Go Generated Code

This page describes exactly what Go code the protocol buffer compiler generates for any given protocol definition. Any differences between proto2 and proto3 generated code are highlighted - note that these differences are in the generated code as described in this document, not the base API, which are the same in both versions. You should read the proto2 language guide and/or the proto3 language guide before reading this document.

Compiler Invocation

The protocol buffer compiler requires a plugin to generate Go code. Install it using Go 1.16 or higher by running:

go install google.golang.org/protobuf/cmd/protoc-gen-go@latest

This will install a protoc-gen-go binary in $GOBIN. Set the $GOBIN environment variable to change the installation location. It must be in your $PATH for the protocol buffer compiler to find it.

The protocol buffer compiler produces Go output when invoked with the go_out flag. The argument to the go_out flag is the directory where you want the compiler to write your Go output. The compiler creates a single source file for each .proto file input. The name of the output file is created by replacing the .proto extension with .pb.go.

Where in the output directory the generated .pb.go file is placed depends on the compiler flags. There are several output modes:

  • If the paths=import flag is specified, the output file is placed in a directory named after the Go package's import path. For example, an input file protos/buzz.proto with a Go import path of example.com/project/protos/fizz results in an output file at example.com/project/protos/fizz/buzz.pb.go. This is the default output mode if a paths flag is not specified.
  • If the module=$PREFIX flag is specified, the output file is placed in a directory named after the Go package's import path, but with the specified directory prefix removed from the output filename. For example, an input file protos/buzz.proto with a Go import path of example.com/project/protos/fizz and example.com/project specified as the module prefix results in an output file at protos/fizz/buzz.pb.go. Generating any Go packages outside the module path results in an error. This mode is useful for outputting generated files directly into a Go module.
  • If the paths=source_relative flag is specified, the output file is placed in the same relative directory as the input file. For example, an input file protos/buzz.proto results in an output file at protos/buzz.pb.go.

Flags specific to protoc-gen-go are provided by passing a go_opt flag when invoking protoc. Multiple go_opt flags may be passed. For example, when running:

protoc --proto_path=src --go_out=out --go_opt=paths=source_relative foo.proto bar/baz.proto

the compiler will read input files foo.proto and bar/baz.proto from within the src directory, and write output files foo.pb.go and bar/baz.pb.go to the out directory. The compiler automatically creates nested output sub-directories if necessary, but will not create the output directory itself.

Packages

In order to generate Go code, the Go package's import path must be provided for every .proto file (including those transitively depended upon by the .proto files being generated). There are two ways to specify the Go import path:

  • by declaring it within the .proto file, or
  • by declaring it on the command line when invoking protoc.

We recommend declaring it within the .proto file so that the Go packages for .proto files can be centrally identified with the .proto files themselves and to simplify the set of flags passed when invoking protoc. If the Go import path for a given .proto file is provided by both the .proto file itself and on the command line, then the latter takes precedence over the former.

The Go import path is locally specified in a .proto file by declaring a go_package option with the full import path of the Go package. Example usage:

option go_package = "example.com/project/protos/fizz";

The Go import path may be specified on the command line when invoking the compiler, by passing one or more M${PROTO_FILE}=${GO_IMPORT_PATH} flags. Example usage:

protoc --proto_path=src \
  --go_opt=Mprotos/buzz.proto=example.com/project/protos/fizz \
  --go_opt=Mprotos/bar.proto=example.com/project/protos/foo \
  protos/buzz.proto protos/bar.proto

Since the mapping of all .proto files to their Go import paths can be quite large, this mode of specifying the Go import paths is generally performed by some build tool (e.g., Bazel) that has control over the entire dependency tree. If there are duplicate entries for a given .proto file, then the last one specified takes precedence.

For both the go_package option and the M flag, the value may include an explicit package name separated from the import path by a semicolon. For example: "example.com/protos/foo;package_name". This usage is discouraged since the package name will be derived by default from the import path in a reasonable manner.

The import path is used to determine which import statements must be generated when one .proto file imports another .proto file. For example, if a.proto imports b.proto, then the generated a.pb.go file needs to import the Go package which contains the generated b.pb.go file (unless both files are in the same package). The import path is also used to construct output filenames. See the "Compiler Invocation" section above for details.

There is no correlation between the Go import path and the package specifier in the .proto file. The latter is only relevant to the protobuf namespace, while the former is only relevant to the Go namespace. Also, there is no correlation between the Go import path and the .proto import path.

Messages

Given a simple message declaration:

message Foo {}

the protocol buffer compiler generates a struct called Foo. A *Foo implements the proto.Message interface.

The proto package provides functions which operate on messages, including conversion to and from binary format.

The proto.Message interface defines a ProtoReflect method. This method returns a protoreflect.Message which provides a reflection-based view of the message.

