How to Define Preprocessor Macros in CMake Using the Recommended Approach?

Defining Preprocessor Macros in CMake

Often, developers require preprocessor macros to be set for their CMake projects. Traditionally, CMake provided the add_definitions command for this purpose. However, since CMake version 3.12, a new approach has been introduced with separate commands for compile definitions, include directories, and compiler options.

Using the New Approach

To define a preprocessor macro using the new syntax, use the add_compile_definitions command as follows:

add_compile_definitions(MACRO_NAME=VALUE)

For example, to define the OPENCV_VERSION macro, you would write:

add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION})

You can also define multiple macros in a single command:

add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION} WITH_OPENCV2)

Advantages

This new approach provides several advantages over the add_definitions command:

• It provides greater control over the definitions that are applied to different parts of your project.
• It avoids the potential for defining macros that are not intended for a particular target.
• It aligns with the best practices for CMake development, as discussed in the CMake documentation (https://cmake.org/cmake/help/latest/command/add_definitions.html).

Note: As noted in the documentation, it is not recommended to use the add_definitions command anymore.

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