C Structures

We've covered most of the primitive types (we're avoid strings for now!). So how about Structs? This will naturally lead us towards the part we're all dreading --- pointers (and strings!). For now, let's have a gentle start.

First of all, some good news. Rust and C structs are really similar. These will have the same representation in memory (assuming you're on a sane platform with i32 for int, and i8 for char!)

struct MyStruct {
    int field;
    int field2;
    char field3;
};

#![allow(unused)]
fn main() {
#[repr(C)]
struct MyStruct {
    field: i32,
    field2: i32,
    field3: i8,
}
}

Note the #[repr(C)]. If you don't specify this, Rust reserves the right to reorder your struct however it feels. That can sometimes improve performance. Forgetting the repr can ruin your day by sometimes working.

Sticking with "pure" C (as opposed to C++) for now, you can use structs a lot like you use them in Rust --- with some differences.

#include <stdio.h>
#include <stdlib.h>

struct MyStruct {
    int field;
    int field2;
    char field3;
};

void print(struct MyStruct s) {
    printf("%d, %d, %d\n", s.field, s.field2, s.field3);
}

void print_ptr(struct MyStruct *s) {
    printf("%d, %d, %d\n", s->field, s->field2, s->field3);
}

int main() {
    struct MyStruct s = {
        1, 2, 3
    };
    print(s);
    
    // "s" is not invalidated because it was copied. C doesn't move!
    printf("%d, %d, %d\n", s.field, s.field2, s.field3);
    
    // Grab a pointer to s and it works like a reference
    print_ptr(&s);
    return 0;
}

Notice how C doesn't have Rust's "move by default"---it copies. You can take a pointer, and use it.