Comparable to the constructor, classes may define a emi(destructor).  This
function is the constructor's counterpart in the sense that it is invoked when
an object ceases to exist. A destructor is usually called automatically, but
that's not always true. The destructors of dynamically allocated objects are
not automatically activated, but in addition to that: when a program is
interrupted by an hi(exit: avoid)tt(exit) call, only the destructors of
already initialized global objects are called.  In that situation destructors
of objects defined em(locally) by functions are also em(not) called. This is
one (good) reason for avoiding tt(exit) in bf(C++) programs.

Destructors obey the following syntactical requirements:
    itemization(
    it() a destructor's name is equal to its class name prefixed by a tilde;
    it() a destructor has no arguments;
    it() a destructor has no return value.
    )
    Destructors are declared in their class interfaces. Example:
        verb(    class Strings
    {
        public:
            Strings();
            ~Strings();     // the destructor
    };)

By convention the constructors are declared first. The destructor is
declared next, to be followed by other member functions.

    A destructor's hi(destructor: main task) main task is to ensure that
memory allocated by an object is properly returned when the object ceases to
exist. Consider the following interface of the class tt(Strings):
        verbinclude(//STRINGSTORE examples/strings.h)

The constructor's task is to initialize the data fields of the object. E.g,
its constructors are defined as follows:
        verbinclude(//CONS examples/stringstore.cc)
    As objects of the class tt(Strings) allocate memory a destructor is
clearly required. Destructors may or may not be called automatically, but note
that destructors are em(only) called (or, in the case of dynamically allocated
objects: should only be called) for fully constructed objects.

bf(C++) considers objects `fully constructed' once at least one of its
constructors could normally complete. It used to be em(the) constructor, but
as bf(C++) supports constructor delegation, multiple constructors can be
activated for a single object; hence `at least one constructor'.  The
remaining rules apply to fully constructed objects;
    itemization(
    it() Destructors of local non-static objects are called automatically when
the execution flow leaves the block in which they were defined; the
destructors of objects defined somewhere in the outer block of a function are
called just before the function terminates.
    it() Destructors of static or global objects are called when the program
itself terminates.
    it() The destructor of a dynamically allocated object is called by
tt(delete) using the object's address as its operand;
    it() The destructors of a dynamically allocated array of objects are
called by tt(delete[]) using the address of the array's first element as its
operand;
    it() The destructor of an object initialized by placement tt(new) is
activated  by explicitly calling the object's destructor.
    )
    The destructor's task is to ensure that all memory that is
dynamically allocated and controlled only by the object
itself is returned. The task of the tt(Strings)'s
destructor would therefore be to delete the memory to which tt(d_string)
points. Its implementation is:
        verbinclude(//DESTR examples/stringstore.cc)

    The next example shows tt(Strings) at work. In tt(process)
a tt(Strings store) is created, and its data are displayed. It returns a
dynamically allocated tt(Strings) object to tt(main). A
tt(Strings *) receives the address of the allocated object and deletes the
object again. Another tt(Strings) object is then created in a block of
memory made available locally in tt(main), and an
    hi(destructor: explicit call)explicit call to tt(~Strings) is required
to return the memory allocated by that object. In the example only once a
tt(Strings) object is automatically destroyed: the local tt(Strings)
object defined by tt(process). The other two tt(Strings) objects require
explicit actions to prevent memory leaks.
        verbinclude(-a examples/stringstoreexample.cc)
