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Question

In my test.proto file I have as below, ie no field inside message HTML, how do I set a value for the _html field of message Page?

message HTML{
    enum Type {
        NO_TYPE = 0;
        HTML5 = 1;
        XHTML = 2;
    }
}

message Page{ 
 optional HTML _html = 1;
}

How can I set the values?

echo _html : **???** | protoc --encode=Page test.proto  > binary.data

https://medium.com/@at_ishikawa/cli-to-generate-protocol-buffers-c2cfdf633dce

Answer 1

How can I set the values?

you can't when it is defined like you have shown. The message HTML only contains a definition of an enum but no actual fields. You can either add a field in the message HTML , or define the type in Page as HTML.Type instead of just HTML :

test.proto:

message HTML{
    enum Type {
        NO_TYPE = 0;
        HTML5 = 1;
        XHTML = 2;
    }
}

message Page{ 
 optional HTML.Type _html = 1;
}

and then create a file test.txt:

_html: HTML5

you can create a binary file like this:

protoc --experimental_allow_proto3_optional --encode Page test.proto < test.txt > binary.data

and decode it :

protoc --experimental_allow_proto3_optional --decode Page test.proto < binary.data

The last command prints:

_html: HTML5

Apart from the enum definition, the HTML message is an Empty message , which even exists as a predefined message :

// A generic empty message that you can re-use to avoid defining duplicated
// empty messages in your APIs. A typical example is to use it as the request
// or the response type of an API method.
// The JSON representation for Empty is empty JSON object {} .
message Empty {}