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do everything that the base class can
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lnheritance Basics
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Public Inheritance Situation
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Base class member function accessing M Derived class member function accessing M
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main,accessing BM
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I main,accessing DM
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Derived class member function accessing 6 M
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B is an object of the base class; D is an object of the publicly derived class; M is a member of the base class
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Access rules that depend on M's visibility in the base class
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The Constructor and Base Class Initialization
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If no constructor is written, a single zeroparameter default constructor is generated that calls the base class zeroparameter constructor for the inherited portion and then applies the default initialization for any additional data members
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Constructors should be defined for each derived class If no constructor is written, a single zero-parameter default constructor is generated This constructor will call the base class zero-parameter constructor for the inherited portion and then apply the default initialization for any additional data members Constructing a derived class object by first constructing its inherited portion is standard practice In fact, it is done by default even if an explicit derived class constructor is given This action is natural because encapsulation means that the inherited portion is a single entity, and the base class constructor tells us how to initialize this single entity Base class constructors can be explicitly called by name in the initializer list Thus the default constructor for a derived class, in reality, is
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The base class initializer can be called with parameters that match a base class constructor As an example, Figure 45 illustrates the Underf 1owException class that could be used in implementing data
1 class UnderflowException : public underflow-error 2 { 3 public: 4 UnderflowException( const string & msg = " " ) 5 : underflow-error( msgc-str( ) ) { } 6 1;
A base class initializerisusedtO call the base class constructor^
Constructor for the new exception class Underflow that uses base class initializer list
structures The Underf lowExcept ion is thrown when an attempt is made to extract something from an empty data structure An Underf 1owException object is constructed by providing an optional string The underf low-error class specification requires a primitive string, so we need to use an initializer list As the Under f 1owException object adds no data members, the construction method is simply to use the underf low-error constructor to construct the inherited portion If the base class initializer is not provided an automatic call to the base class constructor with no parameters is generated If there is no such base class constructor, a compiler error results Thus in this case, initializer lists might be mandatory
Adding Members
Because a derived class inherits from its base class the behavior of the base class, all methods defined for the base class are now defined for the derived class In this section we examine the consequences of adding extra methods and data members Our vector class in Section 342 throws an exception if an out-ofbounds index is detected The only information that it passes back is that an error has occurred Let us look at an alternative that we could have used (note that exception and <stdexcept> are relatively new language additions, which is why we elected not to use them in the remainder of this text) The alternative stores information about what went wrong inside the exception object, providing accessors to get this information However it still IS-A exception,meaning that it can be used any place that an exception can be used The new class is shown in Figure 46 The BadIndex class has one constructor and three methods (in addition to defaults for copying and destruction that we ignore for now) The constructor accepts two parameters It initializes the inherited exception portion by using a zero-parameter constructor It then uses the two parameters to store the index that caused the error and the size of the vector Presumably, the vector has code such as
/ / See Figure 314 Object & operator[]( int index
) ) ) );
if( index < 0 I I index >= size( ) throw BadIndex( index, size( return objects[ index 1 ;