SOLUTIONS ■ 219 if( word1.size() > 1) { cout << "\nEnter the password once more: "; cin >> setw(20) >> word2; if( word1 == word2) // Password confirmed? { // Yes! secret = word1; return true; } } return false; // No new password } // // Password.cpp // Testing the functions getPassword() and // changePassword(). // // After entering the password correctly (max. three // attempts within 60 seconds), the user can change it. // #include <iostream> #include <iomanip> #include <string> #include <cctype> using namespace std; bool getPassword(void); // Read a password. bool changePassword(void); // Change a password. // Inline functions: inline void cls() { cout << "\033[2J"; } inline void go_on() { cout << "\n\nGo on with return! "; cin.sync(); cin.clear(); // Only new input while( cin.get() != '\n') ; } inline char getYesOrNo() // Read character Y or N. { char c = 0; cin.sync(); cin.clear(); // Just new input do { cin.get(c); c = toupper(c); // Permitting lower case letters also. } while( c != 'Y' && c != 'N'); return c; } 220 ■ CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES static string header = "\n\n **** Test password handling ****\n\n"; static string menu = "\n\n B = Booking " "\n\n E = End of program" "\n\n Your choice: "; int main() { char choice = 0; while( choice != 'E') { cls(); cout << header << menu; // Header and Menu cin.get(choice); choice = toupper(choice); cls(); cout << header << endl; // Header switch( choice) { case 'B': // Booking if( !getPassword() ) { cout << "Access denied!" << endl; go_on(); } else { cout << "Welcome!\n\n" << "Do you want to change the password? (y/n)"; if( getYesOrNo() == 'Y') { if( changePassword() ) cout << "Password changed!" << endl; else cout << "Password unchanged!" << endl; go_on(); } // Place statements for booking here. } break; case 'E': cls(); cout << "\n Bye Bye!" << endl; break; } } // End of while return 0; } 221 References and Pointers This chapter describes how to define references and pointers and how to use them as parameters and/or return values of functions. In this context, passing by reference and read-only access to arguments are introduced. chapter 12 222 ■ CHAPTER 12 REFERENCES AND POINTERS // Ref1.cpp // Demonstrates the definition and use of references. // #include <iostream> #include <string> using namespace std; float x = 10.7F; // Global int main() { float &rx = x; // Local reference to x // double &ref = x; // Error: different type! rx *= 2; cout << " x = " << x << endl // x = 21.4 << " rx = " << rx << endl; // rx = 21.4 const float& cref = x; // Read-only reference cout << "cref = " << cref << endl; // ok! // ++cref; // Error: read-only! const string str = "I am a constant string!"; // str = "That doesn't work!"; // Error: str constant! // string& text = str; // Error: str constant! const string& text = str; // ok! cout << text << endl; // ok! Just reading. return 0; } Object names: The object in memory 10.7 x, rx . . . . . . ■ DEFINING REFERENCES Example float x = 10.7, &rx = x; Sample program DEFINING REFERENCES ■ 223 A reference is another name, or alias, for an object that already exists. Defining a refer- ence does not occupy additional memory. Any operations defined for the reference are performed with the object to which it refers. References are particularly useful as parame- ters and return values of functions. ᮀ Definition The ampersand character, &, is used to define a reference. Given that T is a type, T& denotes a reference to T. Example: float x = 10.7; float& rx = x; // or: float &rx = x; rx is thus a different way of expressing the variable x and belongs to the type “reference to float”. Operations with rx, such as Example: rx; // equivalent to x; will automatically affect the variable x. The & character, which indicates a reference, only occurs in declarations and is not related to the address operator &! The address operator returns the address of an object. If you apply this operator to a reference, it returns the address of the referenced object. Example: &rx // Address of x, thus is equal to &x A reference must be initialized when it is declared, and cannot be modified subse- quently. In other words, you cannot use the reference to address a different variable at a later stage. ᮀ Read-Only References A reference that addresses a constant object must be a constant itself, that is, it must be defined using the const keyword to avoid modifying the object by reference. However, it is conversely possible to use a reference to a constant to address a non-constant object. Example: int a; const int& cref = a; // ok! The reference cref can be used for read-only access to the variable a, and is said to be a read-only identifier. A read-only identifier can be initialized by a constant, in contrast to a normal refer- ence: Example: const double& pi = 3.1415927; Since the constant does not take up any memory space, the compiler creates a temporary object which is then referenced. 224 ■ CHAPTER 12 REFERENCES AND POINTERS // Ref2.cpp // Demonstrating functions with parameters // of reference type. // #include <iostream> #include <string> using namespace std; // Prototypes: bool getClient( string& name, long& nr); void putClient( const string& name, const long& nr); int main() { string clientName; long clientNr; cout << "\nTo input and output client data \n" << endl; if( getClient( clientName, clientNr)) // Calls putClient( clientName, clientNr); else cout << "Invalid input!" << endl; return 0; } bool getClient( string& name, long& nr) // Definition { cout << "\nTo input client data!\n" << " Name: "; if( !getline( cin, name)) return false; cout << " Number: "; if( !