Often functions have very small function bodies. The C++ compiler can handle these more efficiently by declaring them to be inline functions, which will be expanded at the point of call rather than implemented as true function definitions.
There are two ways to define inline functions. In the first form the function body is placed directly in the class definition. In the second form the class definition is declared using the inline keyword, and the definition is placed in the interface file.
The first form tends to be more commonly used than the second.