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adding the new Ratio type to the collection of predefined numeric types int , float, etc. We can envision the object x like this:
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x num den 22 7
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Notice the use of the specifier Ratio:: as a prefix to each function name. This is necessary for each member function definition that is given outside of its class definition. The scope resolution operator :: is used to tie the function definition to the Ratio class. Without this specifier, the compiler would not know that the function being defined is a member function of the Ratio class. This can be avoided by including the function definitions within declaration, as shown below in Example 10.2.
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When an object like the Ratio object x in Example 10.1 is declared, we say that the class has been instantiated, and we call the object an instance of the class. And just as we may have many variables of the same type, we may also have may instances of the same class:
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Ratio x, y, z;
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EXAMPLE 10.2
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A Self-Contained Implementation of the Ratio Class
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Here s the same Ratio class with the definitions of its member functions included within the class declaration: class Ratio { public: void assign(int n, int d) { num = n; den = d; } double convert() { return double(num)/den; } void invert() { int temp = num; num = den; den = temp; } void print() { cout << num << '/' << den; } private: int num, den; };
In most cases, the preferred style is to define the member functions outside of the class declaration, using the scope resolution operator as shown in Example 10.1. That format physically separates the function declarations from their definitions, consistent with the general principle of information hiding. In fact, the definitions are often put in a separate file and compiled separately. The point is that application programs that use the class need only know what the objects can do; they do not need to know how the objects do it. The function declarations tell what they do; the function definitions tell how they do it. This, of course, is how the predefined types (int, double, etc.) work: we know what the result should be when we divide one float by another, but we don t really know how the division is done (i.e., what algorithm is implemented). More importantly, we don t want to know. Having to think about those details would distract us from the task at hand. This point of view is often called information hiding and is an important principle in object-oriented programming. When the member function definitions are separated from the declarations, as in Example 10.1, the declaration section is called the class interface, and the section containing the member function definitions is called the implementation. The interface is the part of the class that the programmer needs to see in order to use the class. The implementation would normally be
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concealed in a separate file, thereby hiding that information that the user (i.e., the programmer) does not need to know about. These class implementations are typically done by implementors who work independently of the programmers who will use the classes that they have implemented. 10.3 CONSTRUCTORS The Ratio class defined in Example 10.1 uses the assign() function to initialize its objects. It would be more natural to have this initialization occur when the objects are declared. That s how ordinary (predefined) types work:
int n = 22; char* s = "Hello";
C++ allows this simpler style of initialization to be done for class objects using constructor functions. A constructor is a member function that is invoked automatically when an object is declared. A constructor function must have the same name as the class itself, and it is declared without return type. The following example illustrates how we can replace the assign() function with a constructor. EXAMPLE 10.3 A Constructor Function for the Ratio Class
class Ratio { public: Ratio(int n, int d) { num = n; den = d; } void print() { cout << num << '/' << den; } private: int num, den; }; int main() { Ratio x(-1,3), y(22,7); cout << "x = "; x.print(); cout << " and y = "; y.print(); } x = -1/3 and y = 22/7
The constructor function has the same effect as the assign() function had in Example 10.1: it initializes the object by assigning the specified values to its member data. When the declaration of x executes, the constructor is called automatically and the integers -1 and 3 are passed to its parameters n and d. The function then assigns these values to x s num and den data members. So the declarations Ratio x(-1,3), y(22,7); are equivalent to the three lines Ratio x, y; x.assign(-1,3); y.assign(22,7);
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