Use C++ language binding for ZMQ #13957

ZMQ has a native language binding for C++, which among other things takes advantage of RAII for the context, sockets and messages.

Is there any particular reason why Bitcoin Core's ZMQ interface does not use the C++ language bindings? That would most likely simplify the code a lot and make it more robust against subtle bugs.

I'm aware that the doc/zmq.md file explicitly says that "the C++ language bindings are not necessary" - however, at least on my Debian system, it seems that the default ZMQ package libzmq3-dev already includes the C++ language bindings; so it is not really a separate dependency.

I believe @jmcorgan did the initial ZeroMQ work back in 2014. He might be able to give the history to this :-)

At the time, the C++ wrapper (which is a single, inline C++ header file) was sometimes included in the C package, sometimes not (on different distributions), and the least effort imposed on the build was to do it in C and not worry about it. I don't know if this is still true.

Can you elaborate on the simplifications and bug resilience you're referring to? I've used both the native C API and the C++ wrapper, and there's not much difference.

@jmcorgan: The main point where I think the C++ wrapper improves readability and robustness is through type-safety and RAII. Instead of passing around void* for the ZMQ context and socket, you have a typed object (e.g. zmq::socket_t). And instead of manually calling an initialiser and a destructor for it, this is done by the language - including a guarantee that the destructor is called, which might be forgotten when using the pure C API (especially with things like multiple returns in a function or exceptions).

If there is no longer a big reason not to depend on the C++ wrapper, then I can simply prepare a PR that changes to it and then we can judge based on that how much it simplifies the code (or not).

A PR would be great to test builds.

@domob1812 Sounds great!

Demo PR is in #14017.

Can close?

Yes, I think we can close this since the C++ bindings seem to require separate dependencies on non-Debian systems (which I can't test).