Many classes (e.g., tt(std::string)) offer ti(swap) members allowing us to
swap two of their objects. The em(Standard Template Library) (STL, cf. chapter
ref(STL)) offers various functions related to swapping. There is even a
tt(swap) em(generic algorithm) (cf. section ref(SWAP)), which is commonly
implemented using the assignment operator. When implementing a tt(swap) member
for our class tt(Strings) it could be used, provided that all of tt(String)'s
data members can be swapped. As this is true (em(why) this is true is
discussed shortly) we can augment tt(class Strings) with a tt(swap) member:
        verb(    void Strings::swap(Strings &other)
    {
        std::swap(d_string, other.d_string);
        std::swap(d_size, other.d_size);
    })

Having added this member to tt(Strings) the copy-and-swap implementation
of tt(String::operator=) can now be used.

    When two variables (e.g., tt(double one) and tt(double two)) are swapped,
each one holds the other one's value after the swap. So, if tt(one == 12.50)
and tt(two == -3.14) then after tt(swap(one, two) one == -3.14) and tt(two ==
12.50).

    Variables of primitive data types (pointers and the built-in types) can be
swapped, class-type objects can be swapped if their classes offer a tt(swap)
member.

    So should we provide our classes with a swap member, and if so, how should
it be implemented?

    The above example (tt(Strings::swap)) shows the standard way to implement
a tt(swap) member: each of its data members are swapped in turn. But there are
situations where a class cannot implement a swap member this way, even if the
class only defines data members of primitive data types. Consider the
situation depicted in figure ref(SWAPLIST).

    figure(memory/fswap)(Swapping a linked list)(SWAPLIST)

In this figure there are four objects, each object has a pointer pointing to
the next object. The basic organization of such a class looks like this:
        verb(    class List
    {
        List *d_next;
        ...
    };)

Initially four objects have their tt(d_next) pointer set to the next
object: 1 to 2, 2 to 3, 3 to 4. This is shown in the upper half of the
figure. At the bottom half it is shown what happens if objects 2 and 3 are
swapped: 3's tt(d_next) point is now at object 2, which still points to 4; 2's
tt(d_next) pointer points to 3's address, but 2's tt(d_next) is now at object
3, which is therefore pointing to itself. Bad news!

Another situation where swapping of objects goes wrong happens with classes
having data members pointing or referring to data members of the same
object. Such a situation is shown in figure ref(SWAPSELF).

  figure(memory/fswap2)(Swapping objects with self-referential data)(SWAPSELF)

Here, objects have two data members, as in the following class setup:
        verb(    class SelfRef
    {
        size_t *d_ownPtr;       // initialized to &d_data
        size_t d_data;
    };)

The top-half of figure ref(SWAPSELF) shows two objects; their upper data
members pointing to their lower data members. But if these objects are swapped
then the situation shown in the figure's bottom half is encountered. Here the
values at addresses a and c are swapped, and so, rather than pointing to their
bottom data members they suddenly point to other object's data members. Again:
bad news.

The common cause of these failing swapping operations is easily recognized:
simple swapping operations must be avoided when data members point or refer to
data that is involved in the swapping. If, in figure ref(SWAPSELF) the a and c
data members would point to information outside of the two objects (e.g., if
they would point to dynamically allocated memory) then the simple swapping
would succeed.

However, the difficulty encountered with swapping tt(SelfRef) objects does not
imply that two tt(SelfRef) objects cannot be swapped; it only means that we
must be careful when designing tt(swap) members. Here is an implementation of
tt(SelfRef::swap):
        verb(    void SelfRef::swap(SelfRef &other)
    {
        swap(d_data, other.d_data);
    })

In this implementation swapping leaves the self-referential data member
as-is, and merely swaps the remaining data. A similar tt(swap) member could be
designed for the linked list shown in figure ref(SWAPLIST).
