preparing_recipes_for_conan2.md

Preparing recipes for Conan 2.0

As you may know, Conan 2.0-alpha is already released. Conan Center Index will run Conan 1.X for a long time, but it's time to start preparing recipes to make them as compatible as possible with Conan 2.0. Then, when the time comes that we update to 2.0 in Conan Center, the transition will be smooth.

For more information about Conan 2.0 breaking changes and how to prepare the migration from Conan 1.X to 2.0, please read the migration guide in the Conan documentation.

This document is a practical guide, offering extended information particular to Conan Center Index recipes to get them ready to upgrade to Conan 2.0.

New cpp_info set_property model

New Conan generators like CMakeDeps and PkgConfigDeps, don't listen to cpp_info .names, .filenames or .build_modules attributes. There is a new way of setting the cpp_info information with these generators using the set_property(property_name, value) method.

All the information in the recipes, already set with the current model, should be translated to the new model. These two models will live together in recipes to make recipes compatible with both new and current generators for some time. After a stable Conan 2.0 version is released, and when the moment arrives that we don't support the current generators anymore in Conan Center Index, those attributes (.names, .filenames etc.) will disappear from recipes, and only set_property methods will stay.

We will cover some cases of porting all the information set with the current model to the new one. To read more about the properties available for each generator and how the properties model work, please check the Conan documentation.

⚠️ Note: Please, remember that the new set_property and the current attributes model are completely independent since Conan 1.43. Setting set_property in recipes will not affect current CMake 1.X generators (cmake, cmake_multi, cmake_find_package and cmake_find_package_multi) at all.

CMakeDeps

Update required_conan_version to ">=1.43.0"

If you set the property cmake_target_name in the recipe, the Conan minimum required version should be updated to 1.43.

required_conan_version = ">=1.43.0"

class GdalConan(ConanFile):
    name = "gdal"
    ...

The reason for this change is that in Conan versions previous to 1.43 the cmake_target_name values were not the final CMake target names. Those values were completed by Conan, adding namespaces automatically the final target names. After 1.43 cmake_target_name sets the complete target name that is added to the .cmake files generated by Conan. Let's see an example:

class GdalConan(ConanFile):
    name = "gdal"
    ...
    def package_info(self):
        # Before 1.43 -> Conan adds GDAL:: namespace -> Creates target with name GDAL::GDAL
        # self.cpp_info.set_property("cmake_target_name", "GDAL")

        # After 1.43 -> Conan creates target with name GDAL::GDAL
        self.cpp_info.set_property("cmake_target_name", "GDAL::GDAL")

Translating .names information to cmake_target_name, cmake_module_target_name and cmake_file_name

To translate the .names information to the new model there are some important things to take into account:

• The value of the .names attribute value in recipes is just a part of the final target name for CMake generators. Conan will complete the rest of the target name by pre-pending a namespace (with :: separator) to the .names value. This namespace takes the same value as the .names value. Let's see an example:

class SomePkgConan(ConanFile):
    name = "somepkg"
    ...
    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "some-pkg"
        self.cpp_info.names["cmake_find_package_multi"] = "some-pkg"
        ...

This recipe generates the target some-pkg::some-pkg for both the cmake_find_package and the cmake_find_package_multi generators. Also, please remember that if no .names attribute were set, Conan would create the target somepkg::somepkg for both generators by default.

As we explained before, the cmake_target_name sets the complete target name, so, to translate this information to the new model we should add the following lines:

class SomePkgConan(ConanFile):
    name = "somepkg"
    ...
    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "some-pkg"
        self.cpp_info.names["cmake_find_package_multi"] = "some-pkg"
        # CMakeDeps does NOT add any namespace automatically
        self.cpp_info.set_property("cmake_target_name", "some-pkg::some-pkg")
        ...

• If .filenames attribute is not set, it will fall back on the .names value to generate the files. Both the Find<pkg>.cmake and <pkg>-config.cmake files that store the dependencies will take the .names value to create the complete filename. For the previous example, to translate all the information from the current model to the new one, we should have added one more line setting the cmake_file_name value.

class SomePkgConan(ConanFile):
    name = "somepkg"
    ...
    def package_info(self):
        # These generators fallback the filenames for the .cmake files
        # in the .names attribute value and generate
        self.cpp_info.names["cmake_find_package"] = "some-pkg" # generates module file Findsome-pkg.cmake
        self.cpp_info.names["cmake_find_package_multi"] = "some-pkg" # generates config file some-pkg-config.cmake

        self.cpp_info.set_property("cmake_target_name", "some-pkg::some-pkg")
        self.cpp_info.set_property("cmake_file_name", "some-pkg") # generates config file some-pkg-config.cmake
        ...

