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# Drop & defer

## Drop: your destructor

A struct that defines a method literally named `drop` runs that method on scope exit. The signature is fixed: `fn drop(mut self)`, with no return type.

```cplus
struct Buf { ptr: *u8, len: usize }
impl Buf {
    fn drop(mut self) {
        unsafe { free(self.ptr); }
    }
}

fn main() -> i32 {
    let b: Buf = make_buf();
    // ... use b ...
    return 0;
}                                    // b.drop() runs here
```

Defining `drop` makes the type non-`Copy`, which is necessary: copying a value that owns a resource would lead to a double free. See [Ownership](/docs/ownership) for how `Copy` is derived.

## Raw-pointer accountability

A `drop` frees its struct's fields by hand; the compiler does not synthesize per-field drops. For a raw-pointer field (`*T`) the compiler cannot tell whether the struct owns the memory or only borrows it, so it makes you say which. A raw-pointer field that is neither released by the struct's `drop` nor marked `opaque` is a compile error (**E0510**). There is no silent-leak default.

```cplus
struct Buf { ptr: *u8, len: usize }
impl Buf {
    fn drop(mut self) { unsafe { free(self.ptr); } }   // owned: you free what you own
}

struct View { opaque ptr: *u8, len: usize }            // borrowed: `opaque` means "not mine"
```

Severity tracks what the compiler can prove about the `drop` body. It is a structural check, with no dataflow:

| The field's release in `drop` is... | Result |
|---|---|
| unconditional, or guarded only by a null-test on the same field | clean |
| present but conditional (a refcount, flag, or loop, so it cannot be proven to always run) | **W0002** warning |
| absent, delegated to a helper, or there is no `drop` at all | **E0510** error |
| the field is marked `opaque` | clean (managed elsewhere) |

`free(self.ptr as *u8)` counts as releasing `ptr`, since the cast is transparent. The warning case is for legitimately conditional owners such as refcounted types, which free their control block only on the last reference. Use `opaque` only when another owner truly frees the pointer: an FFI handle the runtime owns, a borrowed view, or a pointer freed by a sibling struct.

## Containers drop their elements

The hand-freeing above is only for the raw-pointer fields you own directly. For the owning containers — `Vec[T]`, `Box[T]`, `Arc[T]`, `Rc[T]`, `HashMap[K, V]` — drop is automatic and **recursive**. Dropping a `Vec[T]` runs each element's `drop` exactly once and then frees the backing buffer, so a `Vec[Buf]` releases every `Buf` before the buffer goes. You write no per-element loop.

```cplus
fn main() -> i32 {
    let mut v: Vec[Buf] = Vec::new();
    v.push(make_buf());
    v.push(make_buf());
    return 0;
}                                    // each Buf.drop() runs, then v frees its buffer
```

## Enum payloads drop when consumed

Matching an owned enum and binding a payload you do not move out drops that payload at the arm's exit, so nothing leaks:

```cplus
match msg {                          // msg is owned
    Message::Text(s) => #println(s.len() as i32),   // s drops at arm exit
    Message::Close   => {}
}
```

Every move-out shape still disarms that drop, so a payload you *do* move out is not dropped twice.

## Loop bodies drop each iteration

A value created inside a `while`, `for`, or `loop` body is its own scope: it drops at the end of every iteration, and on `break` or `continue`, not once at the end of the loop. So a buffer built per iteration is released per iteration, with no accumulation.

```cplus
let mut i: i32 = 0;
while i < n {
    let line: string = read_line();   // line.drop() runs at the end of
    handle(line);                     //   every iteration, including on continue
    i = i +% 1;
}
```

A binding declared *outside* the loop is different: moving it on each iteration is rejected (**E0335**), because after the first iteration moved it away there is nothing left to move. Re-initialize it before the move if you need that pattern.

## `defer`: run at scope exit, LIFO

`defer` schedules an action to run when the scope exits, in last-in-first-out order:

```cplus
fn main() -> i32 {
    #println(1);
    defer #println(4);
    defer #println(3);
    #println(2);
    return 0;
}
// Prints 1, 2, 3, 4
```

`defer` and `Drop` share one scope-exit stack: they interleave in declaration order and are popped LIFO at exit.