Precompilation Directives in C++

In C++, preprocessor directives are special instructions that the preprocessor, a stage prior to compilation, uses to modify the source code before the compiler processes it.

Preprocessor directives are very powerful tools for managing code configuration. However, it is also very common for them to be overused.

Syntax of Preprocessor Directives

Preprocessor directives in C++ are commands that begin with the # symbol and are interpreted by the preprocessor before the code is compiled.

Inclusion Directives

The #include directive is used to include header files or other source files in the current file.

#include <file.h>  // Includes standard header files
#include "file.h"  // Includes local files

#include <iostream>  // Inclusion of the standard input/output library

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

Compilation Conditions

These directives #ifdef, #ifndef, #if, #else, #elif, #endif are used to include or exclude parts of the code depending on certain conditions.

#define DEBUG

int main() {
#ifdef DEBUG
    std::cout << "Debug mode activated" << std::endl;
#endif
    return 0;
}

Macro Definition

The #define directive is used to define macros, which are code snippets or constants that are substituted during the preprocessing stage.

#define MACRO_NAME value

While the #undef directive is used to undefine a macro.

#undef MACRO_NAME