Pointeurs : concept et analogie

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Del curso dictado por École Polytechnique Fédérale de Lausanne

Initiation à la programmation (en C++)

224 calificaciones

École Polytechnique Fédérale de Lausanne

Initiation à la programmation (en C++)

224 calificaciones

Ce cours initie aux bases de la programmation en utilisant le langage C++ : variables, boucles, fonctions, ... Il ne présuppose pas de connaissance préalable. Les aspects plus avancés (programmation orientée objet) sont donnés dans un cours suivant, «Introduction à la programmation orientée objet (en C++)». Il s'appuie sur de nombreux éléments pédagogiques : vidéos sous-titrées, quizz dans et hors vidéos, exercices, devoirs notés automatiquement, notes de cours.

De la lección

Pointeurs et références

Cette semaine nous terminons notre présentation de nouveaux types de données avec les « pointeurs » et « références » qui permettent de faire référence à d'autres données existantes ou d'en créer de nouvelles dynamiquement.

Pointeurs et références : introduction9:56

Références12:04

Pointeurs : concept et analogie6:07

Pointeurs : déclaration et opérateurs de base10:24

Pointeurs : allocation dynamique18:42

Pointeurs « intelligents »12:28

Conoce a los instructores

Jean-Cédric Chappelier
Dr.
School of Computer and Communication Sciences
Jamila Sam
Dr
School of Computer and Communication Sciences

We have seen references in detail.

Now, let's pass to real pointers.

The main difference is that a pointer

is a variable which actually exists in memory.

It is not just a label for a variable which already exists,

it is a new variable.

To understand what pointers are, one must first know that variables,

generally, are identified by an address in memory.

Suppose, for example, that I have a variable <i>x</i> which exists in memory

and which contains something, anything, and that I look

at memory like this as I go along.

At a certain place, the value of <i>x</i>

will be written to memory in binary form,

and at another moment it will be something else, some other variables.

Well, this variable <i>x</i> is identified by its address,

the place in memory where it begins.

So, we can take the address in memory of a variable.

And a pointer is just that, precisely,

it is a variable which contains the address of another object in memory.

A pointer is simply a variable which contains the address of another variable.

It is a sort of variable of variables, just like a bookmark

for a website, which I took as an example in the introduction,

is the address of a site which exists elsewhere.

And so this way,

references and pointers are fundamentally different.

A reference is not a variable which contains an address,

it is just another label.

There is a big difference between having another label on the same drawer

and another drawer which contains the address of something.

A pointer thus has an extra level, a level of abstraction.

A pointer is really like a book from an address book.

A page on which we could write only one address at a time.

And I would now like to use this analogy

to illustrate a whole bunch of practical applications

for pointers in programs.

These different cases will be illustrated with C++ syntax

that you have not encountered yet and that will be presented

in the next video

which presents the more practical, technical aspects.

Still, we left them here

in case you might want to review this video

or review the slides once you will have learnt about this syntax.

It could be useful.

Let's start with the first case: declaring a pointer.

I told you that a pointer was a variable

and so we can declare a pointer.

What does it mean to declare a pointer?

It just means that we are adding a page to our address book.

But careful, adding a new page

does not mean that there is an address written on it.

Adding a new page is just adding a new page

without specifying what will be written on it.

There will likely be something written on it, but we don't know how to interpret it.

Second point: assigning a pointer.

What does it mean to assign a pointer?

It means giving an address,

writing an address on the page of our address book.

We sometimes speak of allocating a pointer.

What does it mean to allocate a pointer?

Allocating a pointer means, to go buy some land somewhere.

It is allocating memory and building a house in it.

That is, putting a value there and getting back the address of this house

to copy it onto the page of our address book.

That's what allocating a pointer means.

But note that here -- this is a common mistake --

the pointer is neither the land nor the house.

Those are two completely different things.

The pointer is just the page of the address book

which contains the address of the house.

It is not the house itself.

In other words, the pointer does not equal the value pointed by it.

Those are two completely different things.

In the case of the page of an address book

which contains the address of the land that we bought, that we allocated,

freeing a pointer simply means

that we authorize the memory to re-use this area,

that this area is no longer our property

and that it can be re-used freely somewhere else.

And so this area is no longer our property.

So careful, freeing a pointer is actually a misnomer,

it actually means freeing the land it points to,

it does not mean that we have erased the page of our address book.

The address is still there, but it is a common mistake

to reuse the address of a piece of land which does not belong to us anymore.

This is why, later on,

we will encourage you to erase the page of the address book

once you have freed the pointer.

Copying a pointer is what you do

when you give an address to someone you know.

When you give an address to friends,

they copy the address that you give them into a page of an address book.

They will not copy the house whose address it is.

You do not give them your house.

You give them only the address, you copy the address.

That's what it means to copy a pointer:

it is an exchange of addresses.

And finally, erasing or cancelling a pointer

simply means that you are erasing the page of your address book.

Note that if this address was the address of a piece of land

that we had allocated in memory,

erasing the pointer would not free this land.

The land is still occupied, it is still used.

So first, we have to destroy the pointer before erasing it.

There, I hope this analogy will help you to use pointers

more easily, to understand what is going on.

The next video will show you

how to apply this practically

by going into detail on how to use pointers.

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