Is there a way to add information about the cmake build mode (debug or release) into a compiled C++ executable?

Question

Until today I have been building a C++ application in debug mode. I have now built it using release mode, and I want to assess what difference this makes to the application performance.

Having done an initial test, I see no change to the application performance.

This might be expected, since the application does a lot of work interacting with network sockets as well as parsing JSON data.

• The JSON library might have already been compiled in release mode, since I am just linking to it, the value of build flags used by cmake to build the code I have written will not have any effect on this
• Since talking to networks involves some latency, it is likely that the build mode has no or at least very little effect on the round trip time of some functions which include network waiting time

Is there a way to inject some information such that I can std::println some information when my application starts up which indicates whether cmake has been ran in debug or release mode?

I'm thinking that perhaps there might be a way to inject the value of CMAKE_CXX_FLAGS_DEBUG or CMAKE_CXX_FLAGS_RELEASE into a C++ char*, and then log one or the other depending on the build mode? But this is just an initial idea. I have no clue how to actually do it, or if this is the right approach.

Answer 1

I think @Dominik Kasewski's answer is more appropriate for what you want to do but there is also the configure_file mechanism

configure_file(myfile.h.in myfile.h)

CMake will take myfile.h.in and expand cmake variables in it:

static constexpr const char * build_type = "${CMAKE_BUILD_TYPE}";

to create a file that

static constexpr const char * build_type = "Debug";

for example.

• This is only useful if you have more than just one thing to do.
• If you have ${...} constructs in the file and dont want those expanded you can also use @CMAKE_BUILD_TYPE@ and add the @ONLY argument to configure_file
• This allows you to do more than just macros that you would set with target_compile_definitions.

Since this is more complicated than what @Dominik Kaszewski suggests, I would go with their answer but if you have more information that you want to pass from CMake to the source code, then this is a convenient way to do it.

Answer 2

What you want can be achieved with #ifdef. Since debug mode commonly sets -DDEBUG=1 (or -DDBG=1, you can easily control it either way), you can check for it to determine the mode you are in:

#ifdef DEBUG
static constexpr const char* mode = "debug";
#else
static constexpr const char* mode = "release";
#endif

printf("You are in %s mode\n", mode);

Other config options can be passed as preprocessor flags in the same way.