OOSC - Summer Semester 2004 - 21 1 Chair of Software Engineering Object-Oriented Software Construction Bertrand Meyer
Chair of Software Engineering OOSC - Summer Semester 2004 - 21 1 Object-Oriented Software Construction Bertrand Meyer.
Dec 21, 2015
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OOSC - Summer Semester 2004 - 21
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Chair of Software Engineering
Object-Oriented Software Construction
Bertrand Meyer
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Chair of Software Engineering
Lecture 21:
Typing issues, covariance
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The America Conjecture
(Pierre America at TOOLS Europe 1990)
One can have at most two of the three properties of: Static typing. Substitutivity. Covariance.
Can we disprove the America conjecture?
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Typing: A simple problem!
The basic operation of object-oriented computation:
x.f (arg)
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The typing problem
When, and how, do we know that:
There is a feature applicable to OBJ and corresponding to f?
arg is an acceptable argument for that feature?
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Terminology
x: Entity.OBJ: The object attached to x at some time during
execution.f: Routine (One of the two forms of feature;
the other is attributes.)
x OBJ
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Typing vs. binding
What do we know about the feature to be called?
Static typing:
At least one
Dynamic binding:
The right one
Example:
my_aircraft.lower_landing_gear
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Inheritance and typing
AIRCRAFT*
PLANE*
COPTER*
BOEING AIRBUS
A_320B_747B_737
B_747_400
* deferred+ effected++ redefined
lower_landing_gear*
lower_landing_gear++
lower_landing_gear+
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Cost of correcting errors (Boehm)
Requirements Design Code Development Acceptance Operationtest test
SMALL PROJECTS
LARGE PROJECTS
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Typing rules (all locally checkable)
Declaration rule. Call rule. Attachment rule.
(For the precise formulations see Eiffel: The Language.)
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Declaration rule
Every entity must be declared as being of a certain type.
For example:
x: AIRCRAFT
n: INTEGER
ba1: BANK_ACCOUNT
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Call rule
If a class C contains the call
x.f
there must be a feature of name f in the base class of the type of x, and that feature must be available (exported) to C.
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Attachment rule
In an assignment x := y, or the corresponding argument passing, the base class of the type of y must be a descendant of the base class of the type of x.
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For typing to be acceptable
No exception to type ee-rules (“casts”).
Multiple inheritance
Unconstrained genericity:
class LIST [G] ...
Constrained genericity:
class VECTOR [G –> NUMERIC] ...
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For typing to be acceptable (cont’d)
Assignment attempt:
x ?= y
Contracts, to clarify the semantics and include constraints other than typing (e.g. numerical ranges).
Covariance for routine arguments.
Anchored declarations (like x) to avoid endless redeclarations.
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Multiple inheritance
DOCUMENT MESSAGE
MAILABLE_ DOCUMENT
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A class hierarchy
DRIVER
BIKERTRUCKER
HEAVY_TRUCKER
partner: DRIVER
share (other: DRIVER)
partner++
share++
partner++
share++
partner++
share++
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Choosing a partner
class DRIVER feature
partner: DRIVER-- This driver’s alternate
share (other: DRIVER) is-- Choose other as alternate.
requireother /= Void
dopartner := other
end
...
end
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d1, d2: DRIVER...d1.share (d2)
A typical call
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Specializing
class TRUCKER inherit
DRIVERredefine
partnerend
feature
partner: TRUCKER-- This driver’s alternate.
...
end
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The need for covariance
class TRUCKER inherit
DRIVERredefine
partner,share
end
feature
partner: TRUCKER-- This driver’s alternate.
share (other: TRUCKER) is-- Choose other as alternate.
requireother /= Void
dopartner := other
end
end
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Direct covariance
DRIVER
BIKERTRUCKER
HEAVY_TRUCKER
partner: DRIVER
share (other: DRIVER)
partner++
share++
partner++
share++
partner++
share++
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Anchored declarations:
class TRUCKER inherit
DRIVERredefine
partner,share
end
feature
partner: TRUCKER-- This driver’s alternate.
share (other: like partner) is-- Choose other as alternate.
requireother /= Void
dopartner := other
end
end
Avoiding redefinition avalanche
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End of lecture 21
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