Header files can run in main class. How is that possible?

Question

When I working with C programming it's normal include method is

#include<stdio.h>
int main()
{
    printf("Hello World");

    return 0;
}

Output: Hello World

Now I have tried to put my #include into my main class and its run perfectly without any errors or warnings.

int main()
{
    #include <stdio.h>
    printf("Hello World\n");

    return 0;
}

Output: Hello World

I have tried this method with C++. I couldn't do that kind of activity in there it gives me lot of errors.

Why only C have this technique?
How is that possible?

Answer 1

As seen here ,inside cdefs.h __BEGIN_DECLS is defined as

#ifdef        __cplusplus
#define __BEGIN_DECLS        extern "C" {
#define __END_DECLS        }
#else
#define __BEGIN_DECLS
#define __END_DECLS
#endif

This __BEGIN_DECLS is used in stdio.h as seen here

For C++, __BEGIN_DECLS expands to extern "C" and linkage specification should be at global scope not inside main