This CMake command configures preprocessor definitions for a specific target, such as an executable or library. These definitions, effectively `-D` options passed to the compiler, allow conditional compilation based on symbols defined during the build process. For example, `target_compile_definitions(my_executable PRIVATE DEBUG_MODE=1)` would define `DEBUG_MODE` to `1` when compiling sources for the `my_executable` target, but only within that target. This differs from global definitions, which affect all targets. Scope modifiers like `PUBLIC`, `PRIVATE`, and `INTERFACE` control the visibility of the definitions to dependent targets.
Employing target-specific definitions promotes modularity and avoids unintended side effects common with globally defined symbols. Isolating definitions to the relevant target improves code clarity, maintainability, and build reproducibility. Historically, developers often relied on global definitions due to simplicity, but this approach increased the risk of conflicts and unexpected behavior across large projects. The advent of target-specific compilation settings, including the subject of this explanation, significantly improved the management of conditional compilation within CMake projects.
