Scope and Members Defined outside the Class

The fact that a class is a scope explains why we must provide the class name as well as the function name when we define a member function outside its class (§ 7.1.2, p. 259). Outside of the class, the names of the members are hidden.

Once the class name is seen, the remainder of the definition—including the parameter list and the function body—is in the scope of the class. As a result, we can refer to other class members without qualification.

For example, recall the clear member of class Window_mgr (§ 7.3.4, p. 280). That function’s parameter uses a type that is defined by Window_mgr:

void Window_mgr::clear(ScreenIndex i)
{
    Screen &s = screens[i];
    s.contents = string(s.height * s.width, ' '),
}

Because the compiler sees the parameter list after noting that we are in the scope of class Window_mgr, there is no need to specify that we want the ScreenIndex that is defined by Window_mgr. For the same reason, the use of screens in the function body refers to name declared inside class Window_mgr.

On the other hand, the return type of a function normally appears before the function’s name. When a member function is defined outside the class body, any name used in the return type is outside the class scope. As a result, the return type must specify the class of which it is a member. For example, we might give Window_mgr a function, named addScreen, to add another screen to the display. This member will return a ScreenIndex value that the user can subsequently use to locate this Screen:

class Window_mgr {
public:
    // add a Screen to the window and returns its index
    ScreenIndex addScreen(const Screen&);
    // other members as before
};
// return type is seen before we're in the scope of Window_mgr
Window_mgr::ScreenIndex
Window_mgr::addScreen(const Screen &s)
{
    screens.push_back(s);
    return screens.size() - 1;
}

Because the return type appears before the name of the class is seen, it appears outside the scope of class Window_mgr. To use ScreenIndex for the return type, we must specify the class in which that type is defined.