If there's any physical representation, then it's not all the way there on the abstraction front. I meant to exclude any definition of storage format with the "

If there's any physical representation, then it's not all the way there on the abstraction front. I meant to exclude any definition of storage format with the "that contain no implementation" qualifier, because specifying data members is necessarily specifying how you expect the storage of that information to be implemented. As a contrasting example, Java and .NET interfaces aren't allowed to contain fields, only method and property signatures. Which really means only method signatures, since Java and .NET properties are just methods + syntactic sugar. Both languages also have ABCs, but they are distinct language constructs that don't get to enjoy the same privileges as interfaces. That may mean that C++ abstract base classes don't quite hit it on the LSP front. It's also true that C++ is a language that predates LSP's formulation by over a decade, so we can't really expect it to be a language that's designed to take LSP into account, anyway. Long story short, though, just because LSP is baked into your programming language doesn't mean it's moot. It just means you're working in a language that's well-designed on that front.