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C++ Class Members

C++ Class Members. Class Definition class Name { public: constructor(s) destructor function members data members protected: function members data members private: function members data members };. C++ Class Members. Class Definition - Data Members Data Members

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C++ Class Members

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  1. C++ Class Members • Class Definition • class Name • { • public: • constructor(s) • destructor • function members • data members • protected: • function members • data members • private: • function members • data members • }; 1 February 2006

  2. C++ Class Members • Class Definition - Data Members • Data Members • It is important to note ….. • Each class instance gets distinct copies of all data members 1 February 2006

  3. C++ Class Members • Class Definition - Data Members • class MyClass • { • public: • MyClass() { myValue = 10; } • int getMyValue () { return myValue;} • int setMyValue (int aValue) { myValue = aValue;} • private: • int myValue; • }; • int main(void) • { • MyClass inst0, inst1; • inst1.setMyValue(20); • cout << inst0.getMyValue() << endl • << inst1.getMyValue() << endl; • return 0; • } 1 February 2006

  4. C++ Class Members • Class Definition - Function Members • Function Members • Unlike data members • Only one instance of each function member exists for a class • Unless the function is declared inline 1 February 2006

  5. C++ Class Members • Class Definition - Function Members • Function Members • If only a single instance….. • How are data members which function manipulates • bound to the function instance????? • What is not shown • The member function has a hidden argument • The arg is a pointer to the object that was sent the message • Pointer is identified by the keyword this 1 February 2006

  6. C++ Class Members • Class Definition - Function Members • Function Members • The this pointer • Has type ClassType • Contains the address of the class instance through which the member function was invoked • Starting address is same as the address of the first variable in the class structure 1 February 2006

  7. C++ Class Members • Class Definition - Function Members • Function Members • Every nonstatic function member has a pointer implicitly declared • ClassName * const this; • this is initialized to point to the object for which the member • function was invoked • Because this is declared const • It cannot be changed • Object pointed to can 1 February 2006

  8. C++ Class Members • Class Definition - Function Members • Function Members • Since this is a pointer to the beginning of the class structure • One can select elements using pointer notation • this -> myElement • To refer to the entire instance….. • *this • return *this • Will produce a copy of the instance as the return value 1 February 2006

  9. C++ Class Members • Class Definition - Function Members • class Link • { • public: • void append (Link* aLink); • void setPrev (Link* aLink); • private: • Link* next; • Link* prev; • }; • void Link :: append(Link* aLink) • { • next = aLink; • aLink -> setPrev(this); • return; • } Next Li Li+1 Previous this 1 February 2006

  10. C++ Class Members • Managing Storage - Object Creation • When a C++ object is created 2 events occur…….. • Storage is allocated for the object • The constructor is called to initialize the storage • Storage allocation occurs in several ways and at different times 1 February 2006

  11. C++ Class Members • Managing Storage - Object Creation • Static Storage • Allocated when the program begins • Constant for the life of the program • Stack • Storage allocated on the stack when an execution point is • reached - Opening curly brace. • Released at the complementary execution point - closing curly • brace. • Heap - Free Store • Storage is dynamically allocated from the heap at runtime 1 February 2006

  12. C++ Class Members • Managing Storage - Object Creation • Declaration • Introduces a name • Tells the compiler what the name means • Usually go into header files • Definition • Allocates storage for the name 1 February 2006

  13. C++ Class Members • Managing Storage - Object Creation • Variable • The compiler determines the variable size and generates space • If there is an initializing value, the value entered into that storage • Compile time storage usually allocated at the opening brace • Constructor not called until sequence point where object defined • Auto storage will be allocated on the stack • Function • Compiler generates code and allocates storage to hold the code • Storage for function has an address that can be produced using • Function name with no arg list • Address of operator • Usually go into implementation files 1 February 2006

  14. C++ Class Members • Managing Storage - Linkage • Linkage • Describes storage to represent an identifier as seen by linker • Internal • Storage created for identifier for file being compiled • Name may be used in other files without conflict • External • Single piece of storage for all files being compiled • Accessed from other files by keyword extern • Variables defined outside functions • Inline function definitions default to external linkage • Automatic (local) Variables • Exist on the stack while function being called • Auto variables have no linkage 1 February 2006

  15. C++ Class Members • Managing Storage - Inline Functions • Inline Functions • C++ permits specification of inline functions • Identified by keyword inline … the definition of function follows • syntax • inline type functionName (argsi) { body } • When inline definition encountered • No code generated • Code remembered and substituted for each function call 1 February 2006

