The qtprotobufgen Tool

The qtprotobufgen tool can be used to generate Qt Protobuf classes from a protobuf schema. The tool is provided by the CMake Qt6::ProtobufTools package. It works as an extension to Google's protoc tool.

find_package(Qt6 COMPONENTS ProtobufTools REQUIRED)

Usage

Qt provides CMake functions that ease the use of the qtprotobufgen tool. When using CMake as a build tool you should prefer using the Qt CMake API. For build systems other than CMake, adapt the commands described in Running manually.

CMake

The following CMake commands integrate a protobuf schema into a Qt project.

Usually qtprotobufgen would be invoked through CMake using the qt_add_protobuf macro.

Using qt_add_protobuf

cmake_minimum_required(VERSION 3.16...3.22)
project(MyThings)

find_package(Qt6 REQUIRED COMPONENTS Protobuf)
qt_standard_project_setup()

qt_add_protobuf(MyMessages
    GENERATE_PACKAGE_SUBFOLDERS
    PROTO_FILES
        path/to/message.proto
        path/to/other_message.proto
    PROTO_INCLUDES
        /path/to/proto/include
)

qt_add_executable(MyApp main.cpp)

target_link_libraries(MyApp PRIVATE MyMessages)

In the example above, we generate a library called MyMessages, which contains the message types defined in the paths passed to the PROTO_FILES option. The GENERATE_PACKAGE_SUBFOLDERS option to generate a folder structure for the generated files. And the PROTO_INCLUDES option tells protoc to look for dependencies or imports in the specified directories. We create a target for an executable called MyApp, which we link to the MyMessages library.

Running manually

protoc --plugin=protoc-gen-qtprotobuf=<path/to/bin/>qtprotobufgen \
    --qtprotobuf_out="[<options>:]<output_dir>" \
    [--qtprotobuf_opt="<options>"] \
    [-I/extra/proto/include/path] \
    <protofile>.proto

The options argument is a semicolon-separated list of Options. It can be passed in two different ways. Either by prepending to the options to the output_dir argument, delimited by a colon. Or through a separate argument, --qtprotobuf_opt. You also can pass the corresponding keys as the QT_PROTOBUF_OPTIONS environment variable. Keys need to be presented as a semicolon-separated list:

export QT_PROTOBUF_OPTIONS="COPY_COMMENTS;GENERATE_PACKAGE_SUBFOLDERS;EXTRA_NAMESPACE=MyTopLevelNamespace"

Options

The generator supports options that can be provided to tune generation. Options have direct aliases in the qt_add_protobuf function. The following options are supported:

• COPY_COMMENTS copies comments from .proto files. If provided in the parameter list, comments related to messages and fields are copied to generated header files.
• GENERATE_PACKAGE_SUBFOLDERS generates a folder structure for the generated files matching the .proto file's package name. For example, package io.qt.test; would put the generated files into io/qt/test/.
• EXTRA_NAMESPACE is an optional namespace that will be used for the generated classes. The classes are always generated in a namespace whose name is the same as the package name specified in the .proto file. If this option is used, then everything will be nested inside the extra namespace.
• EXPORT_MACRO is the base name of the symbol export macro used for the generated code. The generated macro name is constructed as QPB_<EXPORT_MACRO>_EXPORT. If the option is not set, the macro is not generated. Since Qt 6.8, EXPORT_MACRO supports the following format: EXPORT_MACRO=macro_name[:macro_output_file[:<true|false>]]. The new format allows you to set the generated macro file name and explicitly enable or disable the generation of the file containing the macro definitions. If macro_output_file is not set, the option falls back to the old syntax handling.
• QML_URI enables QProtobufMessage types in the QML context by registering them in a QML module via the provided URI import path. The URI option will be used in the line of the generated qmldir file, and also used to form the target path by replacing dots with forward slashes.

  Note: Read Identified Modules for further in-depth discussion of the URI.