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Understanding CMake's GLOBAL_DEPENDS_NO_CYCLES Property


What it Does

  • Setting GLOBAL_DEPENDS_NO_CYCLES to ON prohibits any type of dependency cycle, including those involving static libraries.
  • CMake automatically analyzes these dependencies when generating the build system. By default, it allows cycles as long as they consist entirely of static libraries.
  • GLOBAL_DEPENDS_NO_CYCLES is a CMake property that enforces the absence of cycles (circular dependencies) within the overall dependency graph of your project's targets.

Why You Might Use It

  • Enforcing acyclic dependencies helps to:
    • Improve build clarity and maintainability.
    • Potentially prevent build errors or unexpected behavior that can arise from circular dependencies.
    • Simplify the build process by making it more deterministic.

Example Usage

set(GLOBAL_DEPENDS_NO_CYCLES ON)  # Disallow all dependency cycles

# Your target definitions and dependencies here
add_executable(my_executable source1.cpp source2.cpp)
target_link_libraries(my_executable another_library)

Things to Consider

  • If you encounter issues due to legitimate cycles, consider restructuring your code or using alternative approaches to manage dependencies.
  • Use this property judiciously based on your project's specific requirements.
  • While some projects benefit from acyclic dependencies, others might have legitimate reasons for having them (e.g., header-only libraries).
  • Remember that CMake properties are case-sensitive, so ensure you use the correct capitalization (GLOBAL_DEPENDS_NO_CYCLES).
  • For debugging purposes, you can use the GLOBAL_DEPENDS_DEBUG_MODE property to see detailed information about CMake's dependency graph analysis.

Scenario 1: Disallowing All Cycles (Recommended)

This example shows a project enforcing a strict no-cycle policy:

set(GLOBAL_DEPENDS_NO_CYCLES ON)

# Target definitions (acyclic)
add_library(libraryA fileA.cpp)
add_library(libraryB fileB.cpp)
add_executable(my_executable main.cpp)

target_link_libraries(my_executable libraryA libraryB)

In this case, libraryA and libraryB have a healthy dependency relationship where one doesn't depend on the other (or indirectly through other targets). They are then linked to the executable my_executable.

Scenario 2: Allowed Static Library Cycle (Default Behavior)

# No explicit GLOBAL_DEPENDS_NO_CYCLES setting (default allows static library cycles)

add_library(libraryA fileA.cpp)
add_library(libraryB fileB.cpp)

target_link_libraries(libraryA libraryB)  # Cycle here (static libraries)
target_link_libraries(my_executable libraryA)

In this scenario, libraryA and libraryB have a circular dependency, which is typically allowed for static libraries by default. However, if you set GLOBAL_DEPENDS_NO_CYCLES to ON, this would be flagged as an error.

Scenario 3: Handling Legitimate Cycles

If your project has a well-defined reason for a cycle (e.g., header-only library), you might need to adjust your approach:

Option A: Restructure Code (Preferred)

  • If possible, refactor your code to break the cycle. For example, move some common functionality to a separate header-only library that both targets can include.

Option B: Suppress Error for Specific Target (Less Preferred)

  • If refactoring isn't feasible, you can use the target_link_libraries command with the FORCE_STATIC option for specific libraries within a cycle, but use this with caution as it might lead to unexpected behavior (use only if absolutely necessary).
  • While GLOBAL_DEPENDS_NO_CYCLES can improve build clarity, it's not a silver bullet. Consider your project's specific needs and use it judiciously. If you encounter errors due to legitimate cycles, explore alternative approaches to manage dependencies.

Refactoring Code

  • This is the most recommended approach. By restructuring your code to eliminate circular dependencies, you improve project maintainability and potentially build speed. Here are some strategies:
    • Move common code to header-only libraries
      If two targets depend on each other for header files, create a separate header-only library containing the common code. This breaks the cycle as both targets can include the header-only library.
    • Extract functionality into separate libraries
      If two targets share functionality beyond headers, consider extracting that functionality into a separate library. This creates a more modular design and breaks the cycle.

Using Header-Only Libraries

  • If the reason for the cycle is purely header dependencies, consider creating header-only libraries. These libraries only provide header files for compilation and don't need linking. However, be aware that header-only libraries can increase compile times and might not be suitable for all scenarios.

Manual Dependency Management (Less Preferred)

  • This approach requires careful planning and is generally not recommended. You'd manually ensure build order and manage dependencies through file inclusion and compilation flags. This can be error-prone and difficult to maintain in complex projects.

Accepting Cycles in Specific Cases (Least Preferred)

  • If you have a very well-defined reason for a cycle (e.g., a well-designed singleton pattern), you might choose to accept it. However, proceed with caution as cycles can lead to unexpected build behavior and make debugging more challenging.
  • Accepting cycles should be a last resort and only for well-understood cases.
  • Manual dependency management is generally discouraged due to complexity and error-proneness.
  • Consider header-only libraries if the cycle involves pure header dependencies.
  • Always prioritize refactoring to eliminate cycles whenever possible. This provides the most maintainable and potentially fastest build solution.