Building with Confidence: Leveraging INTERFACE_COMPILE_FEATURES for Feature-Aware Compilation in CMake

What it is

• This information is especially important when other projects or targets depend on the one you're defining.
• It's used to specify the compiler features that are required when compiling a target (e.g., a library or executable) and its public headers.
• INTERFACE_COMPILE_FEATURES is a target property introduced in CMake version 3.1.

How it works

1. • You use the target_compile_features command to set this property on a target.
   • The command takes two arguments: the target name and a list of compiler features.
   • You can use the PUBLIC and INTERFACE keywords to control the scope of the features:
     • PUBLIC: The features are added to the target's COMPILE_FEATURES property, which is private to the target itself.
     • INTERFACE: The features are added to the target's INTERFACE_COMPILE_FEATURES property, which is exported and can be consumed by dependent targets.

2. Consuming the Property
   

   • When another target links against the target with INTERFACE_COMPILE_FEATURES set, CMake checks if the compiler supports the listed features.
   • If any required features are missing, CMake will raise an error during the build process. This helps ensure compatibility between the library/executable and its consumers.

Example

target_compile_features(my_library PUBLIC CXX17 INTERFACE CXX14)

# Target depending on my_library
target_link_libraries(my_program PUBLIC my_library)

• If my_program uses any features introduced in C++17, it would need to have its own COMPILE_FEATURES property set accordingly.
• In this example, my_library itself requires C++17 features for compilation, but its public headers only necessitate C++14 support for dependent targets.

Benefits

• Enhanced code maintainability: Explicitly stating required features makes code more understandable and future-proof.
• Improved build-time error checking: CMake ensures compatibility by verifying compiler feature support at build time.

Additional Notes

• You can use generator expressions (e.g., $<COMPILE_LANGUAGE> or custom expressions) within the feature list for dynamic adjustments.

---

Conditional Compilation Based on Features

This example shows how to use INTERFACE_COMPILE_FEATURES along with preprocessor macros for conditional compilation:

# Define a feature macro based on INTERFACE_COMPILE_FEATURES
target_compile_features(my_library INTERFACE CXX17)
if(TARGET_COMPILE_FEATURES_${my_library} INCLUDE CXX17)
  message(STATUS "my_library supports C++17 features")
  add_definitions(-DUSE_CXX17_FEATURE)
endif()

# Source code using the macro
add_executable(my_program main.cpp)
target_link_libraries(my_program PUBLIC my_library)

# main.cpp
#ifdef USE_CXX17_FEATURE
// Use C++17 features here
#endif

• If supported, a macro USE_CXX17_FEATURE is defined, allowing conditional code in main.cpp.
• The TARGET_COMPILE_FEATURES variable checks if my_library supports CXX17.
• Here, my_library sets CXX17 as an INTERFACE_COMPILE_FEATURE.

Optional Feature with Generator Expression

This example showcases how to handle an optional feature using a generator expression:

# Feature might be enabled based on build type
set(ENABLE_OPTIONAL_FEATURE OFF CACHE BOOL "Enable optional feature")

target_compile_features(my_library
  PUBLIC CXX11
  INTERFACE $<BOOL:${ENABLE_OPTIONAL_FEATURE}>:CXX14)

• If ENABLE_OPTIONAL_FEATURE is ON, CXX14 becomes an INTERFACE_COMPILE_FEATURE.
• The INTERFACE features depend on this variable using a generator expression.
• This defines an optional ENABLE_OPTIONAL_FEATURE variable (off by default).

Customizing Features for Different Targets

This example illustrates how to specify distinct features for different targets:

# Feature requirements for specific targets
target_compile_features(my_library INTERFACE C++11)
target_compile_features(my_executable INTERFACE C++17)

• It independently sets C++17 for my_executable.
• This assigns C++11 as an INTERFACE_COMPILE_FEATURE for my_library.

---

CMAKE_CXX_STANDARD (or equivalent for other languages)

• Drawbacks
  
  • Not as flexible: It's a global setting, not specific to individual targets.
  • Can lead to unnecessary warnings or errors if some targets require a lower standard.
  • Deprecated in newer CMake versions in favor of target_compile_features.
• This variable sets the default compiler standard for all C++ targets in your project.

Compiler-Specific Flags

• Drawbacks
  
  • Less portable: Flags differ between compilers and platforms.
  • Not as readable: The purpose of the flag might not be immediately clear.
  • Doesn't provide an easy way to check for feature support.
• You can directly set compiler-specific flags using target_compile_options or environment variables.

Custom CMake Modules

• Drawbacks
  
  • More complex to maintain and debug.
  • Reinventing the wheel when CMake already provides mechanisms like target_compile_features.
• You can create your own CMake modules that handle feature detection and conditional compilation.

• If you have a strong reason to avoid it, consider the alternatives with caution, understanding their limitations.
• For most cases, INTERFACE_COMPILE_FEATURES is the preferred method due to its clarity, portability, and ability to check for feature support.