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

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.

Object Oriented Programming A methodology of programming Four (Five ?) major principles: 1.Data Abstraction. 2.Encapsulation. 3.Information Hiding. 4.Polymorphism (dynamic binding). 5.Inheritance. (particular case of polymorphism ?) Will describe these using C++, because...

The C++ language An object-oriented, general-purpose programming language, derived from C (C++ = C plus classes). C++ adds the following to C: 1.Inlining and overloading of functions. 2.Default argument values. 3.Argument pass-by-reference. 4.Free store operators new and delete, instead of malloc() and free(). 5.Support for object-oriented programming, through classes, information hiding, public interfaces, operator overloading, inheritance, and templates.

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.

Non Object-Oriented Extensions to C Major improvements over C. 1.Stream I/O. 2.Strong typing. 3.Parameter passing by reference. 4.Default argument values. 5.Inlining. Weve discussed some of these already.

Stream I/O in C++ Input and output in C++ is handled by streams. The directive #include 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 before any characters are processed.

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 while(infile >> value) sum = sum + value; return sum; }

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; }

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); }

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.

Why use I/O streams ? (contd) 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.

C++ Strong Typing There are 6 principal situations in which C++ has stronger typing than C. 1.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();

C++ Strong Typing (contd) 2.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( ); // C++: error, f not defined // C: OK, taken to mean int f()

C++ Strong Typing (contd) 3.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.

C++ Strong Typing (contd) 4.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);

C++ Strong Typing (contd) 5.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;

C++ Strong Typing (contd) 6.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.

Object-Oriented Programming Object-oriented programming is a programming methodology characterized by the following concepts: 1.Data Abstraction: problem solving via the formulation of abstract data types (ADT's). 2.Encapsulation: the proximity of data definitions and operation definitions. 3.Information hiding: the ability to selectively hide implementation details of a given ADT. 4.Polymorphism: the ability to manipulate different kinds of objects, with only one operation. 5.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.

O-O Principles and C++ Constructs O-O ConceptC++ Construct(s) AbstractionClasses EncapsulationClasses Information HidingPublic and Private Members PolymorphismOperator overloading, templates, virtual functions InheritanceDerived Classes

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.

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

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

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

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; }

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(); }

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 }

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

Header File Partitioning

Header File Partitioning (contd)

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