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Widget(); Widget(const Widget&); // default constructor // copy constructor
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The first of these two special constructors is called the default constructor; it is called automatically whenever an object is declared in the simplest form, like this:
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Widget x;
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The second of these two special constructors is called the copy constructor; it is called automatically whenever an object is copied (i.e., duplicated), like this:
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If either of these two constructors is not defined explicitly, then it is automatically defined implicitly by the system. Note that the copy constructor takes one parameter: the object that it is going to copy. That object is passed by constant reference because it should not be changed. When the copy constructor is called, it copies the complete state of an existing object into a new object of the same class. If the class definition does not explicitly include a copy constructor (as all the previous examples have not), then the system automatically creates one by default. The ability to write your own copy constructor gives you more control over your software. EXAMPLE 10.9 Adding a Copy Constructor to the Ratio Class
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class Ratio { public: Ratio(int n=0, int d=1) : num(n), den(d) { reduce(); } Ratio(const Ratio& r) : num(r.num), den(r.den) { } void print() { cout << num << '/' << den; } private: int num, den; void reduce(); }; int main() { Ratio x(100,360); Ratio y(x); cout << "x = "; x.print(); cout << ", y = "; y.print(); } x = 5/18, y = 5/18
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The copy constructor copies the num and den fields of the parameter r into the object being constructed. When y is declared, it calls the copy constructor which copies x into y.
Note the required syntax for the copy constructor: it must have one parameter, which has the same class as that being declared, and it must be passed by constant reference: const X&. The copy constructor is called automatically whenever
an object is copied by means of a declaration initialization; an object is passed by value to a function; an object is returned by value from a function.
EXAMPLE 10.10 Tracing Calls to the Copy Constructor
class Ratio { public: Ratio(int n=0, int d=1) : num(n), den(d) { reduce(); } Ratio(const Ratio& r) : num(r.num), den(r.den) { cout << "COPY CONSTRUCTOR CALLED\n"; } private: int num, den; void reduce(); }; Ratio f(Ratio r) { Ratio s = r; return s; } // calls the copy constructor, copying to r // calls the copy constructor, copying r to s // calls the copy constructor, copying s to
int main() { Ratio x(22,7); Ratio y(x); // calls the copy constructor, copying x to y f(y); } COPY CONSTRUCTOR CALLED COPY CONSTRUCTOR CALLED COPY CONSTRUCTOR CALLED COPY CONSTRUCTOR CALLED In this example, the copy constructor is called four times. It is called when y is declared, copying x to y; it is called when y is passed by value to the function f, copying y to r; it is called when s is declared, copying r to s; and it is called when the function f returns by value, even though nothing is copied there. Note that the initialization of s looks like an assignment. But as part of a declaration it calls the copy constructor just as the declaration of y does.
If you do not include a copy constructor in your class definition, then the compiler generates one automatically. This default copy constructor will simply copy objects bit-by-bit. In many cases, this is exactly what you would want. So in these cases, there is no need for an explicitly defined copy constructor. However, in some important cases, a bit-by-bit copy will not be adequate. The string class, described in 9, is a prime example. In objects of that class, the relevant data member
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