What is #include in C++?

In C++, #include is a preprocessor directive used to include the contents of a file (usually a header file) in the program before compilation. This allows you to access functions, classes, and other definitions from libraries and modules without having to redefine them.

Key Points about #include:

1. Header Files: The most common use of #include is to include standard library header files (e.g., <iostream>, <vector>, <string>) or user-defined header files. For example:

   #include <iostream>  // Standard library for input-output operations
   #include "myHeader.h"  // User-defined header file

2. Angle Brackets vs. Quotes:

   • Angle Brackets (<>): Used for including standard library headers or system files. The compiler searches for these files in standard library directories.
   • Quotes (""): Used for including user-defined header files. The compiler first searches in the current directory and then in standard directories.

3. Compile-Time Inclusion: The contents of the included file are copied into the source file at the point where #include is used, allowing the program to utilize the definitions from the included file.

4. Avoiding Multiple Inclusions: To prevent multiple inclusions of the same header file, which can lead to errors, it's common to use include guards or #pragma once:

   // Using include guards
   #ifndef MYHEADER_H
   #define MYHEADER_H
   // Declarations
   #endif

5. Library Functions: Including libraries using #include gives you access to pre-written functions and classes, facilitating code reuse and improving development efficiency.

Example Usage:

Here’s a simple example that demonstrates the use of #include:

#include <iostream>  // Include the iostream library

using namespace std;

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

Conclusion:

The #include directive is fundamental in C++ programming, allowing developers to include and utilize code from external libraries and their own header files, thus promoting modular programming and code reuse.

Sources:

• GeeksforGeeks: C++ #include
• Cplusplus.com: Preprocessor Directives
• Programiz: C++ Header Files