C++ for Scientists - Technische Universität Dresden

68 CHAPTER 3. CLASSES
int main()
{
complex c1, c2;
// set c1
c1.set r(3.0);
c1.set i(2.0);
// copy c1 to c2
c2.set r(c1.get r());
c2.set i(c1.get i());
return 0;
}
In line 3 we created two objects of type complex. Then we set one of the objects and copied it
to the other one. This works but it is a bit clumsy, isn’t it?
C ++ provides another keyword for defining classes: struct. The only difference 2 is that members
are by default public, therefore the example above is equivalent to:
struct complex
{
double get r() { return r; }
void set r(double newr) { r = newr; }
double get i() { return i; }
void set i(double newi) { i = newi; }
private:
double r, i;
};
Our member variables can only be accessed via functions. This gives the class designer the
maximal control over the behavior. The setter could only accept values in a certain range. We
could count how often the setter and getter is called for each complex number or for all complex
numbers in the execution. The functions could have additional print-outs for debugging. 3 We
could even allow the reading only at certain times of the day or writing only if the program runs
on a computer with a certain IP. We will most likely not do the latter, at least not for complex
numbers, but we could. If the variables are public and accessed directly, such modifications
would not be possible. Nevertheless, handling the real and imaginary part of a complex number
is cumbersome and we will discuss alternatives.
Most C ++ programmer would not implement it this way. What would a C ++ programmer do
first then? Writing constructors.
3.3 Constructors
What are constructors? Constructors initialize objects of classes and create a working environment
for member functions. Sometimes such an environment includes resources like files,
memory or locks that have to be freed after use. We come back to this later.
To start with let us define a constructor for complex:
2 There is really no other difference. One can define operators and virtual functions or derived classes in the
same manner as with class. Performance of class and struct is also absolutely identical.
3 A debugger is usually a better alternative to putting print-outs into programs.