C++ for Scientists - Technische Universität Dresden

3.7. ACCESSING OBJECT MEMBERS 79
This function works only the complex number is not constant. So we also need a function that
takes a constant reference as argument. In return it can only provide a constant reference of
the number’s real part.
inline const double& real(const complex& c) { return c.r; }
This function requires a friend declaration, too.
The functions — in free as well as in member form — can evidently only be called when object
is created. The references of the number’s real part that we use in the statement
real(c)+= 5.;
exist only until the end of the statement. The variable c lives longer. We can create a reference
variable:
double &rr= real(c);
C ++ destroys objects in reverse order. That means that even if rr and c are in the same function
or block, c lives longer than rr.
The same is true for constant references if objects from variable declarations are referred.
Temporary objects can also be passed as constant references enabling the definition of outdated
references:
const double &rr= real(complex()); // Bad thing!!!
cout ≪ ”The real part is ” ≪ rr ≪ ’\n’;
The complex variable is created temporarily and only exist until the end of the first statement.
The reference to its real part lives till the end of the surrounding block.
Advise
Do Not Make Constant References Of Temporary Expressions!
They are invalid before you use them the first time.
3.7.2 Subscript operator
A really stupid way to access vector entries would be writing a function for each one:
class vector
{
public:
double& zeroth() { return data[0]; }
double& first() { return data[1]; }
double& second() { return data[2]; }
// ...
int size() const { return my size; }
};