The optimize_for option does not affect the output of the Go code generator.

Nested Types

A message can be declared inside another message. For example:

message Foo {
  message Bar {
  }
}

In this case, the compiler generates two structs: Foo and Foo_Bar.

Fields

The protocol buffer compiler generates a struct field for each field defined within a message. The exact nature of this field depends on its type and whether it is a singular, repeated, map, or oneof field.

Note that the generated Go field names always use camel-case naming, even if the field name in the .proto file uses lower-case with underscores (as it should). The case-conversion works as follows:

  1. The first letter is capitalized for export. If the first character is an underscore, it is removed and a capital X is prepended.
  2. If an interior underscore is followed by a lower-case letter, the underscore is removed, and the following letter is capitalized.

Thus, the proto field foo_bar_baz becomes FooBarBaz in Go, and _my_field_name_2 becomes XMyFieldName_2.

Singular Scalar Fields (proto2)

For either of these field definitions:

optional int32 foo = 1;
required int32 foo = 1;

the compiler generates a struct with an *int32 field named Foo and an accessor method GetFoo() which returns the int32 value in Foo or the default value if the field is unset. If the default is not explicitly set, the zero value of that type is used instead (0 for numbers, the empty string for strings).

For other scalar field types (including bool, bytes, and string), *int32 is replaced with the corresponding Go type according to the scalar value types table.

Singular Scalar Fields (proto3)

For this field definition:

int32 foo = 1;
The compiler will generate a struct with an int32 field named Foo and an accessor method GetFoo() which returns the int32 value in Foo or the zero value of that type if the field is unset (0 for numbers, the empty string for strings).

For other scalar field types (including bool, bytes, and string), int32 is replaced with the corresponding Go type according to the scalar value types table. Unset values in the proto will be represented as the zero value of that type (0 for numbers, the empty string for strings).

Singular Message Fields

Given the message type:

message Bar {}
For a message with a Bar field:
// proto2
message Baz {
  optional Bar foo = 1;
  // The generated code is the same result if required instead of optional.
}

// proto3
message Baz {
  Bar foo = 1;
}
The compiler will generate a Go struct
type Baz struct {
	Foo *Bar
}

Message fields can be set to nil, which means that the field is unset, effectively clearing the field. This is not equivalent to setting the value to an "empty" instance of the message struct.

The compiler also generates a func (m *Baz) GetFoo() *Bar helper function. This function returns a nil *Bar if m is nil or foo is unset. This makes it possible to chain get calls without intermediate nil checks.

Repeated Fields

Each repeated field generates a slice of T field in the struct in Go, where T is the field's element type. For this message with a repeated field:

message Baz {
  repeated Bar foo = 1;
}

the compiler generates the Go struct:

type Baz struct {
	Foo  []*Bar
}

Likewise, for the field definition repeated bytes foo = 1; the compiler will generate a Go struct with a [][]byte field named Foo. For a repeated enumeration repeated MyEnum bar = 2;, the compiler generates a struct with a []MyEnum field called Bar.

The following example shows how to set the field:

baz := &Baz{
  Foo: []*Bar{
    {}, // First element.
    {}, // Second element.
  },
}

To access the field, you can do the following:

foo := baz.GetFoo() // foo type is []*Bar.
b1 := foo[0] // b1 type is *Bar, the first element in foo.

Map Fields

Each map field generates a field in the struct of type map[TKey]TValue where TKey is the field's key type and TValue is the field's value type. For this message with a map field:

message Bar {}

message Baz {
  map<string, Bar> foo = 1;
}

the compiler generates the Go struct:

type Baz struct {
	Foo map[string]*Bar
}

Oneof Fields

For a oneof field, the protobuf compiler generates a single field with an interface type isMessageName_MyField. It also generates a struct for each of the singular fields within the oneof. These all implement this isMessageName_MyField interface.

For this message with a oneof field:

package account;
message Profile {
  oneof avatar {
    string image_url = 1;
    bytes image_data = 2;
  }
}

the compiler generates the structs:

type Profile struct {
	// Types that are valid to be assigned to Avatar:
	//	*Profile_ImageUrl
	//	*Profile_ImageData
	Avatar isProfile_Avatar `protobuf_oneof:"avatar"`
}

type Profile_ImageUrl struct {
        ImageUrl string
}
type Profile_ImageData struct {
        ImageData []byte
}

Both *Profile_ImageUrl and *Profile_ImageData implement isProfile_Avatar by providing an empty isProfile_Avatar() method.

