Forward CMake BOOL to C++

Question

I want to be able to set a bunch of boolean flags when calling CMake, that are later used in C++ code. Something like:

set(DEBUG_ENABLE false CACHE BOOL "enable debugging")
target_compile_definitions(target PRIVATE DEBUG_ENABLE=${DEBUG_ENABLE})

This actually works fine and produces something equivalent to:

#define DEBUG_ENABLE false

However, the the native CMake literals for BOOLs are ON/OFF and tools like the CMake GUI will use those values, resulting in a define that doesn't work well in C++:

#define DEBUG_ENABLE OFF

What is the best way to forward such a boolean variable from CMake to C++?

I can think of a few things:

• Conversion function in CMake, resulting in three lines per boolean value
• Use STRING instead of BOOL
• Make it work in C++: #define ON true

None of these options seem particularly great to me.

Answer 1

I'd use configure_file. It won't get you a C++ bool directly, but it's something that will actually scale over time.

You'll need to make a config file template for cmake to modify, in this example config.hpp.in.

#cmakedefine01 DEBUG_ENABLE

Next, add a configure_file line to your CMakeLists.txt.

configure_file(config.hpp.in "${CMAKE_BINARY_DIR}/config.hpp")

Any variable in the config template will be replaced with either 0/1 depending on the value in cmake.

Working output:

(after configuring)
$ cat config.hpp 
#define DEBUG_ENABLE 0

$ cmake -DDEBUG_ENABLE=ON .
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/so/cmake-bool/build
$ cat config.hpp 
#define DEBUG_ENABLE 1

This also completely removes the need for your target_compile_definitions line, provided you #include "config.hpp" in the appropriate places.