Heap

The stack is great, but it can't solve all our problems. What about data whose size is not known at compile time? Collections, strings, and other dynamically-sized data cannot be (entirely) stack-allocated. That's where the heap comes in.

Heap allocations

You can visualize the heap as a big chunk of memory—a huge array, if you will.
Whenever you need to store data on the heap, you ask a special program, the allocator, to reserve for you a subset of the heap. We call this interaction (and the memory you reserved) a heap allocation. If the allocation succeeds, the allocator will give you a pointer to the start of the reserved block.

No automatic de-allocation

The heap is structured quite differently from the stack.
Heap allocations are not contiguous, they can be located anywhere inside the heap.

+---+---+---+---+---+---+-...-+-...-+---+---+---+---+---+---+---+
|  Allocation 1 | Free  | ... | ... |  Allocation N |    Free   |
+---+---+---+---+---+---+ ... + ... +---+---+---+---+---+---+---+

It's the allocator's job to keep track of which parts of the heap are in use and which are free. The allocator won't automatically free the memory you allocated, though: you need to be deliberate about it, calling the allocator again to free the memory you no longer need.

Performance

The heap's flexibility comes at a cost: heap allocations are slower than stack allocations. There's a lot more bookkeeping involved!
If you read articles about performance optimization you'll often be advised to minimize heap allocations and prefer stack-allocated data whenever possible.

`String`'s memory layout

When you create a local variable of type String, Rust is forced to allocate on the heap¹: it doesn't know in advance how much text you're going to put in it, so it can't reserve the right amount of space on the stack.
But a String is not entirely heap-allocated, it also keeps some data on the stack. In particular:

The pointer to the heap region you reserved.
The length of the string, i.e. how many bytes are in the string.
The capacity of the string, i.e. how many bytes have been reserved on the heap.

Let's look at an example to understand this better:

let mut s = String::with_capacity(5);

If you run this code, memory will be laid out like this:

      +---------+--------+----------+
Stack | pointer | length | capacity | 
      |  |      |   0    |    5     |
      +--|------+--------+----------+
         |
         |
         v
       +---+---+---+---+---+
Heap:  | ? | ? | ? | ? | ? |
       +---+---+---+---+---+

We asked for a String that can hold up to 5 bytes of text.
String::with_capacity goes to the allocator and asks for 5 bytes of heap memory. The allocator returns a pointer to the start of that memory block.
The String is empty, though. On the stack, we keep track of this information by distinguishing between the length and the capacity: this String can hold up to 5 bytes, but it currently holds 0 bytes of actual text.

If you push some text into the String, the situation will change:

s.push_str("Hey");

      +---------+--------+----------+
Stack | pointer | length | capacity |
      |  |      |   3    |    5     |
      +--|  ----+--------+----------+
         |
         |
         v
       +---+---+---+---+---+
Heap:  | H | e | y | ? | ? |
       +---+---+---+---+---+

s now holds 3 bytes of text. Its length is updated to 3, but capacity remains 5. Three of the five bytes on the heap are used to store the characters H, e, and y.

`usize`

How much space do we need to store pointer, length and capacity on the stack?
It depends on the architecture of the machine you're running on.

Every memory location on your machine has an address, commonly represented as an unsigned integer. Depending on the maximum size of the address space (i.e. how much memory your machine can address), this integer can have a different size. Most modern machines use either a 32-bit or a 64-bit address space.

Rust abstracts away these architecture-specific details by providing the usize type: an unsigned integer that's as big as the number of bytes needed to address memory on your machine. On a 32-bit machine, usize is equivalent to u32. On a 64-bit machine, it matches u64.

Capacity, length and pointers are all represented as usizes in Rust².

No `std::mem::size_of` for the heap

std::mem::size_of returns the amount of space a type would take on the stack, which is also known as the size of the type.

What about the memory buffer that String is managing on the heap? Isn't that part of the size of String?

No!
That heap allocation is a resource that String is managing. It's not considered to be part of the String type by the compiler.

std::mem::size_of doesn't know (or care) about additional heap-allocated data that a type might manage or refer to via pointers, as is the case with String, therefore it doesn't track its size.

Unfortunately there is no equivalent of std::mem::size_of to measure the amount of heap memory that a certain value is allocating at runtime. Some types might provide methods to inspect their heap usage (e.g. String's capacity method), but there is no general-purpose "API" to retrieve runtime heap usage in Rust.
You can, however, use a memory profiler tool (e.g. DHAT or a custom allocator) to inspect the heap usage of your program.

std doesn't allocate if you create an empty String (i.e. String::new()). Heap memory will be reserved when you push data into it for the first time.

The size of a pointer depends on the operating system too. In certain environments, a pointer is larger than a memory address (e.g. CHERI). Rust makes the simplifying assumption that pointers are the same size as memory addresses, which is true for most modern systems you're likely to encounter.

Exercise

The exercise for this section is located in 03_ticket_v1/09_heap

100 Exercises To Learn Rust