ssrs fixed data matrix A Constructor Function for the Rational Class in Software

Creating Quick Response Code in Software A Constructor Function for the Rational Class

EXAMPLE 8.3 A Constructor Function for the Rational Class
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class Rational { public: Rational(int n, int d) { num = n; den = d; } void print0 { tout -CC num << '/' CC den; } private: int num, den; > ; main0 t Rational x(-1,3), y(22,7); tout << "X = I'; x.print(); tout CC 'I and y = 'I; y.print(); x= -If3 and y = 2217
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The constructor function has the same effect as the ass ign ( > function had in Example 8.1: 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
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CLASSES
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function then assigns these values to X'S num and den data members. So the declarations
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Rational x(-1,3), y(22,7);
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are equivalent to the three lines
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Rational x, y; x.assign(-1,3); y.assign(22,7);
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A class s constructor constructs the class objects by allocating and initializing storage for the objects and by performing any other tasks that are programmed into the function. It literally creates a live object from a pile of unused bits. We can visualize the relationshins between the Rational class itself and its instantiated objects like this:
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num I -1
Rational
den c 3 l num I 22
The class itself is represented by a rounded box containing its member functions. Each function maintains a pointer, named this", which points to the object that is calling it. The snapshot here represents the status during the execution of the last line of the program, when the object Y is calling the print ( ) function: y . print ( ) . At that moment, the this" pointer for the constructor points to no object because it is not being called. A class may have several constructors. Like any other overloaded function, these are distinguished by their distinct parameter lists.
EXAMPLE 8.4 Adding More Constructors to the Rational Class
class Rational { public: Rational0 { num = 0; den = 1; Rational(int n) { num = n; den Rational(int n, int d) ( num = void print0 { tout C-C num << private: int num, den;
) = 1; ) n; den = d; } '/I CC den; >
main0 { Rational x, y(4), z(22,7); tout << X = I; x.print 1 . tout << l&y - 1 . I y.print 1 . tout << ll;nz - 1 . I z.print 1 ; 1
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CLASSES
The output looks like this:
This version of the Rat ional class has three constructors. The first has no parameters and initializes the declared object with the default values 0 and 1. The second constructor has one integer parameter and initializes the object to be the fractional equivalent to that integer. The third constructor is the same as in Example 8.2.
Among the various constructors that a class may have, the simplest is the one with no parameters. It is called the default constructor. If this constructor is not explicitly declared in the class definition, then the system will automatically create it for the class. That is what happens in Example 8.1.
8.4 CONSTRUCTOR INITIALIZATION LISTS Most constructors do nothing more than initialize the object s member data. Consequently, C++ provides a special syntactical device for constructors that simplifies this code. The device is an initialization list. Here is the third constructor in Example 8.2, rewritten using an initialization list:
Rational(int n, int d) : mm(n), den(d) { }
The assignment statements in the function s body that assigned n to num and d to den are removed. Their action is handled by the initialization list shown in boldface. Note that the list begins with a colon and precedes the function body which is now empty. Here is the Rat ional class with its three constructors rewritten using initializer lists.
EXAMPLE 8.5 Using Initializer Lists in the Rational Class
class Rational { public: Rational0 : num(O), den(l) { 1 Rational(int n) : num(n), den( 1) l 1 Rational(int n, int d) : num(n.), den(d) private: int num, den;
Of course, these three separate constructors are not necessary. They can be combined into a single constructor, using default parameter values:
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