Please note that if we hadn't set the cmake_file_name property, the CMakeDeps generator would have taken the package name to generate the filename for the config file and the generated filename would have resulted somepkg-config.cmake instead of some-pkg-config.cmake.

• Some recipes in Conan Center Index define different .names values for cmake_find_package and cmake_find_package_multi. For these cases, besides cmake_target_name you should also set the cmake_module_target_name and cmake_find_mode properties. Let's see an example:

class ExpatConan(ConanFile):
    name = "expat"
    ...
    def package_info(self):
        # creates EXPAT::EXPAT target for module files FindEXPAT.cmake
        self.cpp_info.names["cmake_find_package"] = "EXPAT"
        # creates expat::expat target for config files expat-config.cmake
        self.cpp_info.names["cmake_find_package_multi"] = "expat"
        ...

Should translate to the code above. Please note we have added the cmake_find_mode property for the CMakeDeps generator with value both.

class ExpatConan(ConanFile):
    name = "expat"
    ...
    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "EXPAT"
        self.cpp_info.names["cmake_find_package_multi"] = "expat"

        # creates EXPAT::EXPAT target for module files FindEXPAT.cmake
        self.cpp_info.set_property("cmake_target_name", "EXPAT::EXPAT")
        # creates expat::expat target for config files expat-config.cmake
        self.cpp_info.set_property("cmake_module_target_name", "expat::expat")

        # generates module file FindEXPAT.cmake
        self.cpp_info.set_property("cmake_file_name", "EXPAT")
        # generates config file expat-config.cmake
        self.cpp_info.set_property("cmake_module_file_name", "expat")

        # config is the default for CMakeDeps
        # we set cmake_find_mode to both to generate both module and config files
        self.cpp_info.set_property("cmake_find_mode", "both")
        ...

⚠️ Note: There are more cases in which you probably want to set the cmake_find_mode property to both. For example, for the libraries which find modules files are included in the CMake distribution.

Translating .filenames information to cmake_file_name, cmake_module_file_name and cmake_find_mode

Like in the .names case, there are some cases in Conan Center Index of recipes that set different filenames for cmake_find_package and cmake_find_package_multi generators. To translate that information to the set_property model we have to set the cmake_file_name and cmake_find_mode properties. Let's see an example:

class GlewConan(ConanFile):
    name = "glew"
    ...
    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "GLEW"
        self.cpp_info.names["cmake_find_package_multi"] = "GLEW"
        self.cpp_info.filenames["cmake_find_package"] = "GLEW" # generates FindGLEW.cmake
        self.cpp_info.filenames["cmake_find_package_multi"] = "glew" # generates glew-config.cmake
        ...

In this case we have to set the cmake_find_mode property for the CMakeDeps generator with value both. That will make CMakeDeps generator create both module and config files for consumers (by default it generates just config files).

class GlewConan(ConanFile):
    name = "glew"
    ...
    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "GLEW"
        self.cpp_info.names["cmake_find_package_multi"] = "GLEW"
        self.cpp_info.filenames["cmake_find_package"] = "GLEW"
        self.cpp_info.filenames["cmake_find_package_multi"] = "glew"

        self.cpp_info.set_property("cmake_target_name", "GLEW::GLEW")

        self.cpp_info.set_property("cmake_file_name", "GLEW") # generates FindGLEW.cmake
        self.cpp_info.set_property("cmake_module_file_name", "glew") # generates glew-config.cmake

        # generate both modules and config files
        self.cpp_info.set_property("cmake_find_mode", "both")
        ...

Understanding some workarounds with the .names attribute model in recipes

The .names model has some limitations. Because of this, there are some recurrent workarounds in recipes to achieve things like setting absolute names for targets (without the :: namespace), or for setting a custom namespace. These workarounds can now be undone with the set_property model because it allows setting arbitrary names for CMake targets. Let's see some examples of these workarounds in recipes:

• Use of components to get arbitrary target names in recipes. Some recipes add a component whose only role is to get a target name that is not limited by the namespaces added by the current generators automatically. For example, the ktx recipe uses this workaround to get a target with name KTX::ktx.

class KtxConan(ConanFile):
    name = "ktx"
    ...

    def package_info(self):
        # changes namespace to KTX::
        self.cpp_info.names["cmake_find_package"] = "KTX"
        ...
        # the target inherits the KTX:: namespace and sets the target KTX::ktx
        self.cpp_info.components["libktx"].names["cmake_find_package"] = "ktx"
        ...
        # all the information is set via this "fake root" component
        self.cpp_info.components["libktx"].libs = ["ktx"]
        self.cpp_info.components["libktx"].defines = [
            "KTX_FEATURE_KTX1", "KTX_FEATURE_KTX2", "KTX_FEATURE_WRITE"
        ]
        ...

