It’s critical that you be able to hide the underlying implementation of a class because you may want to change that implementation sometime later. You’ll make these changes for efficiency, or because you get a better understanding of the problem, or because some new class becomes available that you want to use in the implementation. Anything that jeopardizes the privacy of the underlying implementation reduces the flexibility of the language. Thus, the inline function is very important because it virtually eliminates the need for preprocessor macros and their attendant problems. With inlines, member functions can be as efficient as preprocessor macros.

The inline function can be overused in class definitions, of course. The programmer is tempted to do so because it’s easier, so it will happen. However, it’s not that big of an issue because later, when looking for size reductions, you can always change the functions to non-inlines with no effect on their functionality. The development guideline should be “First make it work, then optimize it.”