Lecture
A class may be friendly to another class. This means that all the component functions of a class are friendly to another class. A friendly class must be defined outside the body of the class “providing friendship”.
For example,
class X2 {friend class X1; . . .};
class X1
{. . .
void f1 (...);
void f2 (...);
. . .
};
// In this example, functions f1 and f2 of class X1 are friends of class X2, although they
// described without the friend specifier.
Example 1.7.2
Consider the point class — a point in n-dimensional space and its friendly vector class — the radius vector of a point (“vector with the origin at the origin of n-dimensional space”). In the vector class, we define a function for determining the norm of a vector, which is calculated as the sum of the squares of the coordinates of its end.
clas s point
{int N; // dimension
double * x; // pointer to an array of coordinates
friend class vector;
public:
point (int n, double d = 0.0);
};
point:: point (int n, double d)
{N = n;
x = new double [N];
for (int i = 0; i <N; i ++) x [i] = d;
}
class vector
{double * xv;
int N;
public:
vector (point, point);
double norma ();
};
vector:: vector (point begin, point end)
{N = begin.N;
xv = new double [N];
for (int i = 0; i <N; i ++) xv [i] = end.x [i] –begin.x [i];
}
double vector:: norma ()
{double dd = 0.0;
for (int i = 0; i <N; i ++) dd + = xv [i] * xv [i];
return dd;
}
void main (void)
{point A (2,4.0);
point B (2,2.0);
vector V (A, B);
cout << V.norma ();
}
// It will be displayed - 8.
The disadvantage of the proposed point class is that the values of all coordinates of the point x [i] are the same. In order for them to be arbitrary and different, it is necessary to define a constructor as a function with a variable number of parameters, for example:
point:: point (int n, double d, ...)
{
N = n;
x = new double [N];
double * p = & d;
for (int i = 0; i <N; i ++) {x [i] = * p; p ++;}
}
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