How to define an optional field in protobuf 3

Question

I need to specify a message with an optional field in protobuf (proto3 syntax). In terms of proto 2 syntax, the message I want to express is something like:

Answer 1

One way is to optional like described in the accepted answer: https://stackoverflow.com/a/62566052/1803821

Another one is to use wrapper objects. You don't need to write them yourself as google already provides them:

At the top of your .proto file add this import:

import "google/protobuf/wrappers.proto";

Now you can use special wrappers for every simple type:

DoubleValue
FloatValue
Int64Value
UInt64Value
Int32Value
UInt32Value
BoolValue
StringValue
BytesValue

So to answer the original question a usage of such a wrapper could be like this:

message Foo {
    int32 bar = 1;
    google.protobuf.Int32Value baz = 2;
}

Now for example in Java I can do stuff like:

if(foo.hasBaz()) { ... }