In the last post, we built a templated observer framework that let any publisher push strongly-typed messages to its subscribers.
This time we’ll push the design further by trying to let a single publisher handle multiple message types—and we’ll quickly see how that innocent goal invites ambiguity, boilerplate, and some surprising trade-offs.

Discovering Observers - Part 2

by Sandor Dargo

From the article:

Last week, we took the observer pattern from a very simple example and evolved it into a more flexible, template-based implementation in C++. We ended up with abstracted publishers and subscribers, a templated message type for flexibility, and publishers controlling when and how updates are pushed.

This week, we’re going to tackle the challenge of supporting multiple message types in a single publisher.

First attempt: multiple inheritance of templated bases

It’s tempting to provide different specializations for our templates so the same publisher can push different message types.