C++ for Scientists - Technische Universität Dresden

3.5. ASSIGNMENT 75
It must be paid attention that the freed ressources are not used or released somewhere else in
the program afterwards. C ++ generates a default destructor in the same way as the default
constructor: calling the destructor of each member but in the reverse order. 6
3.5 Assignment
Assignment operators are used to enable for user-defined types expressions like:
x= y;
u= v= w= x;
As usual we consider first the class complex. Assigning a complex to a complex requires an
operator like:
complex& operator=(const complex& src)
{
r= src.r; i= src.i;
return ∗this;
}
Evidently, we copy the members ‘r’ and ‘i’. The operator returns a reference to the object
for enabling multiple assignments. ‘this’ is a pointer to the object itself and since we need a
reference for syntactic reasons it is dereferred. What happens if we assign a double?
c= 7.5;
It compiles without the definition of an assignment operator for double. Once again, we have a
implicit conversion: the implicit constructor creates a complex on the fly and assigns this one.
If this becomes a performance issue we can add an assignment for double:
complex& operator=(double nr)
{
r= nr; i= 0;
return ∗this;
}
An assignment operator like the first one that assigns a an object of the same type is called
Copy Assignment and this operator is synthesized by the compiler. In the case of complex
numbers the generated copy assignment operator performs exactly what we need, copying all
members.
As for the vector the synthesized operator is not satisfying because it only copies the address
of the data and not the data itself. The implementation is very similar to the copy constructor:
vector& operator=(const vector& src)
{
if (this == &src)
return ∗this;
assert(my size == src.my size);
for (int i= 0; i < my size; i++)
data[i]= src.data[i];
6 TODO: Good and short explanation why. If possible with example.