What is the memory layout of a vector of arrays?

Question

Are variables of type Vec<[f3; 5]> stored as one contiguous array (of Vec::len() * 5 * sizeof(f32) bytes) or is it stored as a Vec of pointers?

Answer 1

The contents of a Vec<T> is, regardless of T, a single heap allocation, of self.capacity() * std::mem::size_of::<T>() bytes. (Vec overallocates—that’s the whole point of Vec<T> instead of Box<[T]>—so it’s the capacity, not the length, that matter in this calculation.) The actual Vec<T> itself takes three words (24 bytes on a 64-bit machine).

[f32; 5] is just a chunk of memory containing five 32-bit floating-point numbers, with no indirection; this comes to twenty bytes (hence std::mem::size_of::<[f32; 5]>() == 20).