c++if-statementc-preprocessorpreprocessor-directive

Why use preprocessor #if statements instead of if() else?

I see this being done all the time for example in the Linux kernel. What is the purpose of using the preprocessor commands vs. just normal C++ if else block? Is there a speed advantage or something?

Solution

  • A preprocessor changes the C/C++ code before it gets compiled (hence pre processor).

    Preprocessor ifs are evaluated at compile-time.

    C/C++ ifs are evaluated at run-time.

    You can do things that can't be done at run-time.

    Adjust code for different platforms or different compilers:

    #ifdef __unix__ /* __unix__ is usually defined by compilers targeting Unix systems */
#include <unistd.h>
#elif defined _WIN32 /* _Win32 is usually defined by compilers targeting 32 or 64 bit Windows systems */
#include <windows.h>
#endif

    Ensure header file definitions are included only once (equivalent of #pragma once, but more portable):

    #ifndef EXAMPLE_H
#define EXAMPLE_H

class Example { ... };

#endif

    You can make things faster than at run-time.

    void some_debug_function() {
#ifdef DEBUG
    printf("Debug!\n");
#endif
}

    Now, when compiling with DEBUG not defined (likely a command line parameter to your compiler), any calls to some_debug_function can be optimized away by the compiler.