Resize function missing from repeated field in protobuf

Question

Current protoc version: 25.0

I am trying to add x elements into a repeated field at once, and I thought the best way to do it was to use the Resize function. However When I try to compile the following errors emerge:

‘class google::protobuf::RepeatedPtrField<Elem>’ has no member named ‘Resize’; did you mean ‘size’?

Minimum reproducible example ( Neither function call compiles ):

message Elem{
    uint32 a = 1;
}
message Container{
    repeated Elem elems = 5;
}

Container c;
c.mutable_elems()->Resize(5);
c.mutable_elems()->Resize(5, Elem());

Why might this be and is there any way to avoid boilerplate in this case?

(boilerplate like for(int i = 0; i <5; ++i)c.add_elems();)

Answer 1

repeated Elem elems contains a vector-like container of pointers to other repeated messages. c.mutable_elems() returns this container of pointers RepeatedPtrField<Elem>, which does not have the Resize() method. Having this method would have little sense, you unlikely want to fill the container with several null pointers or a same single pointer repeated in several new elements.

RepeatedPtrField<Elem> has the Reserve() method, that can be used before an insertion loop for better performance:

Container c;
c.mutable_elems()->Reserve(5);
for (int i = 0; i < 5; ++i)
  c.add_elem(...);  // or c.AddAllocated(...);