How to keep boxed objects in heap until the end of program execution?

March 31, 2023

I’m doing this, for experimental purposes (I want to allocate many objects in heap and measure the performance of this):

struct Foo {
    x: u32
}

for i in 0..1000 {
  let b = Box::new(Foo {x: i as u32});
  let p = b.deref();
  println!("Pointer: {:p}", p);
}

I’m getting this output:

Pointer: 0x600000780000
Pointer: 0x600000780000
Pointer: 0x600000780000
Pointer: 0x600000780000
Pointer: 0x600000780000
...

Obviously, all addresses are the same, which means that right after an object is allocated it gets destroyed. How to prevent this? I want to keep the memory occupied by objects, not freed immediately.

>Solution :

You can use Box::leak to ensure the box doesn’t get freed.

fn main() {
    struct Foo {
        x: u32
    }

    for i in 0..1000 {
        let b = Box::new(Foo {x: i as u32});
        let p = Box::leak(b);
        println!("Pointer: {:p}", p);
    }
}

More generally std::mem::forget can be used to not run destructors