Objective-C - Disadvantages to Bridging With C++?

So, after all that, would you recommend this design structure in an application? and why.

No.

It is a really nice bit of code; I particularly like the use of imp_implementationWithBlock() (but I admit I might be partial to that particular feature of the runtime ;). And, of course, explorations like this are always an incredibly valuable learning tool.

The issue, in the context of "real world paying project" use, is that you are effectively creating a relatively generic bridge that will then have to have specific bridges at either end to interface with either typical C++ libraries or typical Objective-C APIs/libraries. To put it another way, you have effectively created a new runtime derived from an amalgamation of two existing runtimes.

And, as you point out in the Cons, you pretty much have to touch, wrap, modify and/or debug a shim on top of every C++ class you want to bring into this pattern.

In working with quite a bit of Objective-C++ code over the last 20+ years, a bridge like this is generally more trouble than it is worth. You would likely be better off -- spend less time writing and debugging code -- creating simple Objective-C wrappers around the C++ (or C, frankly) APIs that can then be integrated with and consumed by the targeted system's Objective-C frameworks.