What does slicing mean in C++?

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Quoting this lecture:

Slicing

Suppose that class D is derived from
class C. We can think of D as class C
with some extra data and methods. In
terms of data, D has all the data that
C has, and possible more. In terms of
methods, D cannot hide any methods of
C, and may have additional methods. In
terms of existing methods of C, the
only thing that D can do is to
override them with its own versions.

If x is an object of class D, then we
can slice x with respect to C, by
throwing away all of the extensions
that made x a D, and keeping only the
C part. The result of the slicing is
always an object of class C.

slicing http://webdocs.cs.ualberta.ca/~hoover/Courses/201/201-New-Notes/lectures/slides/slice/slide1.gif

Design Principle: Slicing an object
with respect to a parent class C
should still produce a well-formed
object of class C.

Usage Warning: Even though D is-a C,
you must be careful. If you have a
argument type that is a C and you
supply a D it will be sliced if you
are doing call by value, pointer, or
reference. See the example below.

Note on virtual functions. Their
signatures are used to identify which
one to execute.

Watch out for the sliced = operator,
it can make the lhs inconsistent.
Also, the operator= is never virtual,
it wouldn’t make sense. For example,
suppose classes A, B are both
subclasses of class C. Just because an
A is a C, and a B is a C, it doesn’t
mean you can assign a B object to an A
object. Without run-time type
information you cannot make a safe
assignment.

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