#ifdef

These directives allow you to include or exclude code based on certain conditions

If we include a header file 2 times (maybe in multiple files), we will get error saying multiple declaration found.

To overcome this, we need to ignore multiple inclusion, we define a macro. which we check if ifdef and ignore inclusion if already incuded.

Here is an example:

#define DEBUG

#ifdef DEBUG
    std::cout << "Debug mode is on" << std::endl;
#endif

#ifndef NDEBUG
    // Code for debug builds
#else
    // Code for release builds
#endif

Or another example, we check system config, if system is unix we use cls and if it us linux we use clear and many more.

#ifdef _WIN32
    #define CLEAR_SCREEN "cls"
#else
    #define CLEAR_SCREEN "clear"
#endif

#if defined(__cplusplus) && __cplusplus >= 201703L
    // Use C++17 features
#elif defined(__cplusplus) && __cplusplus >= 201402L
    // Use C++14 features
#else
    #error "This program requires at least C++14"
#endif