How do I force Rust to allocate a big object on the heap without using a Vec?

Asked Jul 20 '20 at 14:16

Active Jul 20 '20 at 14:25

Viewed 182 times

I have a large object

pub(crate) struct Grid {
    pub weights: [[[f32; GRID_SIZE]; GRID_SIZE]; GRID_SIZE],
    // ... other attributes
}

With a large grid size, it does not fit on the stack and causes a stack overflow. I tried to create a Box, but Box::new(Grid { .. }) still overflows since Rust tries to create the entire object on the stack and copy it to the heap afterwards. How do I create some type of box or reference immediately for objects that don't fit on the stack?

I found How to allocate arrays on the heap in Rust 1.0?, but this works only for a array and not for an arbitrary struct. I assume there must be a good way to create a pointer directly without converting back and forth to a vector.

The box Grid {} syntax seems to not work either, Rust complains that that syntax is experimental.

edited Jul 20 '20 at 14:25

Shepmaster

388,571
95
1,107
1,366

asked Jul 20 '20 at 14:16

mousetail

7,009
4
25
45

As suggested, there's no such stable feature yet – Alexey S. Larionov Jul 20 '20 at 14:23
Is it possible to achieve the effect with perhaps some `unsafe{}` memory manipulation? – mousetail Jul 20 '20 at 14:24
Why do you not want to use a `Vec`? A `Vec` is not much more than a pointer to an array in heap memory (in fact, it's a pointer plus a length field). – Jesper Jul 20 '20 at 15:25
2

Well, the object is not only an array – mousetail Jul 20 '20 at 16:18

How do I force Rust to allocate a big object on the heap without using a Vec?

0 Answers0