Setting C++ Defines with CMake

The goal

When building C++ code with CMake, it is very common to want to set some pre-processor defines in the CMake code.

For instance, we might want to set the project's version number in a single place, in CMake code like this:

This sets the CMake variable PROJECT_VERSION to 1.5, which we can then use to pass -DMYAPP_VERSION_STRING=1.5 to the C++ compiler. The about dialog of the application can then use this to show the application version number, like this:

Similarly, we might have a boolean CMake option like START_MAXIMIZED, which the user compiling the software can set to ON or OFF:

If it's ON, you would pass -DSTART_MAXIMIZED, otherwise nothing. The C++ code will then use #ifdef. (We'll see that there's a better way.)

The common (but suboptimal) solution

A solution that many people use for this is the CMake function add_definitions. It would look like this:

Technically, this works but there are a number of issues.

First, add_definitions is deprecated since CMake 3.12 and add_compile_definitions should be used instead, which allows to remove the leading -D.

More importantly, there's a major downside to this approach: changing the project version or the value of the boolean option will force CMake to rebuild every single .cpp file used in targets defined below these lines (including in subdirectories). This is because add_definitions and add_compile_definitions ask to pass -D to all cpp files, instead of only those that need it. CMake doesn't know which ones need it, so it has to rebuild everything. On large real-world projects, this could take something like one hour, which is a major waste of time.

A first improvement we can do is to at least set the defines to all files in a single target (executable or library) instead of "all targets defined from now on". This can be done like this:

We have narrowed the rebuilding effect a little bit, but are still rebuilding all cpp files in myapp, which could still take a long time.

The recommended solution

There is a proper way to do this, such that only the files that use these defines will be rebuilt; we simply have to ask CMake to generate a header with #define in it and include that header in the few cpp files that need it. Then, only those will be rebuilt when the generated header changes. This is very easy to do:

We have to write the input file, myapp_config.h.in, and CMake will generate the output file, myapp_config.h, after expanding the values of CMake variables. Our input file would look like this:

A good thing about generated headers is that you can read them if you want to make sure they contain the right settings. For instance, myapp_config.h in your build directory might look like this:

For larger use cases, we can even make this more modular by moving the version number to another input file, say myapp_version.h.in, so that upgrading the version doesn't rebuild the file with the showMaximized() code and changing the boolean option doesn't rebuild the about dialog.

If you try this and you hit a "file not found" error about the generated header, that's because the build directory (where headers get generated) is missing in the include path. You can solve this by adding set(CMAKE_INCLUDE_CURRENT_DIR TRUE) near the top of your CMakeLists.txt file. This is part of the CMake settings that I recommend should always be set; you can make it part of your new project template and never have to think about it again.

There's just one thing left to explain: what's this #cmakedefine01 thing?

If your C++ code uses #ifdef, you want to use #cmakedefine, which either sets or doesn't set the define. But there's a major downside of doing that -- if you forget to include myapp_config.h, you won't get a compile error; it will just always go to the #else code path.

We want a solution that gives an error if the #include is missing. The generated header should set the define to either 0 or 1 (but always set it), and the C++ code should use #if. Then, you get a warning if the define hasn't been set and, because people tend to ignore warnings, I recommend that you upgrade it to an error by adding the compiler flag -Werror=undef, with gcc or clang.  Let me know if you are aware of an equivalent flag for MSVC.

And these are all the pieces we need. Never use add_definitions or add_compile_definitions again for things that are only used by a handful of files. Use configure_file instead, and include the generated header. You'll save a lot of time compared to recompiling files unnecessarily.

I hope this tip was useful.

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