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.
Fonctions : passage des arguments
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From the course by École Polytechnique Fédérale de Lausanne
Initiation à la programmation (en C++)
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From the lesson
Fonctions
Cette semaine aborde un sujet fondamental en programmation : les « fonctions » qui permettent de beaucoup mieux structurer les programmes et d'éviter d'avoir à récrire plusieurs fois la même portion de code.
Meet the Instructors
Jean-Cédric ChappelierDr.
School of Computer and Communication Sciences
Jamila SamDr
School of Computer and Communication Sciences
In a previous video, we examined the five steps
that follow, in the most general case,
a function call.
In the examples we've seen up until now, the arguments we passed to the function
were either simple values, or expressions to evaluate.
Today, we'll take a closer look at what goes on
when the arguments we pass to a function are variables.
Let's start with a real example. Here, we have a small program
whose first instruction is to declare a variable "val"
and to initialize it with a value of 1.
The 2nd instruction of this program calls the function f, which is defined here,
giving it as an argument the variable val
which we just defined.
We have seen in a previous video that at the moment of this function call,
there is a link being made between the argument we passed to the function
and the list of parameters expected by the function.
So here, we have two entities. We have a variable x used
by the function. And the function "f" modifies this variable,
by incrementing it by one unit.
The question we ask ourselves now, is this:
Are x and val in the same memory area?
In other words, will the modification of x
have an effect on val, or not?
In programming, there are two ways of passing arguments.
Passing by value and passing by reference.
Let's examine, in the same example as earlier,
what happens in either of these cases.
So, we have a small function "f" which takes as parameter an integer "x".
It has a <i>void</i> return type, which simply means
that it does not return any value. As you can see,
there is no <i>return</i> statement.
And what this function does is to increment the value
of its parameter, "x" equals "x+1".
If a function uses passing of arguments by value, this means that
the parameter "x" of the function corresponds to a memory area local to the function.
This means that, when we invoke the function by passing it
as an argument, a variable which is defined elsewhere
in the program, for example here, in the main program.
At the moment we call this function, we will copy the value of the variable
passed as an argument into a memory area local to the function.
This means that when I execute this code, I am going to modify
the local area, in which is stored the value of the variable we passed as an argument.
This is passing by value.
To indicate that we want to use passing by reference,
we have to declare the function header in a specific way, in C++.
We will indicate, using a specific symbol, the "&" (ampersand), that the function "f"
should use references. When we define the header
of the function in this way, we indicate that, when the function is called,
the parameter "x" for this function is a reference to the variable that
we passed as an argument. It is just another name for the variable "val" passed
as argument. Here, the variable "val" was equal to 1, and the name "x"
references the same memory area as the one occupied by "val".
So in this case, incrementing x will also increment val.
When we pass an argument by value, the function will work
on a copy of the argument, which means that any modification that we make
within the function does not affect anything outside the function.
We are working on a local area, and the argument is not modified.
When we use passing by reference, we indicate that the local variable
which is passed as an argument corresponds to a reference on the object
associated to the argument, when the function is called. So, the main difference
is that any modification made within the function also affects
the outside, and is visible when the function ends.
In C++, we indicate that a function uses, for one of its parameters,
passing by reference by using a particular symbol, an ampersand (&).
When we prototype the function, we can use it to indicate that
the parameter x is passed by reference. In this case, it is a double
which is passed by reference.
To summarize, when we pass by value, the parameter of the function
is a memory area local to the function and any modification affects only the
local area, and does not affect the variable which was passed as an argument.
When we pass by reference, we indicate that the parameter for the function
is simply an alias, another name for the variable that was passed as argument
and that any modification of the parameter will also modify the variable
passed as argument. Now, if we return to our little introductory example,
we had a small main program which declared a variable "val" containing
an integer of value 1. Then, there was a function call.
If we examine the header of the function, there is no particular symbol,
no ampersand, which means that we are passing by value.
Since we are using passing by value, this means that x, the parameter
for the function, is a local memory area and thus that the value of "val"
is copied into "x".
Modifications to "x" will affect only "x". If we display "x", we can clearly see
that x was modified. However, when we are done executing the function
and we display the value of "val", we can clearly see that its original value
is unchanged, and that "val" is still equal to 1.
If we take this same example, but indicate that the function uses passing by reference
with the ampersand symbol, we still have a variable local to
the <i>main</i> which contains 1. When we make this function call, since there
is a passing by reference, we simply indicate that "x" is another name for
the argument passed to the function, "val". This means that here, whenever
I modify "x", I also modify "val". Displaying "x" clearly shows that x
was modified. Displaying "val" also shows that "val" was modified.
So, in what situations is it useful to use passing by reference?
Well, simply when we want a function
to be able to modify a variable that is passed to it as an argument.
Suppose for example that I wish to write a function that is able to input an integer.
So the goal of this function would be to ask the user to input an integer
on his keyboard. And of course, we want to retrieve this value
in order to use it elsewhere in the program.
How do we access this input value?
One way to do this would be to use passing by reference.
So, in the program that calls this input function,
we declare a variable which will store the result of the input, and we pass
this variable to the <i>saisie</i> (input) function. So when the function is executed,
the user will input a number, and this number will be
passed as a parameter local to the function. Suppose that the user inputs a 2.
Then, as we have seen when we pass by reference,
the argument that is passed to the function is exactly the same memory area
as the parameter. If we input a 2 into "a_lire" ("to_read"), it will also end up in "i", which means
that when the function ends, "i" will contain the value we input, 2.
Of course, this is not the only way to design an input function.
The most natural way, and no doubt
the best way, is to use the return value to retrieve
the result of the input. So here, our input function would have no argument.
When it is executed, it will ask the user for a value and will assign it
to a variable local to the function. It will then return it so that it can be
used by the outside world. And so here, we would need
to use assignment to retrieve the return value of the input function
and to store it in a variable. For example a variable in the
main program which will be used elsewhere in the main program.
So we can see here that we have two options for designing the input function,
one of which uses passing by reference.
We can also take advantage of this mechanism of passing by reference
to deal with situations where a function "returns" several results.
Let's imagine, for example, that we wish to write a function which converts
cartesian coordinates to polar coordinates. The function would
need, as parameters, the two cartesian coordinates. And the point
would be to calculate, from these two cartesian coordinates, the two polar coordinates.
Since in C++, a function can return only one value,
we can decide to have the function not return anything.
But instead, we can pass it, as parameters, two variables by reference
which will store the final result. This mechanism is also used,
for example, to exchange the content of two variables. Passing by reference,
here, is essential to carry out this switch.
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