Plan Review of variable types C++: Tour 2 Functions But first - - PDF document

C++: Tour 2 Plan

• Review of variable types
• Functions

But first…

• UML tidbits

– Use cases – please include documentation – Associations vs. Attributes – Seq Diagrams in Logical View

Projects

• Any questions?
• Design due March 28th

– Online docs of shapewin library on Web – Can include shapewin classes in UML

• Partners…anyone doesn’t have one?

Finally…

• Special thanks to Prof Swammi for taking

my lecture on Thursday

• Any questions?

Variables in C++

• Variable types

– Basic – Pointer – Reference

Variables in C++

• Basic variable

– Memory associated with a variable with size based on the type of the variable. – Variable declarations are “executable” statements

• Memory is allocated when declaration is made

– No need to use new when instantiating objects

• f a class.

Variables in C++

Basic variable

int foo; float f = 7.0; Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”);

Variables in C++

• Assignment = copy not reference

– In Java

• Student joe = new Student(“Geigel” , “Joe”,

“GCCIS”, “CS”);

• Student fred = joe;

Joe Geigel joe fred

Variables in C++

• Assignment = copy not reference

– In C++

• Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”);
• Student fred = joe;

Joe Geigel joe fred Joe Geigel

Variables in C++

• Basic variable

– Accessing class members

• Uses the . Syntax used in Java
• Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”);
• joe.getGrades();

Variables in C++

• Pointer Variables

– Stores the memory address of an object. – new returns a pointer to an object and allocates memory for it on the heap (free store). – Can have pointers to basic data types. – C++ has no garbage collection! – NULL pointer takes value 0.

Variables in C++

Pointer variable

int *foo; float *f = 7.0; // Invalid float *g = 0; // okay float *h = 0x12345; // actually illegal!! Student *joe = new Student (“Geigel” , “Joe”, “GCCIS”, “CS”);

Variables in C++

• Pointer Variables

– Dereference operator * – If ptr is a pointer

• i.e A variable whose contents is a memory address

– then *ptr refers to the object or data item that is pointed to by ptr

• Can be interpreted as:

– The data item or object at ptr – The object or data item pointed to by ptr

Variables in C++

Pointer variable

float *f = 7.0; // Invalid however float *f = new float; (*f) = 7.0; Student *joe = new Student (“Geigel” , “Joe”, “GCCIS”, “CS”); Student *fred = joe;

Variables in C++

• Address of operator

– You can always get the address of any variable

• r object by using the address of operator &.
• float f = 7.0;
• float *fptr = &f;

– Use with caution!!

• Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”);
• Student *joeptr = &joe;

Variables in C++

• Assignment = copy not reference

– For Pointer Variables as well

• Student *joe = new Student(“Geigel” , “Joe”,

“GCCIS”, “CS”);

• Student *fred = joe;

Joe Geigel

0x345ABC2

joe

0x345ABC2

fred 0x345ABC2

Variables in C++

• Pointer variable

– Accessing class members

• Uses the -> Syntax
• Student *joe = new Student (“Geigel” , “Joe”,

“GCCIS”, “CS”);

• joe->getGrades();
• Which is the same as
• (*joe).getGrades();

Variables in C++

• Reference Variables

– Alias for an already existing object – Usually used to pass function arguments by reference. – Cannot change what they point to

Variables in C++

• Reference Variables
• Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”);
• Student &fred = joe;

Joe Geigel joe fred

Variables in C++

• Reference variable

– Accessing class members

• Uses the . Syntax
• Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”);
• joe.getGrades();
• Student &fred (joe);
• fred.getGrades();

Variables in C++ -- Examples

Student joe (“Geigel” , “Joe”, “GCCIS”, “CS”); Student *joe_ptr (new Student (“Schmoe” , “Joe”, “GCCIS”, “CS”); Student &joe_ref (joe); // okay joe = *joe_ptr; *joe_ptr = joe; joe_ref = *joe_ptr; // changes joe joe = joe_ref; // dangerous joe_ptr = &joe; joe_ptr = &joe_ref;

Variables in C++

• Questions?

static

• static class members like in Java:

– the member has no knowledge of any particular instance of the class – data member: there is only one copy shared by all – member function: cannot access non-static data

• r functions

static

• In header file:

class Tribble { public: Tribble(); ~Tribble(); public: static void report(); static unsigned long count; };

static

• In source file

void Tribble::report() { ... } unsigned long Tribble::count = 0;

static

• Local variables can also be declared as

static.

void foo () { static int a = 0; … a = a + 1; }

static

• Questions?

Functions

• Declaration vs definitions

– Declaration is the function signature

• Required before function can be used
• Often included in a header file
• Can exist in multiple files
• char *strcpy (char * to, char* from);

– Definition

• Actual code
• Must defined once and only once

Functions

• void and void *

– void – indicates that a function does not return a value – void * -- function returns a “generic” pointer

• Must be typecast to correct pointer type
• Use with caution

Functions

• In C++ function arguments are pass by

value:

aClass i(7); foo (i); void foo (aClass a) { a.moo = 12; }

Functions – Pass by value

• Get around this by changing the variable

type of the arguments

aClass i(7); foo (&i); void foo (aClass *a) { a->moo = 12; }

Functions – Pass by value

• Cleaner means is to use reference variables:

aClass i(7); foo (i); void foo (aClass &a) { a.moo = 12; }

const

• Any variable declared as const cannot be

changed

– Like final in Java. – If the variable is a pointer then the object / data item pointed to cannot be changed.

• const aClass *a (new aClass(7))
• a->moo = 7; // error

Functions – Pass by value

• Yet another variable type:

– Constant pointers

• Once defined, the data/object pointed to by the

pointer cannot be changed.

• const aClass *a (new aClass(7))
• a->moo = 7; // error

Functions – Pass by value

• Works for function arguments as well.

aClass i(7); foo (&i); void foo (const aClass *a) { a->moo = 12; // error }

Functions – Pass by value

• Furthermore

const aClass *i (new aClass(7)); foo (i); // error void foo (aClass *a) { a->moo = 13; }

const

• A class method can also be declared as

const

– This means that the method will not change the

• bject.

class aClass {

public printValues () const; public changeValues(); }

const

• Const methods

void foo (const aClass *a) { a->printValues(); // okay a->changeValues(); // error }

Functions

• Question?