Apart from using member initializers to initialize composed objects (be they
tt(const) objects or not), there is another situation where member
initializers must be used. Consider the following situation.

A program uses an object of the class tt(Configfile), defined in tt(main)
to access the information in a configuration file. The configuration file
contains parameters of the program which may be set by changing the values in
the configuration file, rather than by supplying command line arguments.

Assume another object used in tt(main) is an object of the class tt(Process),
doing `all the work'. What possibilities do we have to tell the object of the
class tt(Process) that an object of the class tt(Configfile) exists?
    itemization(
    it() The objects could have been declared as em(global) objects. This
em(is) a possibility, but not a very good one, since all the advantages of
local objects are lost.
    it() The tt(Configfile) object may be passed to the tt(Process) object at
construction time. Bluntly passing an object (i.e., by value) might not
be a very good idea, since the object must be copied into the tt(Configfile)
parameter, and then a data member of the tt(Process) class can be used to make
the tt(Configfile) object accessible throughout the tt(Process) class. This
might involve yet another object-copying task, as in the following situation:
    verb(Process::Process(Configfile conf)   // a copy from the caller
{
    d_conf = conf;                  // copying to d_conf member
})

it() The copy-instructions can be avoided if em(pointers) to
the tt(Configfile) objects are used, as in:
    verb(Process::Process(Configfile *conf)  // pointer to external object
{
    d_conf = conf;                  // d_conf is a Configfile *
})

This construction as such is OK, but forces us to use the `tt(->)' field
selector operator, rather than the `tt(.)' operator, which is (disputably)
awkward. Conceptually one tends to think of the tt(Configfile) object as an
object, and not as a pointer to an object. In bf(C) this would probably have
been the preferred method, but in bf(C++) we can do better.
    it() Rather than using value or pointer parameters, the tt(Configfile)
parameter could be defined as a emi(reference parameter) of tt(Process)'s
constructor. Next, use a tt(Config) reference data member in the
tt(class Process).
    )
    But a reference variable cannot be initialized using an assignment, and so
the following is incorrect:
        verb(    Process::Process(Configfile &conf)
    {
        d_conf = conf;        // wrong: no assignment
    })

The statement tt(d_conf = conf) fails, because it is not an
initialization, but an assignment of one tt(Configfile) object (i.e.,
tt(conf)), to another (tt(d_conf)). An assignment to a reference variable is
actually an assignment to the variable the reference variable refers to. But
which variable does tt(d_conf) refer to? To no variable at all, since we
haven't initialized tt(d_conf).  After all, the whole purpose of the statement
tt(d_conf = conf) was to initialize tt(d_conf)....

    How to initialize tt(d_conf)? We once again use the member initializer
syntax. Here is the correct way to  initialize tt(d_conf):
        verb(    Process::Process(Configfile &conf)
    :
        d_conf(conf)      // initializing reference member
    {})

The above syntax must be used in all cases where reference data members
are used. E.g., if tt(d_ir) would have been an tt(int) reference data member,
a construction like
        verb(    Process::Process(int &ir)
    :
        d_ir(ir)
    {})

would have been required.
