Definitions Included

You can use the definitions-included organization for all inline functions. Place the body of the function together with a repeat (if necessary) of the prototype. The function definition may appear directly within the source file or be included with the source file

class Class {
    int method();
};
inline int Class::method() {
    return 3;
}

This organization separates interface and implementation. You can move definitions easily from header files to source files when the function is no longer implemented inline. The disadvantage is that this organization repeats the prototype of the class, which increases the bulk of source files and the chance for inconsistencies.