To achieve conditional compilation in C++ based on a value from CMake, you can define a macro in your CMake configuration and then use that macro in your C++ code with preprocessor directives. Here's a step-by-step guide:
In your CMakeLists.txt
file, you can set a macro using the add_definitions
function or target_compile_definitions
if you want to set it for a specific target. For example:
# Set a macro in CMake
add_definitions(-DMY_FEATURE=1)
Or for a specific target:
# Assuming your target is named 'my_app'
target_compile_definitions(my_app PRIVATE -DMY_FEATURE=1)
In your C++ code, you can now use this macro to conditionally compile code. The macro MY_FEATURE
will be defined as 1
in this case, and you can check its value using preprocessor directives.
#include <iostream>
int main() {
#ifdef MY_FEATURE
#if MY_FEATURE == 1
std::cout << "Feature is enabled." << std::endl;
#else
std::cout << "Feature is disabled." << std::endl;
#endif
#else
std::cout << "MY_FEATURE is not defined." << std::endl;
#endif
return 0;
}
-
CMake Configuration: In
CMakeLists.txt
, you define a macro usingadd_definitions
ortarget_compile_definitions
. This macro will be passed to the compiler as a preprocessor definition. -
Conditional Compilation in C++: In your C++ source code, you use preprocessor directives (
#ifdef
,#if
,#else
,#endif
) to check whether the macro is defined and, if so, what its value is. Based on these checks, certain parts of the code will be compiled.
Suppose you have a feature that should only be included in a debug build. You can set a macro in CMake based on the build type and then use it in your C++ code to conditionally compile debug-specific code.
CMakeLists.txt
:
if(CMAKE_BUILD_TYPE STREQUAL "Debug")
add_definitions(-DDEBUG_FEATURE=1)
endif()
C++ Code:
#ifdef DEBUG_FEATURE
std::cout << "Debug feature is active." << std::endl;
#endif
In this setup, the debug feature-related code in your C++ source will only be compiled if the build type is set to Debug in CMake. This is a typical use case for conditional compilation in combination with build system configurations.