( cin >> nr)) return false; return true; } // Definition void putClient( const string& name, const long& nr) { // name and nr can only be read! cout << "\n Client Data \n" << "\n Name: "; cout << name << "\n Number: "; cout << nr << endl; } ■ REFERENCES AS PARAMETERS Sample program REFERENCES AS PARAMETERS ■ 225 ᮀ Passing by Reference A pass by reference can be programmed using references or pointers as function parame- ters. It is syntactically simpler to use references, although not always permissible. A parameter of a reference type is an alias for an argument. When a function is called, a reference parameter is initialized with the object supplied as an argument. The function can thus directly manipulate the argument passed to it. Example: void test( int& a) { ++a; } Based on this definition, the statement test( var); // For an int variable var increments the variable var. Within the function, any access to the reference a auto- matically accesses the supplied variable, var. If an object is passed as an argument when passing by reference, the object is not copied. Instead, the address of the object is passed to the function internally, allowing the function to access the object with which it was called. ᮀ Comparison to Passing by Value In contrast to a normal pass by value an expression, such as a+b, cannot be used as an argument. The argument must have an address in memory and be of the correct type. Using references as parameters offers the following benefits: ■ arguments are not copied. In contrast to passing by value, the run time of a pro- gram should improve, especially if the arguments occupy large amounts of mem- ory ■ a function can use the reference parameter to return multiple values to the calling function. Passing by value allows only one result as a return value, unless you resort to using global variables. If you need to read arguments, but not copy them, you can define a read-only reference as a parameter. Example: void display( const string& str); The function display() contains a string as an argument. However, it does not gener- ate a new string to which the argument string is copied. Instead, str is simply a refer- ence to the argument. The caller can rest assured that the argument is not modified within the function, as str is declared as a const. 226 ■ CHAPTER 12 REFERENCES AND POINTERS // Ref3.cpp // Demonstrates the use of return values with // reference type. // #include <iostream> #include <string> using namespace std; // Returns a double& refMin( double&, double&); // reference to // the minimum. int main() { double x1 = 1.1, x2 = x1 + 0.5, y; y = refMin( x1, x2); // Assigns the minimum to y. cout << "x1 = " << x1 << " " << "x2 = " << x2 << endl; cout << "Minimum: " << y << endl; ++refMin( x1, x2); // ++x1, as x1 is minimal cout << "x1 = " << x1 << " " // x1 = 2.1 << "x2 = " << x2 << endl; // x2 = 1.6 ++refMin( x1, x2); // ++x2, because x2 is // the minimum. cout << "x1 = " << x1 << " " // x1 = 2.1 << "x2 = " << x2 << endl; // x2 = 2.6 refMin( x1, x2) = 10.1; // x1 = 10.1, because // x1 is the minimum. cout << "x1 = " << x1 << " " // x1 = 10.1 << "x2 = " << x2 << endl; // x2 = 2.6 refMin( x1, x2) += 5.0; // x2 += 5.0, because // x2 is the minimum. cout << "x1 = " << x1 << " " // x1 = 10.1 << "x2 = " << x2 << endl; // x2 = 7.6 return 0; } double& refMin( double& a, double& b) // Returns a { // reference to return a <= b ? a : b; // the minimum. } The expression refMin(x1,x2) represents either the object x1 or the object x2, that is, the object containing the smaller value. ✓ NOTE ■ REFERENCES AS RETURN VALUE Sample program REFERENCES AS RETURN VALUE ■ 227 ᮀ Returning References The return type of a function can also be a reference type. The function call then repre- sents an object, and can be used just like an object. Example: string& message() // Reference! { static string str = "Today only cold cuts!"; return str; } This function returns a reference to a static string, str. Pay attention to the following point when returning references and pointers: The object referenced by the return value must exist after leaving the function. It would be a critical error to declare the string str as a normal auto variable in the function message(). This would destroy the string on leaving the function and the ref- erence would point to an object that no longer existed. ᮀ Calling a Reference Type Function The function message() (mentioned earlier in this section) is of type “reference to string.” Thus, calling message() represents a string type object, and the following statements are valid: message() = "Let's go to the beer garden!"; message() += " Cheers!"; cout << "Length: " << message().length(); In these examples, a new value is first assigned to the object referenced by the function call. Then a new string is appended before the length of the referenced string is output in the third statement. If you want to avoid modifying the referenced object, you can define the function type as a read-only reference. Example: const string& message(); // Read-only! References are commonly used as return types when overloading operators. The oper- ations that an operator has to perform for a user-defined type are always implemented by an appropriate function. Refer to the chapters on overloading operators later in this book for more details. However, examples with operators from standard classes can be pro- vided at this point. 228 ■ CHAPTER 12 REFERENCES AND POINTERS Reference to cout cout << "Good morning" <<'!'; // Ref4.cpp // Expressions with reference type exemplified by // string assignments. // #include <iostream> #include <string> #include <cctype> // For toupper() using namespace std; void strToUpper( string& ); // Prototype int main() { string text("Test with assignments \n"); strToUpper(text); cout << text << endl; strToUpper( text = "Flowers"); cout << text << endl; strToUpper( text += " cheer you up!\n"); cout << text << endl; return 0; } void strToUpper( string& str) // Converts the content { // of str to uppercase. int len = str.length(); for( int i=0; i < len; ++i) str[i] = toupper( str[i]); } ■ EXPRESSIONS WITH REFERENCE TYPE Example: Operator << of class ostream cout << "Good morning" << '!'; Sample assignments of class string . variable var increments the variable var. Within the function, any access to the reference a auto- matically accesses the supplied variable, var. If an object is passed as an argument when passing. called. ᮀ Comparison to Passing by Value In contrast to a normal pass by value an expression, such as a+ b, cannot be used as an argument. The argument must have an address in memory and be of the. such as Example: rx; // equivalent to x; will automatically affect the variable x. The & character, which indicates a reference, only occurs in declarations and is not related to the address