Pointer arithmetics after cast

Question

Consider the following C++ pseudocode:

// Pointer to contiguous memory block suitably aligned to contain
// an array of type T. Possibly obtained via std::malloc or std::aligned_storage.
void *buffer = ...;
// Now cast as T pointer.
T *ptr = static_cast<T *>(buffer);
// Do some pointer arithmetics. For instance, construct the first two
// elements of the array.
::new (ptr) T();
::new (ptr + 1) T();
// etc.

Is this legal? What does the standard say about doing pointer arithmetics after a cast? 5.7/5 of the C++11 standard talks about arithmetics on pointers to elements of an array object, but can any contiguous memory block be considered as an array?

Answer 1

Yes, it is legal. In terms of arithmetic, ptr + 1 is effectively the same as ((uint8)ptr) + (sizeof(*ptr)*1) for any typed pointer. So yes, given any typed pointer, the memory being pointed at can be treated as a contiguous array of elements of that pointer's type, and you can use type casts to change the behavior for any given pointer-arithmetic operation.