Passing a constant is automatically passed by reference?

Question

If I pass a variable to a function but mark that variable as constant will it automatically get passed by reference?

---

In C++

void foo(const int a)

After testing: No because it prints different memory locations.

void foo(const int a)
{
    cout << &a << endl;
}

int main()
{
   int a = 5;
   foo(a);
   cout << &a << endl;
   return 0;
}

---

C

No right? Because I can only pass a pointer to the memory location, not really by reference. Like here: Passing by reference in C

---

Java

Maybe the compiler can figure out that final should point to the same memory location?

void foo(final int a)

---

C#

Same as Java?

void foo(const int a)

---

---

I know I included 4 languages in my question but since I use those 4 languages a lot I want to know.

Is there a way to force Java and C# to handle do this? I know in C++ I can add & to the function and it will be exactly what I want.

 void f(const int &a)

Pretty much that is what I am hopping Java and C# compilers are doing behind the scenes.

Answer 1

In Java, all the variables are passed by value.

Primitives are actually copied and object variables are references anyway, meaning that the value of an object variable is actually the reference -> you are passing the reference's value.

Nevertheless, you always pass by value, same as in C++, you will pass by value if you don't use the reference explicitly.

I'm almost certain that it's the same in C#.

Edit: I forgot about C

It's the same in C: pass by value by default. Obviously, you can pass a pointer. In C++, this is extended so you can take a reference as a parameter:

void function(int& intByReference) {
    cout<<&intByReference<<endl; //prints x
}

int main() {
    int n = 0;
    cout<<&n<<endl; //prints x
    function(n);
}

As C++ includes classes, there is a default constructor to copy the object:

class MyClass {

    public:
        MyClass(MyClass& objectToCopy) {
            //copy your object here
            //this will be called when you pass an instance of MyClass to a function
        }
};

Answer 2

You are making a mistake in thinking your intuition can guide you to how the details of the languages are implemented. This is quite simply not true. In the case of C++ and C, the spec states quite clearly how this should be dealt with.

Java as a pass by value language might have some provision for this behaviour in the specs for final/const/not touched values. (I don't know) But it is only the specs that can allow for this behaviour.

Answer 3

Before answer your question let me illustrate an example to test your scenario.

Suppose you call a method void called(int a ) from method void calledFrom() by passing a variable int a.

In my understanding :

If change of value of a in any method effect another (change of a in calledFrom effect the value of a in called) then its called by reference otherwise its called by value.

Now lets test this in java using final

public class MyClass
{
    int a;
    public void setA(final int a)
    {
        this.a=a;
    }
    public void printA()
    {
        System.out.println(a);
    }

}

from main method you do this:

int a=10;
MyClass mclass=new MyClass();

mclass.setA(a);
mclass.printA();
a++;
mclass.printA();

the output is : 10 10

so changing in main not changing the value of a in MyClass. so call to public void setA(final int a) was not a called by reference it was call by value.

The key final may be arising some confusion. When you writing final in method signature is not going to effect is behavior in any other way except you cant change the value of that parameter inside that method. as for example if you write

public void setA(final int a)
{
    a++; //error
    this.a=a;
}

you will get a compilation error

but you can esealy do this

 public void printA()
{
    a++; //ok
    System.out.println(a);
}

I think this is not only in java all other languages have the same behavior . you can try in other languages .

to get a clear concept about pass by value and pass by reference in context of java you can read this thread

or read this answer