Object-Oriented Programming

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Programming Language Principles Lecture 30 Prepared by Manuel E. Bermudez, Ph.D. Associate Professor University of Florida Object-Oriented Programming

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Object Oriented Programming Over time, data abstraction has become essential as programs became complicated. Benefits: 1. Reduce conceptual load (minimum detail). 2. Fault containment. 3. Independent program components. (difficult in practice). Code reuse possible by extending and refining abstractions.

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Object Oriented Programming A methodology of programming Four (Five ?) major principles: Data Abstraction. Encapsulation. Information Hiding. Polymorphism (dynamic binding). Inheritance. (particular case of polymorphism ?) Will describe these using C++, because ...

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The C++ language An object-oriented, general-purpose programming language, derived from C (C++ = C plus classes). C++ adds the following to C: Inlining and overloading of functions. Default argument values. Argument pass-by-reference. Free store operators new and delete, instead of malloc() and free(). Support for object-oriented programming, through classes, information hiding, public interfaces, operator overloading, inheritance, and templates.

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Design Objectives in C++ Compatibility. Existing code in C can be used. Even existing, pre-compiled libraries can be linked with new C++ code. Efficiency. No additional cost for using C++. Overheadof function calls is eliminated where possible. Strict type checking. Aids debugging, allows generation of efficient code. C++ designed by Bjarne Stroustrup of Bell Labs (now at TAMU). Standardization: ANSI, ISO.

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Non Object-Oriented Extensions to C Major improvements over C. Stream I/O. Strong typing. Parameter passing by reference. Default argument values. Inlining. We’ve discussed some of these already.

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Stream I/O in C++ Input and output in C++ is handled by streams. The directive #include <iostream.h> declares 2 streams: cin and cout. cin is associated with standard input. Extraction: operator>>. cout is associated with standard output. Insertion: operator<<. In C++, input is line buffered, i.e. the user must press <RTN> before any characters are processed.

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Example of Stream I/O in C++ A function that returns the sum of the numbers in the file Number.in int fileSum(); { ifstream infile("Number.in"); int sum = 0; int value; //read until non-integer or <eof> while(infile >> value) sum = sum + value; return sum; }

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Example of Stream I/O in C++ Example 2: A function to copy myfile into copy.myfile void copyfile() { ifstream source("myfile"); ofstream destin("copy.myfile"); char ch; while (source.get(ch)) destin<<ch; }

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Line-by-line textfile concatenation int ch; // Name1, Name2, Name3 are strings ifstream f1 (Name1); ifstream f2 (Name2); ofstream f3 (Name3); while ((ch = f1.get())!=-1 ) if (ch =='\n') while ((ch = f2.get())!=-1) { f3.put(ch); if (ch == '\n') break; } else f3.put(ch); }

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Why use I/O streams ? Streams are type safe -- the type of object being I/O'd is known statically by the compiler rather than via dynamically tested '%' fields. Streams are less error prone: Difficult to make robust code using printf. Streams are faster: printf interprets the language of '%' specs, and chooses (at runtime) the proper low-level routine. C++ picks these routines statically based on the actual types of the arguments.

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Why use I/O streams ? (cont’d) Streams are extensible -- the C++ I/O mechanism is extensible to new user-defined data types. Streams are subclassable -- ostream and istream (C++ replacements for FILE*) are real classes, and hence subclassable. Can define types that look and act like streams, yet operate on other objects. Examples: A stream that writes to a memory area. A stream that listens to external port.

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C++ Strong Typing There are 6 principal situations in which C++ has stronger typing than C. The empty list of formal parameters means "no arguments" in C++. In C, it means "zero or more arguments", with no type checking at all. Example: char * malloc();

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C++ Strong Typing (cont’d) In C, it's OK to use an undefined function; no type checking will be performed. In C++, undefined functions are not allowed. Example: main() f( 3.1415 ); // C++: error, f not defined // C: OK, taken to mean int f()

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C++ Strong Typing (cont’d) A C function, declared to be value-returning, can fail to return a value. Not in C++. Example: double foo() { /* ... */ return; } main() { if ( foo() ) { ... } ... } // C : OK // C++: error, no return value.

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C++ Strong Typing (cont’d) In C, assigning a pointer of type void* to a pointer of another type is OK. Not in C++. Example: int i = 1024; void *pv = &i; // C++: error, // explicit cast required. // C : OK. char *pc = pv; int len = strlen(pc);

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C++ Strong Typing (cont’d) C++ is more careful about initializing arrays: Example: char A[2]="hi"; // C++: error, // not enough space for '\0' // C : OK, but no '\0' is stored. It's best to stick with char A[] = "hi“;

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C++ Strong Typing (cont’d) Free store (heap) management. In C++, we use new and delete, instead of malloc and free. malloc() doesn't call constructors, and free doesn't call destructors. new and delete are type safe.

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Object-Oriented Programming Object-oriented programming is a programming methodology characterized by the following concepts: Data Abstraction: problem solving via the formulation of abstract data types (ADT's). Encapsulation: the proximity of data definitions and operation definitions. Information hiding: the ability to selectively hide implementation details of a given ADT. Polymorphism: the ability to manipulate different kinds of objects, with only one operation. Inheritance: the ability of objects of one data type, to inherit operations and data from another data type. Embodies the "is a" notion: a horse is a mammal, a mammal is a vertebrate, a vertebrate is a lifeform.

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O-O Principles and C++ Constructs O-O Concept C++ Construct(s) Abstraction Classes Encapsulation Classes Information Hiding Public and Private Members Polymorphism Operator overloading, templates, virtual functions Inheritance Derived Classes

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O-O is a different Paradigm Central questions when programming. Imperative Paradigm: What to do next ? Object-Oriented Programming What does the object do ? (vs. how) Central activity of programming: Imperative Paradigm: Get the computer to do something. Object-Oriented Programming Get the object to do something.

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C vs. C++, side-by-side

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C vs. C++, side-by-side (cont’d) In C++, methods can appear inside the class definition (better encapsulation)

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C vs. C++, side-by-side (cont’d) In C++, no explicit referencing. Could have overloaded <<, >> for Stacks: s << 1; s >> i;

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Structures and Classes in C++ Structures in C++ differ from those in C in that members can be functions. A special member function is the “constructor”, whose name is the same as the structure. It is used to initialize the object: struct buffer { buffer() {size=MAXBUF+1; front=rear=0;} char buf[MAXBUF+1]; int size, front, rear; }

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Structures and Classes in C++ The idea is to add some operations on objects of type buffer: struct buffer { buffer() {size=MAXBUF+1;front=rear=0;} char buf[MAXBUF+1]; int size, front, rear; int succ(int i) {return (i+1)%size;} int enter(char); char leave(); }

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Structures and Classes in C++ The definition (body) of a member function can be included in the structure's declaration, or may appear later. If so, use the name resolution operator (::) int buffer::enter(char x) { // body of enter } char buffer::leave() { // body of leave }

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Public and Private Members Structures and classes are closely related in C++: struct x { <member-dclns> }; is equivalent to class x { public: <member-dclns>}; Difference: by default, members of a structure are public; members of a class are private. So, class x { <member-dclns> }; is the same as struct x { private: <member-dclns> };

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Header File Partitioning

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Header File Partitioning (cont’d)

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Header File Partitioning (cont’d)

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Programming Language Principles Lecture 30 Prepared by Manuel E. Bermudez, Ph.D. Associate Professor University of Florida Object-Oriented Programming