C++ Primer Plus полностью

As you’ve seen several times, you need to provide an explicit assignment operator if class constructors use new to initialize pointers. Because C++ uses the base-class assignment operator for the base part of derived objects, you don’t need to redefine the assignment operator for a derived class unless it adds data members that require special care. For example, the baseDMA class defines assignment explicitly, but the derived lacksDMA class uses the implicit assignment operator generated for that class.

Suppose, however, that a derived class does use new, and you have to provide an explicit assignment operator. The operator must provide for every member of the class, not just the new members. The hasDMA class illustrates how this can be done:

hasDMA & hasDMA::operator=(const hasDMA & hs)

{

    if (this == &hs)

        return *this;

    baseDMA::operator=(hs);  // copy base portion

    delete [] style;         // prepare for new style

    style = new char[std::strlen(hs.style) + 1];

    std::strcpy(style, hs.style);

    return *this;

}

What about assigning a derived-class object to a base-class object? (Note that this is not the same as initializing a base-class reference to a derived-class object.) Take a look at this example:

Brass blips;                                                  // base class

BrassPlus snips("Rafe Plosh", 91191,3993.19, 600.0, 0.12); // derived class

blips = snips;                      // assign derived object to base object

Which assignment operator is used? Remember that the assignment statement is translated into a method that is invoked by the left-hand object:

blips.operator=(snips);

Here the left-hand object is a Brass object, so it invokes the Brass::operator=(const Brass &) function. The is-a relationship allows the Brass reference to refer to a derived-class object, such as snips. The assignment operator only deals with base-class members, so the maxLoan member and other BrassPlus members of snips are ignored in the assignment. In short, you can assign a derived object to a base object, and only the base-class members are involved.

What about the reverse? Can you assign a base-class object to a derived object? Take a look at this example:

Brass gp("Griff Hexbait", 21234, 1200);   // base class

BrassPlus temp;                           // derived class

temp = gp;   // possible?

Here the assignment statement would be translated as follows:

temp.operator=(gp);

The left-hand object is a BrassPlus object, so it invokes the BrassPlus::operator=(const BrassPlus &) function. However, a derived-class reference cannot automatically refer to a base-class object, so this code won’t run unless there is also a conversion constructor:

BrassPlus(const Brass &);

It could be, as is the case for the BrassPlus class, that the conversion constructor is a constructor with a base-class argument plus additional arguments, provided that the additional arguments have default values:

BrassPlus(const Brass & ba, double ml = 500, double r = 0.1);

If there is a conversion constructor, the program uses this constructor to create a temporary BrassPlus object from gp, which is then used as an argument to the assignment operator.

Alternatively, you could define an assignment operator for assigning a base class to a derived class:

BrassPlus & BrassPlus ::operator=(const Brass &) {...}

Here the types match the assignment statement exactly, and no type conversions are needed.

In short, the answer to the question “Can you assign a base-class object to a derived object?” is “Maybe.” You can if the derived class has a constructor that defines the conversion of a base-class object to a derived-class object. And you can if the derived class defines an assignment operator for assigning a base-class object to a derived object. If neither of these two conditions holds, then you can’t make the assignment unless you use an explicit type cast.

Private Versus Protected Members

Remember that protected members act like public members as far as a derived class is concerned, but they act like private members for the world at large. A derived class can access protected members of a base class directly, but it can access private members only via base-class member functions. Thus, making base-class members private offers more security, whereas making them protected simplifies coding and speeds up access. Stroustrup, in his book The Design and Evolution of C++, indicates that it’s better to use private data members than protected data members but that protected methods are useful.

Virtual Method Considerations