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Here’s our completed header file:

add.h:

// We really should have a header guard here, but will omit it for simplicity (we'll cover header guards in the next lesson)

// This is the content of the .h file, which is where the declarations go
int add(int x, int y); // function prototype for add.h -- don't forget the semicolon!
In order to use this header file in main.cpp, we have to #include it (using quotes, not angle brackets).

main.cpp:

#include "add.h" // Insert contents of add.h at this point.  Note use of double quotes here.
#include <iostream>

int main()
{
    std::cout << "The sum of 3 and 4 is " << add(3, 4) << '\n';
    return 0;
}
add.cpp:

#include "add.h" // Insert contents of add.h at this point.  Note use of double quotes here.

int add(int x, int y)
{
    return x + y;
}

When the preprocessor processes the #include "add.h" line, it copies the contents of add.h into the current file at that point. Because our add.h contains a forward declaration for function add(), that forward declaration will be copied into main.cpp. The end result is a program that is functionally the same as the one where we manually added the forward declaration at the top of main.cpp.