Const and Pointers.

[learn2adv]

The questions are directed toward the second line in main(). I ran this as displayed and compared it to removing the const and reruning. No difference. Why? No impact with a variable change????

What is actually kept constant when using const in this example? If the results are the same, then const has no influence.

Thanks.

// Exercise to observe pointer and const combinations.

#include <iostream>

using namespace std;

int main ()
{
int Var = 18; // case 1.

const int * pVar = & Var;

cout << "pVar = " << pVar << endl;
cout << "*pVar = " << *pVar << endl;

Var = 20; // case 2.

cout << "pVar = " << pVar << endl;
cout << "*pVar = " << *pVar << endl;

system("pause");
return 0;
}

[Cubbi]

What is actually kept constant when using const in this example?

The access path.

Given a modifiable object (Var), you may construct a const or a non-const access path (reference or pointer, possibly multilevel). The compiler will prevent you from using a const-qualified access path to modify the object, but it's perfectly OK to modify that object using a non-const access path.

Note that this is different from having a const object, e.g. const int Var. Doesn't matter if you have a pointer-to-const or pointer-to-nonconst, if it leads to the const object, modifying that object is undefined behavior.

[learn2adv]

I don't understand your response.

Basically, the const is directed toward Var or &Var? If it is Var, then there should have been a compiler error when Var was reassigned. If it is &Var, then the results indicate const has no influence and therefore why include it?

Not sure how to relate your response to my question.

Thanks.

[learn2adv]

Can anyone explain why next to my title is an icon of a person while everyone else has a new icon?

[Cubbi]

The const is directed towards pVar. It is a pointer-to-const-int. It means, it's a pointer, through which one cannot modify the int.

[learn2adv]

When you say "cannot modify the int.", the int means the type couldn't be change to float or char? Trying to understand what can't be modified.

Thanks.

[Cubbi]

I mean the value of int, not its type. Types are always immutable.

int main()
{

        int Var = 18;             // modifiable int

        const int * pcVar = &Var; // pointer to a non-modifiable int
        int * pmVar = &Var;       // pointer to a modifiable int
        const int& rcVar = Var;   // reference to a non-modifiable int
        int& rmVar = Var;          // reference to a modifibale int

//      *pcVar = 7; // illegal
        *pmVar = 7; // legal
//      rcVar = 7;  // illegal
        rmVar = 7;  // legal
}

[learn2adv]

In my example, for:

const int * pVar = & Var;

does the syntax to the left of * mean that const int is applied to Var and Var is immutable?

Const to the right of * from my understanding means the const is applied to the pointer and the address must be constant?

If this is true, I don't understand why no compiler error when Var changed to 20?

Thanks.

[cire]

const int *

and

int const *

are the same. pointers to const int.

They mean that the value pointed to cannot be modified through the pointer. It does not mean the value pointed to is immutable.

[coder777]

does the syntax to the left of * mean that const int is applied to Var and Var is immutable?

No, the type of 'Var' isn't changed. If you set Var=20; // Ok the dereference of 'pVar' will also change -> if((*pVar) == 20) // true