qt_add_protobuf

Generates Qt-based C++ source code using a protobuf schema

This command was introduced in Qt 6.5.

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

qt_add_protobuf(<target>
    PROTO_FILES <file> ...
    [PROTO_INCLUDES <path> ...]
    [QML [QML_URI <uri>]]
    [OUTPUT_DIRECTORY <dir>]
    [GENERATE_PACKAGE_SUBFOLDERS]
    [COPY_COMMENTS]
    [EXPORT_MACRO <infix>]
    [OUTPUT_HEADERS <var>]
    [OUTPUT_TARGETS <var>]
)

The source files generated by qtprotobufgen are then added to the target. If the target already exists, the files are added to the target source list. If the target doesn't exist, it is created as a library which you must link to.

Arguments

• PROTO_FILES specifies the list of .proto files used by the code generation procedure.
• PROTO_INCLUDES specifies the list of directories to be searched for protobuf dependencies.

  Note: The location of the PROTO_FILES is implicitly considered part of the protobuf include path.

• QML enables the generation of QML-compatible message types from protobuf definitions and registers them as a QML module. If qt_add_protobuf is called on a non-existent target or a target that is not a QML module, a new QML module will be created implicitly.

  qt_add_executable(target
      ...
  )
  // creates a new QML module
  qt_add_protobuf(target
      QML
      ...
  )

  If target is an existing QML module, qt_add_protobuf will attach the generated protobuf types to that module.

  qt_add_qml_module(target
      ...
  )
  // adds to existing QML module
  qt_add_protobuf(target
      QML
      ...
  )

• QML_URI defines the URI used for the QML module.

  Every QML module must define a URI, which is used in import statements to expose its protobuf types to QML.

  qt_add_protobuf(target
      QML
      QML_URI proto.uri.example
  )

  If QML_URI is omitted, the protobuf package name will be used as the module's URI.

  Note: If the QML_URI is omitted, all .proto files specified in the PROTO_FILES section must share the same protobuf package name, as it will be used as the default URI for the resulting QML module.

  Note: You should avoid creating several protobuf QML modules with the same QML_URI or proto package name, because it leads to import error in the QML context.

  Note: If QML_URI is passed to the qt_add_protobuf function but the target already exists, the QML_URI argument will be ignored.

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

• OUTPUT_DIRECTORY defines the directory where the generated files will be placed. By default, the current build directory is used.
• GENERATE_PACKAGE_SUBFOLDERS uses the package name specifier from the .proto files to create the folder structure for the generated files. For example, if the package is defined as: package io.qt.test, the generated files will be placed in OUTPUT_DIRECTORY/io/qt/test/.
• COPY_COMMENTS copies comments from the .proto files into the generated code.
• EXPORT_MACRO applies only when creating a new shared library from the <target>. This option specifies the base name for the export macro used in the generated code. The final macro name is constructed as QPB_<EXPORT_MACRO>_EXPORT. If this option is not set, the target name is used as EXPORT_MACRO.

  Read Creating Shared Libraries for further in-depth information.

• OUTPUT_HEADERS specifies a variable that will store the list of generated headers. This list can be useful for defining custom project install rules.
• OUTPUT_TARGETS specifies a variable that will store the list of generated targets. This list can be useful for defining custom project install rules.

Resolving dependencies between protobuf targets

The qt_add_protobuf command doesn't consider the dependencies between .proto files that are used to generate code for different targets.

The project may have two or more .proto files with dependencies:

// test_messages.proto
syntax = "proto3";

package test.messages;

message MyMessage {
    int32 myField = 1;
}

// test_extensions.proto
syntax = "proto3";

import "test_messages.proto";

package test.extensions;

message MyExtension {
    test.messages.MyMessage baseMessage = 1;
    int32 extension = 2;
}

The above .proto files can be used to generate the standalone libraries:

qt_add_protobuf(test_messages
    PROTO_FILES
        test_messages.proto
)
...
qt_add_protobuf(test_extensions
    PROTO_FILES
        test_extensions.proto
)
...

Since the test_extensions target depends on messages from the test_messages target, users need to link to such targets manually in their CMake scripts:

target_link_libraries(test_extensions PUBLIC test_messages)

Example

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.

QML extended protobuf example

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

find_package(Qt6 REQUIRED COMPONENTS Protobuf Quick)
qt_standard_project_setup()

qt_add_protobuf(MyMessagesPlugin
    QML
    QML_URI my.messages.module.uri
    PROTO_FILES
        path/to/message.proto
        path/to/other_message.proto
    PROTO_INCLUDES
        /path/to/proto/include
)

qt_add_protobuf(MyApp
    QML
    PROTO_FILES
        path/to/internal_message.proto
    PROTO_INCLUDES
        /path/to/proto/include
)

qt_add_qml_module(MyApp
    URI example.uri
    VERSION 1.0
    QML_FILES qml/main.qml
)

qt_add_executable(MyApp main.cpp)
target_link_libraries(MyApp PRIVATE Quick)

In the QML extended example above, by the first qt_add_protobuf call, we generate a QML module called MyMessagesPlugin, which contains the message types defined in the paths passed to the PROTO_FILES option. We use the QML option, that enables proto message types registration in the QML context. The registered types will be available in QML by importing a path that is set by the QML_URI. By second qt_add_protobuf call we add auto-generated code into the existing MyApp QML module. The QML_URI is not required in such cases. Finally, we create a target for an executable called MyApp, which has a QML module for the graphical part and loads MyMessagesPlugin into the main.qml file via the my.messages.module.uri import.

Installing standalone Qt Protobuf library

The qt_add_protobuf command also produces lists of artifacts for further installation. You can read these artifacts by specifying OUTPUT_HEADERS, and OUTPUT_TARGETS arguments as follows:

qt_add_protobuf(MyProtoLib
    PROTO_FILES
        mylib.proto
    OUTPUT_HEADERS
        public_headers
    OUTPUT_TARGETS
        generated_targets
)

The command stores the list of the header files and targets produced by the qt_add_protobuf command to the public_headers and generated_targets variables accordingly.

Use the standard CMake install command to install the artifacts and generate the config files for your library:

include(GNUInstallDirs)
set_target_properties(MyProtoLib PROPERTIES
    PUBLIC_HEADER
        "${public_headers}"
    INTERFACE_INCLUDE_DIRECTORIES
        "$<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}>"
)
install(TARGETS ${generated_targets} EXPORT MyProtoLibTargets
    PUBLIC_HEADER
        DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
)
install(EXPORT MyProtoLibTargets NAMESPACE MyProtoLib:: DESTINATION lib/cmake/MyProtoLib)

Then use the generated MyProtoLibTargets config in the package config file. You can read more about the package creation process in the official CMake documentation.

After installation the library is available as the standalone CMake package:

find_package(Qt6 COMPONENTS Protobuf)
find_package(MyProtoLib CONFIG)

add_executable(MyApp main.cpp)
target_link_libraries(MyApp PRIVATE MyProtoLib::MyProtoLib Qt6::Protobuf)