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qt_add_protobuf

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

Note: This command is in technology preview and may change in future releases.

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> ...
     [COPY_COMMENTS]
     [GENERATE_PACKAGE_SUBFOLDERS]
     [EXTRA_NAMESPACE <namespace>]
     [EXPORT_MACRO <infix>]
     [QML [QML_URI <uri>]]
     [PROTO_INCLUDES <path> ...]
     [OUTPUT_DIRECTORY <dir>]
     [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

  • 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.
  • QML enables QProtobufMessage types in QML context by registering them as a QML module. Protobuf types can extend an existing QML module. See code examples:
     qt_add_qml_module(targetname
     ...
 )

 qt_add_protobuf(targetname
     QML
     ...
 )

     qt_add_executable(targetname
     ...
 )

 qt_add_protobuf(targetname
     QML
     ...
 )

    If qt_add_protobuf is called with a non-existing target or the target is not a QML module, a new QML module will be created implicitly. In case of the new QML module creation, the qt_add_qml_module command is called inside the qt_add_protobuf command. See code example:

     qt_add_executable(targetname
     ...
 )

 qt_add_protobuf(targetplugin
     QML
     QML_URI proto.uri.example
 )

    Every QML module has to define a URI name. Other QML modules may use this name in import statements to import the module into a user application. Use the QML_URI option to set the URI name. It has to be specified in dotted notation, e.g. Qt.Protobuf.Package. If QML_URI is omitted, then the protobuf package name will be used as the module URI.

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

    Note: If the QML_URI is skipped, all *.proto files specified in the qt_add_protobuf command should have the same protobuf package name, since it shall be used as a default URI for the resulting QML module.

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

  • 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.

  • PROTO_FILES is the list of .proto files that will be used in the generation procedure.
  • PROTO_INCLUDES is the list of directories that will be searched for dependencies.
  • OUTPUT_DIRECTORY is the directory where the generated files will be put. By default, the current directory (while evaluating the function) is used.
  • OUTPUT_HEADERS can be used to specify a variable that will hold the list of headers created by the function. This list can be useful for custom project install rules.
  • OUTPUT_TARGETS can be used to specify a variable that will hold the list of targets created by the function. This list can be useful for 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:

 syntax = "proto3";

 package test.messages;

 message MyMessage {
     int32 myField = 1;
 }

 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)

Note: It's recommended to use the PUBLIC linking scope, since messages from test_messages target are referenced in header files that belong to the test_extensions target, so targets that link to test_extensions should have the test_messages target as a transitive dependency.

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)

Note: qt_add_protobuf doesn't implicitly add Qt Protobuf module as the transitive dependency, neither for the MyProtoLib target nor for the MyProtoLib CMake package. Therefore, the Qt Protobuf module lookup and the explicit linking of MyApp to Qt6::Protobuf are mandatory.

See also The qtprotobufgen Tool.

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