Could a very long Class function member defined in header file?

I defined a class in header file and implemented its function in same header file. I didn't put inline keyword with function definition because I think compiler will regard it as a inline function by default -- but inline is only a hint to compiler, right? What if compiler doesn't regard it as inline function because of its length? I never get error message 'multiple definitions' in reality.

struct tmp {
    void print() {
       ...(very long)
     }
};