Why C + + uses member functions

First look at the following code

1#include <iostream>2 3 using namespacestd;4 5 classCircle6 {7  Public:8     DoublePI =3.1415926;9     DoubleR;Ten     DoubleArea = pi*r*R; One  A     DoubleSetarea () -     { -         returnArea = pi*r*R; the     } - }; -  - intMain () + { - Circle C1; +cout <<"Please enter a radius:"<<Endl; ACIN >>C1.R; at  -cout << C1.area <<Endl; -cout << C1.setarea () <<Endl; -  -System"Pause"); -     return 0; in}

Operation Result:

Please enter radius:2.69149e+124314.159 Press any key to continue ...

As shown in the results of the operation, if the area areas are calculated with member variables, the result of the calculation is an error, because after the class circle has created the object C1, because the compiler does not actively call pi*r*r to calculate, but only from the memory to take the value, so the print out of the space is a very large garbage value.

Why C + + uses member functions