Duplicate symbols with header only implementation

Question

I have a C++ class that is declared and implemented in a header file. I chose this because one cannot easily move between Debug and Release builds due to _GLIBCXX_DEBU

Answer 1

inline bool operator==(const Foo& a, const Foo& b) {
    ..
  }

Member functions are implicit inline provided they are defined inside their class. The same stuff is true for them true: If they can be put into the header without hassle, you can indeed do so.

Because the code of the function is put into the header and visible, the compiler is able to inline calls to them, that is, putting code of the function directly at the call site (not so much because you put inline before it, but more because the compiler decides that way, though. Putting inline only is a hint to the compiler regarding that). That can result in a performance improvement, because the compiler now sees where arguments match variables local to the function, and where argument doesn't alias each other - and last but not least, function frame allocation isn't needed anymore.