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Continuing our discussion of the tt(NavCom) class, introduced in section
ref(MULTIPLE), we now define two objects, a base class and a derived class
object:
verb(
ComSet com(intercom);
NavComSet navcom(intercom2, dial2);
)
The object tt(navcom) is constructed using an tt(Intercom) and a
tt(VHF_Dial) object. However, a tt(NavComSet) is at the same time a
tt(ComSet), allowing the i(assignment) em(from) tt(navcom) (a derived class
object) em(to) tt(com) (a base class object):
verb(
com = navcom;
)
The effect of this assignment is that the object tt(com) now
communicates with tt(intercom2). As a tt(ComSet) does not have a tt(VHF_Dial),
the tt(navcom)'s tt(dial) is ignored by the assignment. When assigning a
base class object from a derived class object only the base class data members
are assigned, other data members are dropped, a phenomenon called
emi(slicing). In situations like these slicing probably does not have serious
consequences, but when passing derived class objects to functions defining
base class parameters or when returning derived class objects from functions
returning base class objects slicing also occurs and might have unwelcome
side-effects.
The assignment from a base class object to a derived class object is
problematic. In a statement like
verb(
navcom = com;
)
it isn't clear how to reassign the tt(NavComSet)'s tt(VHF_Dial) data
member as they are missing in the tt(ComSet) object tt(com). Such an
assignment hi(assignment: refused) is therefore refused by the
compiler. Although derived class objects are also base class objects, the
reverse does not hold true: a base class object is not also a derived class
object.
The following general rule applies: in assignments in which base class
objects and derived class objects are involved, assignments in which data are
dropped are legal (called em(slicing)). Assignments in which data remain
unspecified are em(not) allowed. Of course, it is possible to overload an
assignment operator to allow the assignment of a derived class object from a
base class object. To compile the statement
verb(
navcom = com;
)
the class tt(NavComSet) must have defined an overloaded assignment
operator accepting a tt(ComSet) object for its argument. In that case it's up
to the programmer to decide what the assignment operator will do with the
missing data.
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