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Note that on line 74 Dog's default constructor calls Mammal's default constructor. Although it is not strictly necessary to do this, it serves as documentation that you intended to call the base constructor, which takes no parameters. The base constructor would be called in any case, but actually doing so makes your intentions explicit. |
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The implementation for the Dog constructor, which takes an integer, is on lines 8085. In its initialization phase (lines 8182), Dog initializes its base class, passing in the parameter; and then it initializes its breed. |
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Another Dog constructor is on lines 8793. This one takes two parameters. Once again, it initializes its base class by calling the appropriate constructor, but this time it also assigns weight to its base class's variable itsWeight. Note that you cannot assign to the base class variable in the initialization phase. Because Mammal does not have a constructor that takes this parameter, you must do this within the body of the Dog's constructor. |
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Walk through the remaining constructors to make sure you are comfortable with how they work. Note what is initialized and what must wait for the body of the constructor. |
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The output has been numbered so that each line can be referred to in this analysis. The first two lines of output represent the instantiation of Fido, using the default constructor. |
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In the output, lines 3 and 4 represent the creation of rover. Lines 5 and 6 represent buster. Note that the Mammal constructor that was called is the constructor that takes one integer, but the Dog constructor is the constructor that takes two integers. |
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After all the objects are created, they are used and then go out of scope. As each object is destroyed, first the Dog destructor and then the Mammal destructor is called; there are five of each in total. |
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A Dog object has access to all the member functions in class Mammal, as well as to any member functions, such as WagTail(), that the declaration of the Dog class might add. It can also override a base class function. Overriding a function means changing the implementation of a base class function in a derived class. When you make an object of the derived class, the correct function is called. |
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New Term: When a derived class creates a function with the same return type and signature as a member function in the base class, but with a new implementation, it is said to be overriding that method. |
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When you override a function, it must agree in return type and in signature with the function in the base class. The signature is the function prototype other than the return type: that is, the name, the parameter list, and the keyword const, if used. |
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