#if defined ||

`#if defined` is a preprocessor directive in C/C++, used to check if a macro has been defined or not. If the macro has been defined, it evaluates to true, otherwise false. The syntax is as follows:

#if defined MACRO_NAME
    // code to be executed if MACRO_NAME is defined
#else
    // code to be executed if MACRO_NAME is not defined
#endif

For example:

#define DEBUG_MODE

#if defined DEBUG_MODE
    printf("Debug mode is enabled\n");
#else
    printf("Debug mode is disabled\n");
#endif

In this example, since `DEBUG_MODE` has been defined, the first block of code will be executed, printing "Debug mode is enabled" to the console.