C++ for Scientists - Technische Universität Dresden

52 CHAPTER 2. C++ BASICS
Encapsulate your dynamic memory management in classes. Then you have to deal with it
only once per class. 21 If all memory allocated by an object is released when the object
is destroyed then it does not matter how many memory you allocate. If you have 738
objects with dynamic memory then it will be released 738 times. If you have called new
738 times, partly in loops and branches, can you be sure that you have called delete 738
times? We know that there are tools for this but these are errors you better prevent than
fix. Even with the encapsulation there is probably something to fix inside the classes but
this is orders of magnitude less work than having pointers spread all over your program.
We have shown two main purposes of pointers:
• Dynamic memory management; and
• Referring other objects.
For the former there is no alternative to pointers, dynamic memory handling needs pointers,
either directly or using classes that contain pointers. To refer to other objects, there exist
another kind of types called reference (surprise, surprise) that we will introduce in the next
section.
2.10.2 References
The following code introduces a reference:
int i= 5;
int& j= i;
j= 4;
std::cout ≪ ”j = ” ≪ j ≪ ’\n’;
The variable j is referring to i. Changing j will also alter i and vice versa, as in the example. i
and j will always have the same value. One can think of a reference as an alias. Whenever one
defines a reference, one must directly say what it is referring to (other then pointers). It is not
possible to refer to another variable later.
So far, that does not sound extremely useful. References are extremely useful for function
arguments (§ 2.6), for refering parts of other objects (e.g. the seventh entry of a vector), and
for building views ( 22 ).
2.10.3 Comparison between pointers and references
The advantage of pointers over references is the ability of dynamic memory management and
address calculation. On the other hand, references refer to defined locations 23 , they always
must refer to something, they do not leave memory leaks (unless you play really evil tricks),
and they have the same notation in usage as the referred object.
21
It is save to assume that there are many more objects than classes; otherwise there is something wrong with
the program.
22
TODO: reref to a section when it is written
23
References can refer to arbitrary addresses but one must work hard to achieve this. For your own savefy we
will not show you how to make reference to behave as badly as pointers.