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Demystifying CMake's CMP0078: Best Practices for SWIG Target Naming


Purpose

CMP0078 governs how target names are generated when using the UseSWIG command to create SWIG interface files for your project in CMake. It ensures consistency and clarity in target naming conventions.

Behavior Options

  • STANDARD (recommended)

    • Generates target names that directly correspond to the SWIG module name.
    • This provides a more intuitive and predictable naming convention.

  • OLD (deprecated)
    (Default if policy not set)

    • Relies on the UseSWIG_TARGET_NAME_PREFERENCE variable to determine the naming strategy.
    • If not set, uses a "legacy" approach where the target name might not directly reflect the module name.
    • To retrieve the actual target name, you'd need to use the SWIG_MODULE_<name>_REAL_NAME variable.

Setting the Policy

You can control the behavior using the following CMake commands in your CMakeLists.txt file:

cmake_policy(CMP0078 NEW)  # Enforce STANDARD behavior
cmake_policy(CMP0078 OLD)   # Use deprecated OLD behavior (not recommended)

Recommendation

It's generally recommended to set the policy to NEW (STANDARD behavior) for better clarity and consistency in your project's build process. This ensures that target names accurately reflect the corresponding SWIG modules.

Additional Considerations

  • If you encounter issues with existing code relying on the OLD behavior, consider temporarily setting the policy to OLD for compatibility. However, strive to migrate to the STANDARD approach for future-proofing your project.
  • CMake versions before 3.13 won't recognize CMP0078. Your project might need adjustments if targeting older CMake versions.

Scenario 1: Default (deprecated) Behavior (OLD policy)

# CMakeLists.txt

# (No explicit policy setting) - Defaults to OLD

find_package(SWIG REQUIRED)

# Module name: my_module.i
swig_add_library(my_library INTERFACE my_module.i)

# Target name might not directly reflect the module name
# (You might need to use SWIG_MODULE_my_module_REAL_NAME to retrieve it)

In this case, the target name for the generated SWIG interface file might be something like "SWIG_my_module" (depending on your project setup). The actual target name wouldn't be immediately clear from just looking at the code.

Scenario 2: Setting STANDARD Policy (recommended)

# CMakeLists.txt

cmake_policy(CMP0078 NEW)  # Enforce STANDARD behavior

find_package(SWIG REQUIRED)

# Module name: my_module.i
swig_add_library(my_library INTERFACE my_module.i)

# Target name will directly correspond to the module name ("my_library" in this case)

With the NEW policy set, the target name for the generated interface file will be "my_library," which directly reflects the module name (my_module.i). This makes the build process more intuitive and easier to understand.

  • If you encounter issues with existing code using the OLD behavior, consider temporarily setting the policy to OLD for compatibility, but aim to migrate to NEW eventually.
  • Remember to adjust the policy setting based on your project's requirements and compatibility needs. For new projects, using NEW (STANDARD) is recommended.
  • These are simplified examples. Your specific target names might vary depending on your project structure and other factors.

  1. Manual Target Naming

    • Instead of relying on the policy, you can explicitly specify the target name you want using the TARGET_NAME option within swig_add_library:

      swig_add_library(my_swig_interface INTERFACE my_module.i TARGET_NAME my_specific_target)

    This approach offers complete control over target naming, but requires you to manage it manually throughout your project.

    • If you have complex target naming requirements, you could create a custom script that generates the desired names based on your project's logic.
    • This script could be invoked before or after the swig_add_library command.
    • This approach provides flexibility but requires additional development and maintenance effort.

  3. Modern CMake Approach (CMake >= 3.13)

    • Given that CMake versions before 3.13 won't recognize CMP0078, the best alternative is to upgrade your project to use CMake 3.13 or newer.
    • With the STANDARD behavior of CMP0078 in these versions, target names automatically reflect module names, offering a clean and intuitive solution.

Choosing the Right Approach

  • If you have a strong need for custom target names or are working with older CMake versions, consider manual target naming or scripting approaches, but be aware of the increased management overhead.
  • For most projects, especially new ones, using the STANDARD behavior of CMP0078 (CMake >= 3.13) is the recommended approach. It provides clear and consistent target naming conventions.