6.5. Overloading, Hiding, and Overriding

6.5. Overloading, Hiding, and Overriding
	Chapter 6. Inheritance and Polymorphism

[ fromfile: overloadhide.xml id: overloadhide ]

First, recall the definitions of two terms that often get confused:

When two or more versions of a function foo exist in the same scope (with different signatures), we say that foo has been overloaded.
When a virtual function from the base class also exists in the derived class, with the same signature and return type, we say that the derived version overrides the base class version.

Example 6.18 demonstrates overloading and overriding and introduces another relationship between functions that have the same name.

Example 6.18. src/derivation/overload/account.h

[ . . . . ]
class Account {
public:
    Account(unsigned acctno, double bal, QString owner);
    virtual ~Account() { }
    virtual void deposit(double amt);
    virtual QString toString() const;
    virtual QString toString(char delimiter);   
protected:
    unsigned  m_AcctNo;
    double    m_Balance;
    QString   m_Owner;
};

class InsecureAccount: public Account {
public:
    InsecureAccount(unsigned acctno, double bal, QString owner);
    QString toString() const;                   
    void deposit(double amt, QDate postDate);   
};
[ . . . . ]

	Overloaded function.
	Overrides base method and hides toString(char).
	Does not override any method, but hides all Account::deposit() methods.

Function Hiding

A member function of a derived class with the same name as a function in the base class hides all functions in the base class with that name. In such a case

Only the derived class function can be called directly.
The class scope resolution operator :: must be used to call hidden base functions explicitly.

Example 6.19 shows the difference between a hidden and an inaccessible member.

Example 6.19. src/derivation/overload/account-client.cpp

#include  "account.h"
#include  <QTextStream>

int main() {
    InsecureAccount acct(12345, 321.98, "Luke Skywalker");
    acct.deposit(6.23);             
    acct.m_Balance += 6.23;         
    acct.Account::deposit(6.23);    
    // ... more client code
    return 0;
}

	Error! No matching function - hidden by deposit(double, int).
	Error! Member is protected, inaccessible.
	Hidden does not mean inaccessible. You can still access hidden public members via scope resolution.


6.4. Inheritance Design		6.6. Constructors, Destructors, and Copy Assignment Operators