#pragma once in main file what is it for

Explanation:

The #pragma once directive in C++ is a non-standard preprocessor directive that is used to prevent a header file from being included multiple times in a translation unit (source file).

When #pragma once is included at the beginning of a header file, it serves as an include guard. It ensures that the contents of the header file are only included once during the compilation process, even if the header file is included multiple times in different source files.

Here is a step-by-step explanation of how #pragma once works:

1. The #pragma once directive is typically placed at the beginning of a header file, before any other content.

2. When the preprocessor encounters #pragma once, it marks the current header file as "seen" and remembers its unique identifier.

3. When the preprocessor sees an #include directive that references the same header file later in the compilation process, it checks if the header file has already been included before.

4. If the header file has already been included before (based on its unique identifier), the preprocessor skips the inclusion of the header file and continues with the next line of code.

5. If the header file has not been included before, the preprocessor includes the content of the header file at the location of the #include directive, and marks the header file as "seen".

By using #pragma once, you can effectively prevent circular dependencies and avoid redefinition errors that can occur when a header file is included multiple times.

Example:

Consider the following example:

// myfile.h
#pragma once

int add(int a, int b);

// myfile.cpp
#include "myfile.h"

int add(int a, int b) {
   return a + b;
}

// main.cpp
#include "myfile.h"
#include <iostream>

int main() {
   int result = add(2, 3);
   std::cout << "Result: " << result << std::endl;
   return 0;
}

In this example, the #pragma once directive in myfile.h ensures that the contents of the header file are only included once, even though it is included in both myfile.cpp and main.cpp. This prevents any duplication or redefinition errors.

Note that while #pragma once is widely supported by most compilers, it is not part of the official C++ standard. However, it is supported by most major compilers and is widely used in practice.