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 intoio/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 asQPB_<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:qt_add_qml_module(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 theqt_add_protobuf
command.qt_add_executable(targetname ... ) qt_add_protobuf(targetname QML ... )
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 theQML_URI
option to set theURI
name. It has to be specified in dotted notation, e.g.Qt.Protobuf.Package
.qt_add_executable(targetname ... ) qt_add_protobuf(targetplugin QML QML_URI proto.uri.example )
If
QML_URI
is omitted, then the protobuf package name will be used as the moduleURI
.Note: If
QML_URI
is passed toqt_add_protobuf
command buttarget
already exists,QML_URI
argument will be ignored.Note: If the
QML_URI
is skipped, all *.proto files specified in theqt_add_protobuf
command should have the sameprotobuf
package name, since it shall be used as a defaultURI
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 providedURI
import path. TheURI
option will be used in the line of the generatedqmldir
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:
// 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)
Note: Messages from test_messages
target are used 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. It's recommended to use the PUBLIC
linking scope, to have the proper INTERFACE_INCLUDE_DIRECTORIES
and INTERFACE_LINK_LIBRARIES
properties for protobuf library targets.
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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