C++ for Scientists - Technische Universität Dresden

2.10. POINTERS AND REFERENCES 51
Furthermore, the programmer is responsible for releasing the memory when not needed anymore.
This is done by
delete[] v;
As we came from arrays, we made the second step before the first one regarding pointer usage.
The simple use of pointers is allocating one single data item.
int∗ ip = new int;
Releasing such memory is performed by
delete ip;
Note the duality of allocation and release: the single-object allocation requires a single-object
release and the array allocation demands an array release. 20
Pointers can also refer to other variables
int i= 3;
int∗ ip2= &i;
The operator & takes an object and returns its address. The reverse operator is ∗ that takes
an address and returns object.
int j= ∗ip2;
This is called dereferencing. It is clear from the context whether the symbol ∗ represents a
dereference or a multiplication.
A danger of pointers are memory leaks. For instance, our array y became too small and we
want assign a new array
int∗ y = new int[15];
We can now use more space in y. Nice. But what happened with the memory that we allocated
before? It is still there but we have no access to it anymore. We cannot release it anymore.
This memory is lost for the rest of our program execution. Only when the program is finished
the operation system will be able to free it. In the example it is only 40 byte out of how many
Gigabyte you might have. But if this happens with larger data in an iterative process the dead
memory grows and at some point the program crashes when all memory is used.
The warnings above are not intended as fun killers. And we do not discourage the use of
pointers. Many things can be only achieved with pointers: lists, queues, trees, graphs, . . . But
pointers must be used with utter care to avoid all the really serious problems mentioned above.
There are two strategies to minimize pointer-related errors:
Use standard implementations from the standard library or other validated libraries. std::vector
from the standard library provides you all the functionality of dynamic arrays, including
resizing and range check, and the memory is released automatically, see § 4.9. Smart pointers
from Boost provide automatic resource management: dynamically allocated memory
that is not referred by a smart pointer is released automatically, see § 11.2.
20 TODO: Otherwise?