C++ for Scientists - Technische Universität Dresden

72 CHAPTER 3. CLASSES
• Compilers might find more optimizations.
There are cases where the default copy constructor does not work, especially when the class
contains pointers. Say we have a simple vector class with a copy constructor:
class vector
{
public:
vector(const vector& v)
: size(v.size), data(new double[size])
{
for (unsigned i= 0; i < size; i++)
data[i]= v.data[i];
}
// ...
private:
unsigned size;
double ∗data;
};
If we omit this copy constructor the compiler would not complain and voluntarily built one
for us. We are glad that our program is shorter and sexier but sooner or later we find that it
behaves bizarrely. Changing one vector, modifies another one as well and when we observe this
strange behavior we have to find the error in our program. This is particularly difficult because
there is no error in what we have written but in what we have omitted.
Another problem we can observe is that the run-time library will complain that we freed the
same memory twice. 4 The reason for this is the way pointers are copied. Only the address is
copied and the result is that both pointers point to the same memory. This might be useful in
some cases but most of the time it is not, at least in our domain. Some pointer-addicted geeks
might see this differently.
3.3.1 Explicit and implicit constructors
In C ++ we distinguish implicit and explicit constructors. Implicit constructors enable in addition
to object initialization implicit conversions and assignment-like notation for construction.
Instead of:
complex c1(3.0);
we can also write:
or
complex c1= 3.0;
complex c1= pi∗pi/6.0;
This notation is for many scientifically educated people more readable. Older compilers might
generate more code in initializations using ‘=’ (the object is first created with the default
constructor and the value is copied afterwards) while current compiler generate the same code
for both notations.
4 This is an error message every programmer experiences at least once in his/her life (or he/she is not doing
serious business).