispatch mechanism is used. The only entry point
in the system is through a primary dispatch function that won't
resolve for non-helped types. Then, recursive calls go through the
secondary dispatch interface that sort between helped and non-helped
types. Helped typed will enter the system back through the primary
dispatch interface while other types will look for operator<< through
regular lookup, especially ADL.

Change-Id: I1609dd6e85e25764f393458d736ec228e025da32
Reviewed-by: Daniel Carvalho <>
Tested-by: kokoro <>
Maintainer: Bobby Bruce <>
4 files changed