rust - 是否调用析构函数重新分配可变变量？

Question

假设我已经定义了一个可变变量，它可能是一些复杂的结构，包含向量或其他具有 Drop 特性的动态分配数据。在重新分配该变量时，是否会在重新分配后立即调用析构函数？

let mut x = some_complex_struct;
while some_condition {
    // ...
    x = new_complex_struct;
    // ...
}

我的解释是 x 获得了 new_complex_struct 的所有权，它之前拥有的值变成了无主，因此它的析构函数将在重新分配后立即被调用。我的解释正确吗？

Answer 1

My interpretation is x gains ownership on new_complex_struct, its previously owned value becomes unowned, so its destructor would be called right after reassignment. Is my interpretation correct?

是的。 This can easily be checked:

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Foo::drop");
    }
}

fn main() {
    let mut f = Foo;

    for i in 0..5 {
        println!("Before {}", i);
        f = Foo;
        println!("After {}", i);
    }
}

将按预期打印:

Before 0
Foo::drop
After 0
Before 1
Foo::drop
After 1
Before 2
Foo::drop
After 2
Before 3
Foo::drop
After 3
Before 4
Foo::drop
After 4
Foo::drop

Rust 中的析构函数是确定性的，因此这种行为是一成不变的。