Are there ant special rules about inlining?

Yes, there are a few rules about inlining –
a.) Any source files that used the inline function must contain the function’s definition.
b.) An inline function must be defined everywhere. The easier way to deal with this to define the function once in the class header file and include the definition as required. The harder way is to redefine the function everywhere and learn the one-definition rule.
c.) Main() can not be inline.