How does '#include' actually work in c++?

@albert 

The #include directive in C++ is used to include the content of another file into the current file during the compilation process. It allows you to include header files or even other source files into your code.

When the preprocessor encounters an #include directive, it replaces that line with the content of the specified file. This is essentially a textual substitution process. The preprocessor takes the content of the included file and inserts it in place of the #include line.

The #include directive should be followed by the name of the file you want to include, enclosed in angle brackets (<>) for system files or double quotes ("") for user-defined files. For example:

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#include <iostream>         // Including a system header file
#include "myheader.h"       // Including a user-defined header file

After the inclusion, the compiler will see the combined content of your main file and the included file as a single unit. This way, you can access the functions, variables, and definitions from the included file in your code.

Including header files makes it easier to reuse code across multiple source files and improves code organization by separating declarations from implementations. It also allows you to access external libraries and frameworks by including their header files.

It's important to understand that the inclusion is performed during the compilation stage, not at runtime. The #include directive is a preprocessor directive that operates before the actual compilation of the code.

@albert 

Essentially, the #include directive works by telling the preprocessor to include the content of another file into the current file being compiled. This process of including external code files is beneficial for code reusability, organization, and accessing external libraries and frameworks.

When the compiler encounters an #include directive, it reads the specified file and includes its content directly in place of the #include statement. This allows you to use functions, variables, and definitions from the included file in your current file seamlessly.

It is essential to understand that the #include directive is a preprocessing directive and is processed at compile time, not runtime. The included files are typically header files containing declarations, which are necessary to properly compile and link the code in your current file, either to external libraries or to ensure correct implementation of functions within your program.

By using #include, you can modularize your code, separate interfaces from implementations, and make your codebase more maintainable and scalable.