Modern architectures require applications to make effective use of caches to achieve high performance and hide memory latency. This in turn requires careful consideration of placement of data in memory to exploit spatial locality, leverage hardware prefetching and conserve memory bandwidth. In unmanaged languages like C++, memory optimisations are common, but at the cost of losing object abstraction and memory safety. In managed languages like Java and C#, the abstract view of memory and proliferation of moving compacting garbage collection does not provide enough control over placement and layout. We have proposed SHAPES, a type-driven abstract placement specification that can be integrated with object-oriented languages to enable memory optimisations. SHAPES preserves both memory and object abstraction. In this paper, we formally specify the SHAPES semantics and describe its memory safety model.