What's the point of RepeatedField?

[edre]

Some repeated types are implemented with Vec, and some are implemented with RepeatedField. RepeatedField seems to be just using a Vec as a growable capacity, but Vec already does this internally. Is there a plan to replace all remaining RepeatedField with Vec at some point?

[stepancheg]

The idea behind RepeatedField is to reuse allocated memory for nested structures.

Suppose you have:

message A {
    repeated B b = 1;
}

message B {
    optional string s = 1;
}

If you parse A in loop, like this:

let mut a = A::new();
loop {
    a.clear();
    stream.merge_message(&mut a).unwrap();
    // process a
}

In that case after couple of iterations malloc won't be used, becase .clear() does not release memory back to malloc.

If you are using Vec, memory for B::s will be allocated and released on each iteration.

Google protobuf C++ uses this trick.

So there is no plan to replace RepeatedField with Vec by default, but if there's demand, it is possible to implement protobuf option to generate Vec.