CMAKECXXFLAGS

CMAKE_CXX_FLAGS is a CMake cache variable that stores extra command-line options to pass to the C++ compiler for all targets built by a project. It serves as the CMake analogue to CMAKE_C_FLAGS for C, and is commonly used to apply uniform warnings, standards, or other global compiler options across targets. In modern CMake, it is considered a legacy mechanism, and per-target options are preferred, but CMAKE_CXX_FLAGS remains widely seen in older projects.

Usage and behavior: CMAKE_CXX_FLAGS is a single string. You can append flags by modifying the variable, for example using set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra"). The flags are applied to all C++ targets unless overridden by other means. For more explicit control, modern projects use target_compile_options or add_compile_options to attach flags to specific targets.

Per-configuration and generator considerations: CMake also defines per-configuration variants such as CMAKE_CXX_FLAGS_DEBUG, CMAKE_CXX_FLAGS_RELEASE, CMAKE_CXX_FLAGS_RELWITHDEBINFO, and CMAKE_CXX_FLAGS_MINSIZEREL. These are applied according to the build configuration on single-configuration generators (like Makefile or Ninja). On multi-configuration generators (such as Visual Studio or Xcode), these per-configuration variables map to the corresponding configurations, but the recommended approach is to use generator-feature aware methods like target_compile_options with configurable expressions where possible.

Examples and best practices: A typical practice in older projects is to append common flags via CMAKE_CXX_FLAGS, e.g., set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17 -Wall"). However, contemporary CMake guidelines favor setting the language standard with CMAKE_CXX_STANDARD and using target_compile_options or target_compile_features to apply flags on a per-target basis, which improves portability and maintainability.

See also: CMAKE_C_FLAGS, CMAKE_CXX_STANDARD, target_compile_options, add_compile_options.