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Example Code: Using Compile Features Effectively


Compile Features in CMake

Compile features are a mechanism in CMake that allows you to specify language extensions or functionalities that your code requires during compilation. This helps CMake determine the appropriate compiler flags to enable these features while ensuring compatibility with the compiler you're using.

Key Concepts

  • Optional and Conditional Features
    You can mark compile features as optional or make their inclusion conditional based on certain conditions within your CMakeLists.txt file.
  • Language Standard Flags
    CMake automatically translates feature requirements into the corresponding compiler flags to enable the requested standard. For instance, if you specify cxx_constexpr (a C++11 feature), CMake might add -std=gnu++11 to the compilation line.
  • Feature Requirements
    You can specify the compile features your code needs using CMake commands like target_compile_features(). These features can encompass language standards (e.g., C++11, C++17) or compiler-specific extensions.

Benefits of Using Compile Features

  • Clearer Code Dependencies
    By documenting the compile features your code needs, you improve code maintainability and make it easier for others to understand your project's requirements.
  • Reduced Build Errors
    CMake's compatibility check helps you avoid errors arising from using unsupported features, saving you time and effort.
  • Improved Build Portability
    By explicitly stating your code's requirements, you ensure that your project builds correctly on different systems with compatible compilers that support the specified features.
  • The specific list of supported compile features and their corresponding compiler flags depends on the compiler you're using. CMake provides variables like CMAKE_C_KNOWN_FEATURES and CMAKE_CXX_KNOWN_FEATURES to list the known features for the C and C++ compilers, respectively.

Requiring a Specific Feature (C++11 constexpr)

add_library(mylib requires_constexpr.cpp)
target_compile_features(mylib PRIVATE cxx_constexpr)

This code defines a library named mylib that requires the cxx_constexpr feature (available in C++11 and later). The target_compile_features() command ensures the compiler uses the appropriate flag (e.g., -std=gnu++11 for GCC) to enable constexpr during compilation.

Optional Feature (C++14 Range-based for loop)

add_library(mylib optional_range_for.cpp)
target_compile_features(mylib PUBLIC OPTIONAL cxx_range_for)

This code defines a library named mylib that can optionally use C++14's range-based for loop (specified by cxx_range_for). The OPTIONAL keyword indicates the feature is not strictly required but can be used if the compiler supports it.

Conditional Feature Based on Compiler (C99 vs. C11)

if(CMAKE_C_COMPILER_ID MATCHES "GNU")
  target_compile_features(mylib PRIVATE c_variadic_macros)
else()
  target_compile_features(mylib PRIVATE c11_variadic_macros)
endif()

This code checks the compiler ID. If it's GCC, it enables c_variadic_macros (available in C99). Otherwise, it enables c11_variadic_macros (a C11 equivalent), assuming it's another compiler with different feature names or support levels.


Manual Compiler Flags

  • Cons
    • Less portable: You need to know the exact flags for different compilers.
    • Can lead to inconsistencies if you don't document the flags clearly.
    • Doesn't benefit from CMake's compatibility checks.
  • Pros
    • Fine-grained control over compiler flags.
    • May be necessary if the feature you need isn't supported by CMake's Compile Features.
  • Description
    This approach involves directly setting the compiler flags in your CMakeLists.txt using variables like CMAKE_C_FLAGS and CMAKE_CXX_FLAGS.

Preprocessor Definitions

  • Cons
    • Doesn't directly control compiler flags in the same way as Compile Features.
    • Requires more manual work within your code.
  • Pros
    • Can be used within your code to conditionally compile code based on features.
    • May be helpful for internal feature detection.
  • Description
    You can define preprocessor macros like -D flags to signal compiler support for specific features. This approach doesn't directly set compiler flags but relies on conditional compilation within your code.

Build Configuration (if applicable)

  • Cons
    • Requires managing multiple sets of compiler flags, increasing complexity.
    • Doesn't replace the concept of compile features themselves.
  • Pros
    • Can tailor builds for different purposes (e.g., enabling more warnings in Debug mode).
  • Description
    If your project uses build configurations (e.g., Debug, Release), you can set compiler flags specific to each configuration.

Choosing the Right Approach

The best approach depends on your project's needs and complexity. Compile Features offer a good balance between portability, convenience, and control. Manual flags might be necessary for very specific features not supported by CMake, but they require more attention to compatibility. Preprocessor definitions offer a way to conditionally compile code, while build configurations can be helpful for managing different build settings.

  • Build Configurations: Manage different build settings, not a replacement for Compile Features.
  • Preprocessor Definitions: Use for conditional code compilation based on features.
  • Manual Flags: Consider for very specific or unsupported features, but be cautious about portability.
  • Compile Features: Recommended for most cases due to its ease of use and portability benefits.