How to associate an enum value with a string constant in protobuf?

The association of Protobuf enumeration type and string constants

This article discusses how to define enum types in protobuf and associate a string constant for each enum value, which is convenient for use in different programming languages.

You may have defined a simple enum type, for example:

 enum Types {
  TYPE_0 = 0;
  TYPE_1 = 1;
  TYPE_2 = 2;
}

It is impossible to add string constants to enum values ​​directly in the .proto file. Protobuf itself does not directly support the explicit definition of such mapping relationships. However, the generated code provides indirect access.

Solutions for different programming languages:

The Protobuf compiler generates language-specific code based on your .proto file. These codes contain mapping relationships between enum values ​​and string names.

• C: You can use the Foo_Name(value) function (where Foo is the enum type name) to get the string name corresponding to the given enum value. If the enum value does not exist, an empty string is returned. If multiple enum values ​​correspond to the same numeric value, the first definition name is returned.

• Go: The compiler will automatically generate two mappings: Foo_name (int32 to string) and Foo_value (string to int32). You can directly use these mappings to convert enum values ​​and string constants.

• Other languages: Other languages ​​that support protobuf (such as Java, Python) also provide similar mechanisms. For specific methods, please refer to the protobuf documentation of the corresponding language.

Example (Go):

Suppose your .proto file defines the Types enumeration above. There will be a mapping similar to the following in the generated Go code:

 var Types_name = map[int32]string{
    0: "TYPE_0",
    1: "TYPE_1",
    2: "TYPE_2",
}

var Types_value = map[string]int32{
    "TYPE_0": 0,
    "TYPE_1": 1,
    "TYPE_2": 2,
}

You can use these mappings like this:

 stringValue := Types_name[Types.TYPE_1] // stringValue will be "TYPE_1"
intValue := Types_value["TYPE_0"] // intValue will be 0

Summary: Although the association between enum values ​​and string constants cannot be defined directly in the .proto file, the generated code will provide a convenient mechanism to implement this mapping, thus avoiding the work of manually maintaining the mapping table. Please refer to the protobuf documentation for the programming language you are using for specific implementation details.

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