In Rust, Does Modifying a Borrowed Pointer Change the Original Value?

Question

This is Rust 0.4

I believe it does, but it doesn't seem to be true for my particular use case.

I have an owned pointer to a vector of owned pointer to vector's.

I construct it as the following.

let mut table = ~[];
for [0, ..10].each |i| {
    let mut row = ~[];
    for [0, ..128].each |j| {
        row.push(A_CERTAIN_ENUM);
    }
    table.push(row);
}

Then, I pass this two dimensional vector to another function for further initialization.

The function looks something like this.

fn setT (table: &mut ~[~[SomeEnumType]]) {
    // Demonstration purposes...
    for [0, ..10].each |i| {
        for [0, ..128].each |j| {
            table[*i][*j] = A_DIFFERENT_ENUM;
        }
    }
}

I call the function with the following code.

setT(&table);

However, then upon inspection, the content of table is not changed at all! Everything inside is still A_CERTAIN_ENUM rather than A_DIFFERENT_ENUM .

Does anyone know how to have another function modify the parameter you pass to it?

Any help is appreciated!

Answer 1

Which version of Rust are you using? Didn't the type checker complain about using

setT(&mut table);

I am a little stumped too. Of course, pointers should work as expected!

Answer 2

Here is a working version of a similar code (rustc 0.5, commit 19f5f91):

enum Direction {
    North,
    East,
    South,
    West
}

fn main() {
    let mut table = ~[];
    for [0, ..10].each |i| {
        let mut row = ~[];
        for [0, ..128].each |j| {
            row.push(North);
        }
        table.push(copy row);
    }
    io::print(fmt!("before: %d\n", table[0][0] as int));
    setT(&mut table);
    io::print(fmt!("after: %d\n", table[0][0] as int));
}

fn setT(table: &mut ~[~[Direction]]) {
    for [0, ..10].each |i| {
        for [0, ..128].each |j| {
            table[*i][*j] = East;
        }
    }
}

Output:

before: 0
after: 1