In these cases, the recommendation is to add the cmake_target_name property for both the root and component cpp_info. In the end the target that the consumer will get is the one created for the component, but it will avoid creating an "unwanted" target if we add the property just to the component or to the root cpp_info. Please note that when the migration to Conan 2.0 is done, there will be no need for that component anymore and it should dissapear. At that moment, the information from the component will be set in the root cpp_info and the self.cpp_info.components[] lines removed.

class KtxConan(ConanFile):
    name = "ktx"
    ...

    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "KTX"
        ...
        # FIXME: Remove the libktx component in Conan 2.0, this is just needed for
        # compatibility with current generators
        self.cpp_info.components["libktx"].names["cmake_find_package"] = "ktx"
        ...
        self.cpp_info.components["libktx"].libs = ["ktx"]
        self.cpp_info.components["libktx"].defines = [
            "KTX_FEATURE_KTX1", "KTX_FEATURE_KTX2", "KTX_FEATURE_WRITE"
        ]

        # Set the root cpp_info target name as KTX::ktx for the root and the component
        # In Conan 2.0 the component should be removed
        # and those properties should be added to the root cpp_info instead
        self.cpp_info.set_property("cmake_target_name", "KTX::ktx")
        self.cpp_info.components["libktx"].set_property("cmake_target_name", "KTX::ktx")
        ...

• Use build modules to create aliases with arbitray names for targets. Similar to the previous example, some recipes use a build module with an alias to set an arbitrary target name. Let's see the example of the tensorflow-lite recipe, that uses this workaround to define a tensorflow::tensorflowlite target.

class TensorflowLiteConan(ConanFile):
    name = "tensorflow-lite"
    ...

    def package_info(self):
        # generate the target tensorflowlite::tensorflowlite
        self.cpp_info.names["cmake_find_package"] = "tensorflowlite"
        self.cpp_info.filenames["cmake_find_package"] = "tensorflowlite"
        # this build module defines an alias tensorflow::tensorflowlite to the tensorflowlite::tensorflowlite generated target
        self.cpp_info.build_modules["cmake_find_package"] = [os.path.join(self._module_subfolder, self._module_file)]
        ...

To translate this information to the new model, just check which aliases are defined in the build modules and define those for the new model. In this case it should be enough with adding the tensorflow::tensorflowlite target with cmake_target_name to the root cpp_info (besides the cmake_file_nameproperty).

class TensorflowLiteConan(ConanFile):
    name = "tensorflow-lite"
    ...

    def package_info(self):
        self.cpp_info.names["cmake_find_package"] = "tensorflowlite"
        self.cpp_info.filenames["cmake_find_package"] = "tensorflowlite"
        self.cpp_info.build_modules["cmake_find_package"] = [os.path.join(self._module_subfolder, self._module_file)]

        # set the tensorflowlite::tensorflowlite target name directly for CMakeDeps with no need for aliases
        self.cpp_info.set_property("cmake_target_name", "tensorflow::tensorflowlite")
        self.cpp_info.set_property("cmake_file_name", "tensorflowlite")
        ...

Translating .build_modules to cmake_build_modules

Previously we saw that some recipes use a build module with an alias to set an arbitrary target name. But sometimes the declared ".build_modules" come from the original package that declares useful CMake functions, variables etc. We need to use the property cmake_build_modules to declare a list of cmake files instead of using cpp_info.build_modules:

class PyBind11Conan(ConanFile):
    name = "pybind11"
    ...

    def package_info(self):
        ...
        for generator in ["cmake_find_package", "cmake_find_package_multi"]:
            self.cpp_info.components["main"].build_modules[generator].append(os.path.join("lib", "cmake", "pybind11", "pybind11Common.cmake"))
        ...

To translate this information to the new model we declare the cmake_build_modules property in the root cpp_info object:

class PyBind11Conan(ConanFile):
    name = "pybind11"
    ...

    def package_info(self):
        ...
        self.cpp_info.set_property("cmake_build_modules", [os.path.join("lib", "cmake", "pybind11", "pybind11Common.cmake")])
        ...

PkgConfigDeps

The case of PkgConfigDeps is much more straight forward than the CMakeDeps case. This is because the current pkg_config generator suports the new set_property model for most of the properties. Then, the current model can be translated to the new one without having to leave the old attributes in the recipes. Let's see an example:

class AprConan(ConanFile):
    name = "apr"
    ...
    def package_info(self):
        self.cpp_info.names["pkg_config"] = "apr-1"
    ...

In this case, you can remove the .names attribute and just leave:

class AprConan(ConanFile):
    name = "apr"
    ...
    def package_info(self):
        self.cpp_info.set_property("pkg_config_name",  "apr-1")
    ...

For more information about properties supported by PkgConfigDeps generator, please check the Conan documentation.