Ensuring Compiler Compatibility with COMPILE_FEATURES in CMake

What is COMPILE_FEATURES?

In CMake, the COMPILE_FEATURES property of a target specifies the compiler features that are required or enabled when compiling the source files associated with that target. These features can include:

• Optional features
  Functionality that might not be universally supported by all compilers.
• Compiler extensions
  Features specific to a particular compiler, like vendor-specific extensions.
• Language standards
  For example, cxx_constexpr for C++11 constexpr functions.

Setting COMPILE_FEATURES

The target_compile_features command is used to manage the COMPILE_FEATURES property. You can specify features as arguments to this command, categorized as either:

• PUBLIC
  Features that other targets linking against this target must also support. These are added to the target's INTERFACE_COMPILE_FEATURES property. Other targets can then query this property to ensure compatibility.
• PRIVATE
  Features required only for the current target's compilation. These are added to the target's COMPILE_FEATURES property.

target_compile_features(my_target
  PRIVATE cxx_constexpr  # Requires C++11 compiler
  PUBLIC  cxx17        # Publicly exposes C++17 support
)

CMake's Role

• It automatically adds the necessary compiler flags (e.g., -std=c++11) to the compilation command based on the features you specify.
• CMake checks if the compiler you're using supports the requested features. If not, it generates a fatal error.

Benefits of Using COMPILE_FEATURES

• Clear communication
  Publicly exposed features help other targets understand what language standards and compiler support is necessary.
• Improved maintainability
  By explicitly declaring feature dependencies, you make the build process more transparent and easier to maintain.
• Ensures compatibility
  It guarantees that the compiler used for building the target and any targets linking against it support the required features.

• Refer to the cmake-compile-features(7) manual for a list of supported compilers, language standard flags, and more details.
• You can use generator expressions with <...> syntax in features for conditional compilation based on CMake variables or other conditions.

---

Requiring a Specific C++ Standard

This example ensures that the my_library target is compiled with a compiler that supports C++14 features:

add_library(my_library source1.cpp source2.cpp)
target_compile_features(my_library PRIVATE cxx_std_14)

Optional Feature with Preprocessor Check

This code enables range-based for loops in my_program if the compiler supports C++11 features (using a preprocessor check). Otherwise, it falls back to a traditional loop:

add_executable(my_program main.cpp)
target_compile_features(my_program PRIVATE cxx_range_for)  # Optional feature

if (CMAKE_CXX_COMPILE_FEATURES MATCHES "cxx_range_for")
  message(STATUS "Compiler supports range-based for loops.")
else()
  message(STATUS "Compiler does not support range-based for loops. Using traditional loop.")
endif()

# Code in main.cpp
#if CMAKE_CXX_COMPILE_FEATURES MATCHES "cxx_range_for"
#for (int i : some_vector) {
#  // ...
#}
#else
for (auto it = some_vector.begin(); it != some_vector.end(); ++it) {
  int i = *it;
  // ...
}
#endif

Publicly Exposing Feature Requirements

This example showcases a library (my_header_only) that only works with C++17 features and exposes this requirement to clients:

add_library(my_header_only INTERFACE my_header.hpp)
target_compile_features(my_header_only INTERFACE cxx17)

An executable linking against my_header_only must also be compiled with a C++17-supporting compiler.

Feature Detection with Generator Expressions

This example dynamically determines the required language standard based on a CMake variable:

set(MIN_REQUIRED_STANDARD 11)  # Can be set from command line

target_compile_features(my_target PRIVATE cxx_std_<${MIN_REQUIRED_STANDARD}>)

---

CMAKE_CXX_STANDARD and CMAKE_CXX_EXTENSIONS (Legacy)

These were the traditional ways to specify language standards and compiler extensions in CMake. However, they have limitations:

• They only handle a limited set of features compared to COMPILE_FEATURES.
• They are global variables, affecting all targets in your project unless overridden individually. This can be error-prone in large projects.

Preprocessor Checks (Conditional Compilation)

You can use preprocessor directives like #ifdef and #ifndef to check for specific compiler features at compile time. However, this approach has drawbacks:

• It doesn't guarantee build-time compatibility as CMake won't automatically check for compiler support.
• It requires manual checks and maintenance within your code.

Custom Compile Flags (Less Recommended)

You could directly set compile flags using target_compile_options or add_compile_options. This is not recommended as:

• It doesn't leverage CMake's feature detection capabilities.
• It requires knowledge of specific compiler flags, which can vary between compilers.

When to Consider Alternatives

• Very Simple Projects
  In very small projects with a single target and limited feature requirements, preprocessor checks might be a simpler solution. However, as your project grows, COMPILE_FEATURES becomes a more maintainable approach.
• Legacy CMake Projects
  If you're working with a very old CMake project that predates COMPILE_FEATURES, you might need to use these older methods for compatibility.