[shadowgovt]

This has been my experience dealing with portability and C++ as well. In theory, very portable. In practice, "portable" as long as you don't care about the details of how features work and didn't use any undefined behaviors that just happened to work in the first architecture but are implemented totally differently in the new architecture (and C++, of course, embraces undefined behavior instead of keeping the developer out of it).

Things as simple as "believing you know the byte-width of a primitive data type" are dangerous.