How to Effectively Define Preprocessor Macros in CMake?

Defining Preprocessor Macros with CMake

Defining preprocessor variables with CMake has evolved over time. Let's explore how to do it effectively.

add_definitions

Previously, CMake primarily relied on the add_definitions command for this purpose. The equivalent C code would be #define foo.

add_definitions(OPENCV_VERSION=${OpenCV_VERSION} WITH_OPENCV2)

add_compile_definitions

However, starting with CMake version 3.12, the add_compile_definitions command offers a more refined approach. It specifically targets compile definitions, separating them from include directories and compiler options.

add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION})
add_compile_definitions(WITH_OPENCV2)

Benefits of add_compile_definitions

This newer approach eliminates the need for tricks often employed with add_definitions. It provides a more structured and error-prone way of managing compile definitions.

Target-Specific Definitions

As Jim Hunziker mentioned, you can also define preprocessor macros on a per-target basis. Refer to his answer for more details.

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