Originally Posted by The same FAQ I linked
From a practical standpoint, this separation makes life easier and safer for your users. Say Chuck wants to simply "use" your class. Because you read this FAQ and used the above separation, Chuck can read your class's public: part and see everything he needs to see and nothing he doesn't need to see. His life is easier because he needs to look in only one spot, and his life is safer because his pure mind isn't polluted by implementation minutiae.
Back to inline-ness: the decision of whether a function is or is not inline is an implementation detail that does not change the observable semantics (the "meaning") of a call. Therefore the inline keyword should go next to the function's definition, not within the class's public: part.
NOTE: most people use the terms "declaration" and "definition" to differentiate the above two places. For example, they might say, "Should I put the inline keyword next to the declaration or the definition?" Unfortunately that usage is sloppy and somebody out there will eventually gig you for it. The people who gig you are probably insecure, pathetic wannabes who know they're not good enough to actually acomplish something with their lives, nonetheless you might as well learn the correct terminology to avoid getting gigged. Here it is: every definition is also a declaration. This means using the two as if they are mutually exclusive would be like asking which is heavier, steel or metal? Almost everybody will know what you mean if you use "definition" as if it is the opposite of "declaration," and only the worst of the techie weenies will gig you for it, but at least you now know how to use the terms correctly.