HP Integrity NonStop J-Series - Manual (Page 271)

//4 The function setGlobal() sets the value of g, the global RWCString , to the same value.
Now the reference count is up to four, and there is still only one copy of the string
kernel.
//5 Finally, object b tries to change the value of the string. It looks at the reference count
and sees that it is greater than one, implying that the string is being shared by more
than one object. At this point, a clone of the string is made and modified. The reference
count of the original string drops back down to three, while the reference count of the
newly cloned string is one.
A More Comprehensive Example
Because copies of RWCString s are so inexpensive, you are encouraged to store them by value
inside your objects, rather than storing a pointer. This will greatly simplify their management, as
the following comparison demonstrates. Suppose you have a window whose background and
foreground colors can be set. A simple-minded approach to setting the colors would be to use
pointers as follows:
class SimpleMinded {
const RWCString* foreground;
const RWCString* background;
public:
setForeground(const RWCString* c) {foreground=c;}
setBackground(const RWCString* c) {background=c;}
};
On the surface, this approach is appealing because only one copy of the string need be made. In
this sense, calling setForeground() seems efficient. However, a closer look indicates that the
resulting semantics can be muddled: what if the string pointed to by foreground changes? Should
the foreground color change? If so, how will class Simple know of the change? There is also a
maintenance problem: before you can delete a color string, you must know if anything is still
pointing to it.
Here's a much easier approach:
class Smart {
RWCString foreground;
RWCString background;
public:
setForeground(const RWCString& c) {foreground=c;}
setBackground(const RWCString& c) {background=c;}
Now the assignment foreground=c will use value semantics. The color that class Smart should
use is completely unambiguous. Copy on write makes the process efficient, too, since a copy of
the data will not be made unless the string should change. The next example maintains a single
copy of white until white is changed:
Smart window;