Why would struct initialization order matter?

help
1

I have this code:

struct Chunk{
    chunk_type: ChunkType,
    data: Vec<u8>,
    crc: u32
}

impl Chunk {
    fn new(chunk_type: ChunkType, data: Vec<u8>) -> Self {
        let crc = Crc::<u32>::new(&CRC_32_CKSUM);

        Chunk {
            crc: crc.checksum(&[&chunk_type.bytes()[..], &data].concat()),
            chunk_type,
            data
        }
    }
}

It compiles just fine. However, if I just change the order of params in the initialization of Chunk, I get the classic "borrow of a moved value" error:

impl Chunk {
    fn new(chunk_type: ChunkType, data: Vec<u8>) -> Self {
        let crc = Crc::<u32>::new(&CRC_32_CKSUM);

        Chunk {
            chunk_type, // value moved here
            data,
            crc: crc.checksum(&[&chunk_type.bytes()[..], &data].concat()) // borrow of moved value
        }
    }
}

I wonder, why would it throw such an error in this case? Won't the value of crc property be calculated before any move occurs?

2

No. Initializer expressions are evaluated in order, top down. The move of chunk_type happens before the checksum call.

1 Like
3

It makes sense, I just expected the compiler to reorder the calls when the order wouldn't matter, but probably I expected too much.

4

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