Abstract : Object layout - the concrete in-memory representation of objects - raises many delicate issues in the case of the C++ language, owing in particular to multiple inheritance, C compatibility and separate compilation. This paper formalizes a family of C++ object layout scheme and mechanically proves their correctness against the operational semantics for multiple inheritance of Wasserrab et al. This formalization is flexible enough to account for space-saving techniques such as empty base class optimization and tail-padding optimization. As an application, we obtain the first formal correctness proofs for realistic, optimized object layout algorithms, including one based on the popular GNU C++ application binary interface. This work provides semantic foundations to discover and justify new layout optimizations; it is also a first step towards the verification of a C++ compiler front-end.