[knorker]

I read the parent commenter as proposing headroom.

Because if it's an exact match, then it doesn't help at all with ABI compat, and arguably doesn't add anything. Well, aside from compile time, but that's maybe better solved with modules?

[feelamee]

pimpl can solve several problems and allowing to extend struct without breaking ABI is the one is them. But even such implementation of pimpl breaks compile time and ABI dependency. You can't change size without breaking ABI, but still can change members, etc (e.g. replace current fields with heap allocated to extend without breaking ABI :))

[knorker]

Sure, but you could have done that without pimpl, too. At least without defaulted or inlined constructors/destructors/etc.