Heap Memory

Step by step

1. 01Before anything runsThe compiled program has been loaded into memory, but no function has run yet. Nothing is on the stack and nothing is on the heap
2. 02main's frame is pushedmain's frame reserves room for both locals. A Vec on the stack is really three fields, ptr, len and cap, and a Box is a single pointer. None of them are written yet
3. 03Box the valueBox::new asks the allocator for a free spot big enough for an i32, writes 5 there, and stores that address in boxed.ptr
4. 04Allocate the Vecvec! allocates one block holding three i32s at 0x2468, 0x246c and 0x2470. The stack gets the pointer, the length (3 in use) and the capacity (3 reserved)
5. 05Print the valuesprintln! is drawn as a function call here (it is really a macro). It borrows boxed and vector, then follows their pointers to the heap to read 5 and [9, 8, 7]
6. 06main's return valuemain has no declared return type, so it returns the unit type (), written into its return slot before the frame is cleaned up
7. 07Drop the VecBefore the frame goes away, each owner must free what it points to. Locals are dropped in reverse order, so vector goes first: its destructor frees the buffer at 0x2468
8. 08Drop the BoxNext, boxed's destructor frees the four bytes at 0x1234. The heap is now completely empty again
9. 09Program endsWith the heap cleaned up, main's frame is popped. All memory is free and the program exits, without any garbage collector involved

Key takeaways

• The heap data lives elsewhere; the stack only holds a pointer to it
• A Vec on the stack is three fields: ptr, len and cap
• Owners free their heap memory in their destructor
• Locals are dropped in reverse order of declaration

Resources

• The Rust Book: Using Box<T> to point to data on the heapWhen a single boxed value is useful
• The Rust Book: Storing lists of values with vectorsCreating, updating and reading a Vec
• std::vec::Vec, guaranteesWhy a Vec is a (pointer, capacity, length) triple
• The Rust Reference: DestructorsThe exact order values are dropped in

Research notes

• A Box<T> for a sized T is a single pointer. Boxes of unsized types such as Box<[T]> or Box<dyn Trait> are fat pointers that also carry a length or a vtable pointer
• The addresses 0x1234 and 0x2468 are made up. The real allocator picks any free spot, and the addresses change from run to run
• Vec doesn't promise a growth strategy, only that push is amortised O(1)