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Using
this account class hierarchy, whenever we need an
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account
object we can use a checking, student, or savings
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object
in its place.
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This
is because every checking, student, and savings
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object
contains an account object.
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This
is the essential characteristic of a hierarchy.
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Notice
that it does not work the other way around.
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We
cannot use an account object when we need a
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checking,
student, or savings object because an account
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object
does not have the necessary data members or
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member
functions.
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If we
attempt to do that a compile error will result.
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