The following example shows how to set the field:

p1 := &account.Profile{
  Avatar: &account.Profile_ImageUrl{"http://example.com/image.png"},
}

// imageData is []byte
imageData := getImageData()
p2 := &account.Profile{
  Avatar: &account.Profile_ImageData{imageData},
}

To access the field, you can use a type switch on the value to handle the different message types.

switch x := m.Avatar.(type) {
case *account.Profile_ImageUrl:
	// Load profile image based on URL
	// using x.ImageUrl
case *account.Profile_ImageData:
	// Load profile image based on bytes
	// using x.ImageData
case nil:
	// The field is not set.
default:
	return fmt.Errorf("Profile.Avatar has unexpected type %T", x)
}

The compiler also generates get methods func (m *Profile) GetImageUrl() string and func (m *Profile) GetImageData() []byte. Each get function returns the value for that field or the zero value if it is not set.

Enumerations

Given an enumeration like:

message SearchRequest {
  enum Corpus {
    UNIVERSAL = 0;
    WEB = 1;
    IMAGES = 2;
    LOCAL = 3;
    NEWS = 4;
    PRODUCTS = 5;
    VIDEO = 6;
  }
  Corpus corpus = 1;
  ...
}

the protocol buffer compiler generates a type and a series of constants with that type.

For enums within a message (like the one above), the type name begins with the message name:

type SearchRequest_Corpus int32

For a package-level enum:

enum Foo {
  DEFAULT_BAR = 0;
  BAR_BELLS = 1;
  BAR_B_CUE = 2;
}

the Go type name is unmodified from the proto enum name:

type Foo int32

This type has a String() method that returns the name of a given value.

The Enum() method initializes freshly allocated memory with a given value and returns the corresponding pointer:

func (Foo) Enum() *Foo

The protocol buffer compiler generates a constant for each value in the enum. For enums within a message, the constants begin with the enclosing message's name:

const (
	SearchRequest_UNIVERSAL SearchRequest_Corpus = 0
	SearchRequest_WEB       SearchRequest_Corpus = 1
	SearchRequest_IMAGES    SearchRequest_Corpus = 2
	SearchRequest_LOCAL     SearchRequest_Corpus = 3
	SearchRequest_NEWS      SearchRequest_Corpus = 4
	SearchRequest_PRODUCTS  SearchRequest_Corpus = 5
	SearchRequest_VIDEO     SearchRequest_Corpus = 6
)

For a package-level enum, the constants begin with the enum name instead:

const (
	Foo_DEFAULT_BAR Foo = 0
	Foo_BAR_BELLS   Foo = 1
	Foo_BAR_B_CUE   Foo = 2
)

The protobuf compiler also generates a map from integer values to the string names and a map from the names to the values:

var Foo_name = map[int32]string{
	0: "DEFAULT_BAR",
	1: "BAR_BELLS",
	2: "BAR_B_CUE",
}
var Foo_value = map[string]int32{
	"DEFAULT_BAR": 0,
	"BAR_BELLS":   1,
	"BAR_B_CUE":   2,
}

Note that the .proto language allows multiple enum symbols to have the same numeric value. Symbols with the same numeric value are synonyms. These are represented in Go in exactly the same way, with multiple names corresponding to the same numeric value. The reverse mapping contains a single entry for the numeric value to the name which appears first in the .proto file.

Extensions (proto2)

Given an extension definition:

extend Foo {
  optional int32 bar = 123;
}

The protocol buffer compiler will generate an protoreflect.ExtensionType value named E_Bar. This value may be used with the proto.GetExtension, proto.SetExtension, proto.HasExtension, and proto.ClearExtension functions to access an extension in a message. The GetExtension function and SetExtension functions respectively accept and return an interface{} value containing the extension value type.

For singular scalar extension fields, the extension value type is the corresponding Go type from the scalar value types table.

For singular embedded message extension fields, the extension value type is *M, where M is the field message type.

For repeated extension fields, the extension value type is a slice of the singular type.

For example, given the following definition:

extend Foo {
  optional int32 singular_int32 = 1;
  repeated bytes repeated_string = 2;
  optional Bar repeated_message = 3;
}

Extension values may be accessed as:

m := &somepb.Foo{}
proto.SetExtension(m, extpb.E_SingularInt32, int32(1))
proto.SetExtension(m, extpb.E_RepeatedString, []string{"a", "b", "c"})
proto.SetExtension(m, extpb.E_RepeatedMessage, &extpb.Bar{})

v1 := proto.GetExtension(m, extpb.E_SingularInt32).(int32)
v2 := proto.GetExtension(m, extpb.E_RepeatedString).([][]byte)
v3 := proto.GetExtension(m, extpb.E_RepeatedMessage).(*extpb.Bar)

Extensions can be declared nested inside of another type. For example, a common pattern is to do something like this:

message Baz {
  extend Foo {
    optional Baz foo_ext = 124;
  }
}

In this case, the ExtensionType value is named E_Baz_Foo.

Services

The Go code generator does not produce output for services by default. If you enable the gRPC plugin (see the gRPC Go Quickstart guide) then code will be generated to support gRPC.