Question

Let’s we have a simple structure (POD).

struct xyz
{
    float x, y, z;
};

May I assume that following code is OK? May I assume there is no any gaps? What the standard says? Is it true for PODs? Is it true for classes?

xyz v;
float* p = &v.x;
p[0] = 1.0f;
p[1] = 2.0f; // Is it ok?
p[2] = 3.0f; // Is it ok?

Answer 1

The answer here is a bit tricky. The C++ standard says that POD data types will have C layout compatability guarantees (Reference). According to section 9.2 of the C spec the members of a struct will be laid out in sequential order if

1. There is no accessibility modifier difference
2. No alignment issues with the data type

So yes this solution will work as long as the type float has a compatible alignment on the current platform (it’s the platform word size). So this should work for 32 bit processors but my guess is that it would fail for 64 bit ones. Essentially anywhere that sizeof(void*) is different than sizeof(float)