OO Synthetic Design Concepts & Reuse
Lecture 1: Program Fragments and Abstract Classes Topics:
Economics of software development Design for change and program families Abstract operations (and pure virtual functions) Abstract classes Composite object structures
Synthetic OO Concepts and Reuse
Foundational OO concepts:
Identity Classification Inheritance Polymorphism
Synthetic concepts: New design patterns and abstractions built by composing existing concepts in some novel way Often motivated by desire to produce assets of value to an organization
Economics of Software Development
Problem: Financing development of software systems
Big software is notoriously expensive over long term Many software projects run out of money before they ever yield a product
Question: What can a project manager do to minimize cost and maximize value?
Economics of Software Development
Problem: Financing development of software systems
Big software is notoriously expensive over long term Many software projects run out of money before they ever yield a product
Answer: The manager can...
Outsource development to cheaper programmers Buy vs. build Amortize costs over long term: Design for change Create assets that have intrinsic value and that pay dividends: Reusable libraries
Buy vs. Build (Brooks 16)
Dominant cost of software has always been development cost, not replication cost
Sharing that cost among even a few clients is a win Use of n copies of a system multiplies the productivity of the programmer by n.
Any existing product is cheaper to buy than to build from scratch
Even at $100K, a product costs only about as much as one programmer year Delivery is immediate Such products tend to be much better documented
Key issue: Applicability
Can I use an off-the-shelf product to do my task?
Economics of Software Development
Problem: Financing development of software systems
Big software is notoriously expensive over long term Many software projects run out of money before they ever yield a product Note: Assumes level of
design skill and Answer: The manager can... discipline among Outsource development to cheaper programmers developers.
Buy vs. build Amortize costs over long term: Design for change Create assets that have intrinsic value and that pay dividends: Reusable libraries
Design for Change
Idea: Amortize development costs over the long term of a project or organization Model for thinking about how to design for change based on the notion of program families: We consider a set of programs to constitute a family, whenever it is worthwhile to study programs from this set first by studying their common properties and then determining the special properties of the individual members. D.L. Parnas (TSE1976)
What Is a Program Family?
Is Netscape a program?
Netscape .9 Mosaic
Netscape 1.1
All of these are internet browsers that do about the same thing = family
Navigator 2.0 Navigator 3.0
Netscape 6.0
Netscape 1.2 Communicator 2.0 Communicator 3.0
How Are Families Developed?
Sequential-completion model of program-family evolution (Parnas, 1976)
Program fragments: incomplete programs; exist only in mind of the developer
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Initial spec
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Programs in the family
Problems with Model
Much of the important design knowledge is not represented (may even be obscured) in the code. Thus: Very expensive to produce new members of the family
Involves reverse engineering to infer designers intent
Expensive Difficult to do without introducing errors
No easy way to put a boundary around what needs to be tested
Example
void func (Item a[], unsigned n) { if (n == 1) return; for (unsigned i = n-2; i >= 0; --i) { for (unsigned j = i; j < n-1; ++j) { if (a[j] > a[j+1]) { swap(a[j], a[j+1]); } } } }
Question: What does this procedure compute?
Design Knowledge: A Terrible Thing Waste!
Idea: to Reduce cost by recording and using these intermediate program fragments
Amortizes costs over development of whole family
Dramatically reduces costs involved in reverse engineering from code to determine designers intent Dramatically reduces likelihood of introducing faults [recall that testing consumes up to 50% of development costs]
May increase cost in developing first member of the family
A savage finds his way skillfully through a wilderness by reading certain obscure indications; civilized man builds a highway which shows the road to all. John Dewey
Example Fragment
void func (Item a[], unsigned n) { if (n == 1) return;
Note: Method for "bubbling" replaced with an abstract operation
for (unsigned i = n-2; i >= 0; --i) { a := a' such that a'[0..i-1] = a[0..i-1], and a[i..n-1] and a'[i..n-1] have the same elements, but a'[i..n-1] is in sorted order } }
Abstract Operations
Invented by designer of a program fragment
Signify designers intentional deferment of detail Named and documented so as to declare what is to be done without defining how that goal is to be accomplished
Subsequent refinements must flesh out the operation
Example Fragment (More Abstract)
void func (Item a[], unsigned n) { a := a' such that a' has the same elements as a, only the elements of a' are in sorted order }
Abstract Decisions Model of ProgramFamily Evolution
Program fragments:
retained; refined to produce new fragments and programs defer implementation/design decisions to fragments or programs down the tree. abstract operations specify what, not how
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Abstract Operations in OOP
Recall: A base class might declare an operation for which there is no reasonable default method Example: Class Shape
Defines an area() operation But there is no general method for computing area of a shape
Such an operation should be declared abstract In C++, abstract operations are called pure virtual functions
Pure Virtual Function
Definition: Mechanism by which a class can declare an operation w/o providing a method
class BaseClass { public: virtual void pvf() = 0; }; class DerivedClass : public BaseClass { public: void pvf() { } };
Example: Class Shape
class Shape { public: virtual unsigned area() = 0; }; Class Rectangle : public Shape { public: Rectangle(unsigned l, unsigne...