  16. char a static int b class M m2 char a m0 char a int b m3 char a m1 char a C++ Class Members • Static Data Members • A class is a type not a data object • When an instance of a class is defined….. • The instance gets a copy of all data members • If data member declared static • Single copy • Shared by all instances of the class • Smalltalk • Pool variables 1 February 2006

  17. C++ Class Members • Static Data Members • syntax • Declaration • static type varName; • Definition • type ClassName :: varName < = value > • Static member exists even if no class instances exist • Remain in scope of class • Can only be accessed from outside if declared public 1 February 2006

  18. C++ Class Members • Static Data Members • Storage for a static variable • Must be explicitly reserved and initialized • Static can be declared within a class • Definition and initialization must occur outside Class • Can be only single definition • Static (nonclass) variables • Initialized to 0 by default • Static class variables • Cannot be initialized by the constructor 1 February 2006

  19. C++ Class Members • Initializing Static Data Members... • class SimpleClass • { • public: • SimpleClass () {} • SimpleClass (int aValue){myValue = aValue;} • // static char myChar = 'c'; This is illegal • static char myChar; • private: • static int myValue; • }; • // Define and initialize the static variables • int SimpleClass::myValue = 10; • char SimpleClass::myChar = 'a'; 1 February 2006

  20. C++ Class Members • Static Data Members • Static Data Member • Can appear as a default arg to a member function • Non-static Data Member • Cannot appear as a default arg to a member function • Within a class body….. • int a; • static int b; • int f1 ( int myValue0 = a ); // illegal • int f2 ( int myValue1 = b ); // ok 1 February 2006

  21. C++ Class Members • Static Data Members • A static data member can be an instance of the class of which it is a • member….. • A non-static can only point to an instance of it’s class. • Within a class body • class F1 • { • public: • F1 *a; // ok • static F1 b; // ok • F1 c; // illegal • }; 1 February 2006

  22. C++ Class Members • Const Data Members • Class data members can be specified as const • Consequence • Storage always allocated and must be initialized at point of • definition • Initialization must occur in the constructor • Implemented via the initialization list • Storage not allocated until instance created…... • At that time, the const must be • Created • Initialized • Const variable persists for the lifetime of the instance 1 February 2006

  23. C++ Class Members • Static Function Members • Function members can be declared static • A static function has no this pointer • static function can access • Directly • static members • enums and typedefs • Indirectly • Regular members via . or -> through an explicit object 1 February 2006

  24. C++ Class Members • Static Function Members • class SimpleClass • { • public: • //static int getMyNumber() {return myNumber;} requires an object • static int getMyNumber() { return myValue;} • static int getMyStuff(SimpleClass *myClass) {return myClass->myNumber;} • static int getMyVal() {return myValue;} • static char myChar; • char myLetter; • private: • static int myValue; • int myNumber; • }; • int SimpleClass::myValue=10; • char SimpleClass::myChar = 'a'; 1 February 2006

  25. C++ Class Members • Static Function Members • int main() • { • SimpleClass myClass, yourClass, *myClassPtr = &myClass ; • cout << SimpleClass::getMyStuff(myClassPtr) << endl; • cout << myClass.myChar << " " << SimpleClass::myChar << endl; • return 0; • } 1 February 2006

  26. C++ Class Members • Const Function Members • The keyword const can be placed after argument list of member • function • syntax • returnType functionName ( argsi ) const { function body } • Tells compiler that function • Can only read data members • Cannot write them • Member functions declared const • Can be called for non-const objects • Non-const member functions • Cannot be called for const objects. 1 February 2006

  27. C++ Class Members • Overloading Function Members • Functions can use same name in the same scope • If signature is unique • Name plus signature • Uniquely identify function • Same holds true for member functions Same rules applied to disambiguate Must provide Implementation for each overloaded function 1 February 2006

  28. C++ Class Members • Operator Function Members • C++ defined operators can be overloaded to work on Class type • operands • Must take at least one class type argument • syntax • operator X (args) { body } • X is the operator symbol being overloaded • args - as appropriate for implementation • body - implements the function 1 February 2006

  29. C++ Class Members • Operator Function Members • Invocation • Assume + is overloaded • Infix expression syntax • A + B • A, B can be member or non-member instances • Function call syntax • A.operator+ (B) 1 February 2006

