You have only declared that the function add
exists, which is why you can call it. But you don't actually define the function anywhere.
In C++ you have to differ between declaration and definition. Sometimes those are done at the same time, like when declaring a local variable. Other times you separate them, like when having a function prototype in a header file (like you do) then that's the declaration. The definition of the function is then the implementation of the function.
After the edit, you now have another problem. And that is if you include the header file in multiple source files, each of those source files (formally known as translation units) will have the function defined and you will get an error because of that.
It can be solved by making the function static
or inline
. Or better yet, create a new source file (like add.cpp
) where you have the definition of the add
function, and keep only the declaration (prototype) in the header file.