Pointeurs : déclaration et opérateurs de base

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En provenance du cours de École Polytechnique Fédérale de Lausanne

Initiation à la programmation (en C++)

204 notes

École Polytechnique Fédérale de Lausanne

Initiation à la programmation (en C++)

204 notes

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.

À partir de la leçon

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

Rencontrer les enseignants

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

With our address book analogy,

you now know of most of the operations we can do

on pointers.

In this episode, we will go over all these operations

one after the other and discuss

how to use them in C++ language syntax.

To declare a variable in C++, we know we can

use the following syntax.

So, name of the type, for example "int" for integers

followed by the identifier associated to the variable, i.e "x".

For a pointer, the notation is the same, except that the type

will indicate that it is a pointer to a particular data type.

So for example if I want to declare a pointer

on an integer, the associated type will be: "int*"

to mean "pointer to an integer", followed by the name of the pointer,

so of the pointer-type variable we are creating.

Similarly, if I want to declare a pointer on a "double"

I will use the following syntax.

I will write "double*" as the type

followed by the identifier for the variable, "ptr".

So here, I declared a variable named "ptr", the type of which is pointer

on a variable of type "double".

This is for simple declaration. If I want to initialize at the same time,

the syntax is analogous to the one I used up until now.

For example, if I want to write, declare and initialize a variable "x"

of integer type, initialized to 4, I use the following syntax.

Well, the syntax is quite analogous for pointers.

I must indicate in parentheses the value of the pointer.

And this value must be an address.

So here are three examples of declaration-initialization situations

on pointer-type variables.

Here, we are taking a bit of ahead start on what will follow in this episode

and the following ones, by seeing

how to obtain the address of a given memory area,

in this case the address of the memory area named "i", the variable "i";

and how to dynamically allocate a memory area

capable of storing data of a certain type, in this case an integer.

So of course, we will review these concepts

in much further detail when the time comes.

First line here, I am declaring-initializing

a "ptr" variable of type pointer to integer

So the "ptr" variable can contain the address of an integer

and we initialize this variable using a special value

which is "nullptr" meaning that the "ptr" pointer

points to nothing.

So remember our address book analogy.

Here, we have a page of our address book which is empty,

which is initialized to nothing, which contains nothing.

Second instruction, I initialize a "ptr" pointer

using the address of an existing memory area,

of a variable which exists in memory.

We will come back to this later.

And here, I initialize my pointer using the address of

a dynamically allocated memory area.

These concepts will be the focus of the following episodes

and of the rest of this sequence.

By the way, please note that the special value "nullptr" can be assigned

to any type of pointer -- be it a pointer to a "double",

pointer to an integer, pointer to any other type

of data defined in C++.

In either case, it means that

the pointer points to nothing.

C++ provides two fundamental operators

for pointer manipulation.

The '&' operator (ampersand), and the '*' operator (star).

The role of the '&' operator is to return the memory address

of the value of a variable.

Let's see what this means on a real example.

Here, we have a small program which starts by declaring

a variable 'x' of integer type, and initializes it to 3.

We know that to each variable of a program,

an address in memory is assigned.

Let's name this address "adr".

Then, we declare a second variable "px"

which is of type pointer-to-integer.

Meaning that it is capable of containing the address

of a memory area containing an integer.

So we have this situation.

And here, we will start by initializing the variable "px"

using the special value "nullptr".

Which basically means that "px"

is capable of pointing to a memory area containing an integer,

but that at this moment, it points towards nothing.

It points to nothing at all.

Now, the third line of the program.

And this is where our "&" operator comes in.

We assign, to the variable "px", the address of the variable "x".

We have seen that this address here is "adr",

so this is what we end up with.

We will assign the address of the variable "x" to "px".

We are establishing the link between pointer and pointed-to area

with this assignment.

So to sum up, "px" returns the address associated to the variable "x".

The '*' operator allows us to get the value pointed to

by a pointer-type variable.

Earlier, we learned about the '&' operator.

So "&x", where "x" is a variable,

will return the address of "x" in memory.

Now, imagine that we have a pointer "px".

This pointer points to, contains the address of, a memory area

containing a certain value, for example 3.

And we wish to find the value pointed to by using the pointer.

For this, we need to use the '*' operator

applied to a pointer-type variable. This is what will allow us

to get the value pointed to by this pointer.

So if we go over our small example from the previous episode again,

we go through the first instructions

and we find ourselves in the following situation.

A variable "x" initialized to a value of 3, of integer type and which has an address.

A second variable "px" which is of pointer-to-integer type

which we begin by initializing to "nullptr",

meaning that it points to nothing.

And to which we then assign the address of "x"

which will establish the link between pointer

and pointed-to variable.

Then, last instruction, and this is where we call

our famous little '*' operator

which will allow us to establish the link between pointer

and pointed-to value.

So here, we have "*px".

we use the '*' operator on the pointer, which will allow us to get,

through the pointer, the value pointed to by this pointer.

Now, imagine that I have a variable of type "int"

named "i" which contains a certain value and which has an address in memory.

If I write "&i", I will get a value of type pointer-to-int

which is the address "adr".

Now, if I write " *&i ",

I will apply the '*' operator to this pointer

which will return the value stored at that address,

which is 3.

So writing " *&i " is exactly equivalent to writing "i"

since I directly access the value contained in "i".

C++ has the reputation of being a language with a somewhat complicated syntax.

The '*' and '&' operators which we have just seen

are a first example of that.

In fact, depending on the context in which they are used, these operators

do not necessarily always mean the same thing.

Let's see why.

There are now two contexts in which we use the '&' operator.

The first context is when the ampersand follows a type name

and the second is when the &

precedes the name of a pointer-type variable.

In the first case, we will use the '&' to declare

a reference, either when we pass

a variable by reference to a function, or when we declare a reference on its own.

For example, I can declare a variable "i" of integer type

and I can declare a reference

to this variable named "id".

This simply means that "id" is now

another name for the variable "i".

So, first context of use,

we use the "&" to indicate that we are declaring a reference.

Second context of use: when the ampersand symbol

precedes a variable name. We have just seen

that in this case, we will simply obtain

the address of this variable.

And this address can of course be assigned to a pointer-type variable

to establish the link between a pointer and a pointed-to variable.

So here, we have a context of use which

is completely different from the first and which

leads us to a totally different situation.

So, first potential for confusion: two possible contexts for use

of the ampersand which means different things in C++.

Second example where C++ uses the same symbol

to represent two different things: the star '*'.

Indeed, we saw earlier that

when a '*' follows a type name as it is the case here,

it is simply used to declare a variable of pointer type.

For example, if I write something like this in a program,

I am declaring a variable named "id"

and this variable is defined as having a pointer-to-int type

thanks to this '*'.

This means that it can store, potentially,

the address of a memory area containing an integer.

Like so.

So "id"

is a variable of pointer type

declared using '*'.

If, however, the '*' precedes the name of a variable,

this supposes that the variable considered

is a pointer-type variable.

And the '*' will serve to return the value pointed to by the pointer.

And here, this is the situation we are in.

We have a pointer-type variable which contains

the address of a certain memory area, which contains a value.

Let's say that this variable is an integer.

So "*id" will simply return the value of 3,

the value pointed to by the "id" pointer.

So here, the '*' serves to find the value pointed to by a pointer.

It is a context of use

which is quite different from what we have seen up until now.

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