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conditional_compilation.md

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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:

Step 1: Define a Macro in CMakeLists.txt

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)

Step 2: Use the Macro in C++ Code

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;
}

Understanding the Process

  • CMake Configuration: In CMakeLists.txt, you define a macro using add_definitions or target_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.

Example Scenario

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.