  30. C++ Class Members • Operator Function Members • Rules • Because operator functions are invoked using the same syntax • Operator functions... • 1. Must have the same number of arguments as language defined versions • 2. Must have the same precedence as the builtin operators…This cannot be overridden • 3. Cannot have default parameters • 4. Cannot override the predefined meaning 1 February 2006

  31. C++ Class Members • Operator Function Members • Limitations • Because of the operator function syntax • 1. Certain user defined operators cannot behave • analogously to built in versions • Prefix version • operator++ () {} • Postfix version • operator++ (int) {} • 2. Some operators cannot be overloaded. , ?: :: . * • 3. Some operators must be members only =, [ ] , ( ), -> 1 February 2006

  32. C++ Class Members • Overloading Operators • class Complex • { • public: • int im, re; • Complex(int r=0, int i=0):re(r), im(i) { } • Complex operator+ (Complex a1) {return Complex(re+a1.re, im+a1.im);} • Complex& operator++ () • { • re += 1, im += 1; • cout << “prefix" << endl; • return Complex(re -1, im -1); • } • Complex& operator++ (int) • { • re+=1, im+=1; • cout << "postfix" << endl; • return *this; • } • }; 1 February 2006

  33. C++ Class Members • Operator Function Members • Member • Left hand operand must be an instance of class overloading • the operator • Unary • x.operator@() • x is class instance • Binary • x.operator(y) • x operator@ y • x is class instance 1 February 2006

  34. C++ Class Members • Operator Function Members • Non-Member • If operator requires left hand operand of a different type….. • Must use non-member syntax • Must be declared friend if the operator requires access to • protected or private members • Unary • operator@(x) • Binary • operator@(x,y) • x operator@ y 1 February 2006

  35. C++ Class Members • Operators in Action • The Copy Constructor and the Assignment Operator • There are many occasions when a copy of an object is needed • Object passed by value • x.func(y) • A copy of y is made and passed • Object returned by value • Object initialized by another object • ClassName x1 = x2 • A copy of x2 is made • Object assigned to another object • x1 = x2 • x2 is copied to x1 and • x1 is destroyed 1 February 2006

  36. C++ Class Members • Operators in Action • The Copy Constructor • A constructor that….. • Takes a reference to an object of the same class as an • argument • Performs initialization on newly created objects • Used by compiler to copy objects by value • syntax • class X ( const class X& ) • class X ( class X& ) 1 February 2006

  37. C++ Class Members • Operators in Action • The Copy Constructor • If a copy constructor is not defined • Shallow bitwise copy performed - memcpy • If a copied object uses new to allocate memory when created • One must write a copy constructor • Copy constructor need only be written • If class object is passed by value • To prevent pass by value • Define a private copy constructor 1 February 2006

  38. C++ Class Members • Operators in Action • The Assignment Operator - operator=() • Called when one initialized object is to be assigned to a second. • The operator must serve as a destructor to the assignee object. • Can have a return value… • The most common is *this • Is not called for the declarations such as • ClassName object1 = object2 1 February 2006

  39. C++ Class Members • Class Members - Friends • A class Definition…. • Provides 3 levels of access to data and function members • public • protected • private Closed Open 1 February 2006

  40. C++ Class Members • Class Members - Friends • C++ provides means for more liberal access to non-public members • under specified set of conditions….. • Nonmember designated as a friend of the class • Class friend is designated by Keyword friend Followed by class name 1 February 2006

  41. C++ Class Members • Class Members - Friends • Once an object is designated as a friend…. • It has access to non-public members as if they were public • If B is designated as friend of A • B can access A’s non-public members • A cannot access B’s 1 February 2006

  42. C++ Class Members • Class Members - Friends • Friend may be: • 1. Class • friend class X; • 2. A particular function • friend int sub (int y); • 3. A particular class member • friend int Mult::get(); 1 February 2006

  43. C++ Class Members • Class Members - Friends • A friend does not have a this pointer unless it is a member function in • its own right • Question: • If it is a member function which this pointer does it have • Class A a; • Class B b; • Let B be declared as a friend of A • The this pointer of a function bFunct() contains • the address of the class instance b 1 February 2006

  44. C++ Class Members • Class Members - Friends • Caveats and Problems,,, • Friends can have access to everything … this defeats data • hiding. • Introduces extra coupling between classes. • Permits change of internal state from the outside. • Should use member functions not friends to change state. 1 February 2006

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