1 C++ Classes: Access (II) Ying Wu Electrical Engineering & Computer Science Northwestern University ECE230 Lectures Series

2 Thinking in C Buy_Materials() Hire_Workers() Build_Floor() Build_Bedrooms() Build_Kitchen() Build_Roof() Assembly() $$$

3 Thinking in C++ $$$ HomeDepot Material Worker Floor Bedroom Kitchen Roof House

4 CVariable class CVariable { doublem_dValue; char*m_sName; public: // constructors and destructors CVariable(); CVariable(const char*name, const double& v = 0.0); ~CVariable(); CVariable(const CVariable& var); // copy constructor const CVariable& operator=(const CVariable& var); // overload = // getting and setting double Value() { return m_dValue; }; char* Name() const { return m_sName; }; void SetValue(const double& v) { m_dValue = v; }; bool SetName(const char* name); };

5 A good point of C++: “package” Data encapsulation –Class = Data + Functions Data Functions Class access Is it good to access all the members from outside?

6 What to learn today? Access to data members Access to member functions Header files

7 Syntax for accessing Rules of thumb –(I) we do not differentiate data members and member functions. (since both are members!) –(II) if the client is an object, use “.” –(III) if the client is a pointer of a object, use “  ”

8 void main() { CVariable a; a.SetName(“var_1”); a.SetValue(1.5); CVariable *b; b = &a; b->SetValue(3.3); (*b).SetName(“vvv”); CVariable &c; c = a; c.SetValue(0.0); cout << a.Name() << a.Value() << endl; cout Name() Value() << endl; }

9 Question? (1)Is it good? (2)Why do I need to manipulate the data directly? No, I don’t want my class to be too transparent to other users! I don’t want to give users too much rights to manipulate the data in my package! void main() { CVariable a; a.m_dValue = 1.0; char name[] = “var_1”; a.m_sName = name; }

10 Another good point of C++ Information Hiding –Clients can only access some specific members in a class Data Functions access Data Functions Class

11 Controlling Access to Members public –Presents clients with a view of the services the class provides (interface) –Data and member functions are accessible private –Default access mode –Data only accessible to member functions and friend s –private members only accessible through the public class interface using public member functions

12 Access Functions and Utility Functions Utility functions –private functions that support the operation of public functions –Not intended to be used directly by clients Access functions –public functions that read/display data or check conditions –Allow public functions to check private data –“set” functions –“get” functions –Predicate functions

13 // salesp.h // SalesPerson class definition // Member functions defined in salesp.cpp #ifndef SALESP_H #define SALESP_H class SalesPerson { public: SalesPerson(); // constructor void getSalesFromUser(); // get sales figures from keyboard void setSales( int, double ); // User supplies one month's // sales figures. void printAnnualSales(); private: double totalAnnualSales(); // utility function double sales[ 12 ]; // 12 monthly sales figures }; #endif

14 // salesp.cpp // Member functions for class SalesPerson #include using namespace std; #include "salesp.h" // Constructor function initializes array SalesPerson::SalesPerson() { for ( int i = 0; i < 12; i++ ) sales[ i ] = 0.0; } // Function to get 12 sales figures from the user // at the keyboard void SalesPerson::getSalesFromUser() { double salesFigure; for ( int i = 1; i <= 12; i++ ) { cout << "Enter sales amount for month " << i << ": "; cin >> salesFigure; setSales( i, salesFigure ); }

15 // Function to set one of the 12 monthly sales figures. // Note that the month value must be from 0 to 11. void SalesPerson::setSales( int month, double amount ) { if ( month >= 1 && month 0 ) sales[ month - 1 ] = amount; // adjust for subscripts 0-11 else cout << "Invalid month or sales figure" << endl; } // Print the total annual sales void SalesPerson::printAnnualSales() { cout << setprecision( 2 ) << setiosflags( ios::fixed | ios::showpoint ) << "\nThe total annual sales are: $" << totalAnnualSales() << endl; } // Private utility function to total annual sales double SalesPerson::totalAnnualSales() { double total = 0.0; for ( int i = 0; i < 12; i++ ) total += sales[ i ]; return total; }

16 87// Fig. 6.7: fig06_07.cpp 88// Demonstrating a utility function 89// Compile with salesp.cpp 90#include "salesp.h" 91 92int main() 93{ 94 SalesPerson s; // create SalesPerson object s s.getSalesFromUser(); // note simple sequential code 97 s.printAnnualSales(); // no control structures in main 98 return 0; 99} OUTPUT Enter sales amount for month 1: Enter sales amount for month 2: Enter sales amount for month 3: Enter sales amount for month 4: Enter sales amount for month 5: Enter sales amount for month 6: Enter sales amount for month 7: Enter sales amount for month 8: Enter sales amount for month 9: Enter sales amount for month 10: Enter sales amount for month 11: Enter sales amount for month 12: The total annual sales are: $ Create object s, an instance of class SalesPerson 1. Load header file and compile with the file that contains the function definitions 2. Create an object 2.1 Use the object’s member functions to get and print sales Program Output Use access functions to gather and print data ( getSalesFromUser and printAnnualSales ). Utility functions actually calculate the total sales, but the user is not aware of these function calls. Notice how simple main() is – there are no control structures, only function calls. This hides the implementation of the program.

17 Separating Interface from Implementation Separating interface from implementation –Makes it easier to modify programs –Header files Contains class definitions and function prototypes Using #ifndef #define … #endif –Source-code files Contains member function definitions

18 // variable.h Header file #ifndef _VARIABLE_H_ #define _VARIABLE_H_ class CVariable { doublem_dValue; char*m_sName; public: // constructors and destructors CVariable(); CVariable(const char*name, const double& v = 0.0); ~CVariable(); CVariable(const CVariable& var); // copy constructor const CVariable& operator=(const CVariable& var); // overload = // getting and setting double Value() { return m_dValue; }; char* Name() const { return m_sName; }; void SetValue(const double& v) { m_dValue = v; }; bool SetName(const char* name); }; #endif

19 // variable.cpp source file #include “variable.h” CVariable::CVariable() : m_dValue(0.0), m_sName(NULL) { // empty } CVariable::CVariable(const char* name, const double& v) { m_dValue = v; m_sName = new char[strlen(name)+1]; strcpy(m_sName, name); } CVariable::CVariable(const CVariable& var) { m_dValue = var.m_dValue; m_sName = new char[strlen(var.m_sName)+1]; strcpy(m_sName, var.m_sName); } CVariable::~CVariable() { if(m_sName!=NULL){ delete [] m_sName; }

20 const CVariable& CVariable::operator=(const CVariable& var) { if(&var != this){ // check for self-assignment m_dValue = var.m_dValue; SetName(var.m_sName); } return *this; } bool CVariable::SetName(const char* name) { boolcode = true; if(m_sName!=NULL) delete [] m_sName; m_sName = new char [strlen(name) + 1]; if(m_sName){ strcpy(m_sName, name); } else code = false; return code; }