Why can I modify the class with a const function in C++11?

Question

I'm a bit newbie to CPP and I don't know why the setValue() const works meanwhile it's a const.

Why the class allows modification from a const public

It seems really odd, there is no error on g++ -Wall or with MS Visual C++

Here is my code:

main.cpp

#include <iostream>
#include <cassert>
#include "DArray.h"

int main(void)
{
    DArray darray(1);
    darray.setValue(0, 42);
    assert(darray.getValue(0) == 42);
    darray.~DArray();
    system("pause");
    return 0;
}

DArray.h

class DArray
{
private:
    int* tab;

public:
    DArray();
    DArray(unsigned int n);
    ~DArray();

    int& getValue(unsigned int n) const;
    void setValue(unsigned int n, int value) const;
};

DArray.cpp

#include "DArray.h"

DArray::DArray()
{

}

DArray::DArray(unsigned int n)
{
    tab = new int[n];
}

DArray::~DArray()
{
    delete[] tab;
    tab = nullptr;
}

int& DArray::getValue(unsigned n) const
{
    return tab[n];
}

void DArray::setValue(unsigned n, int value) const // HERE
{
    tab[n] = value;
}

Answer 1

It is because you do not modify it. When you do:

int* tab

tab contains only an address. Then in

void DArray::setValue(unsigned n, int value) const // HERE
{
    tab[n] = value;
}

You do not modify this address, you modify some memory after it. Thus you do not modify your class.

If instead, you used

std::vector<int> tab

You would have an error in setValue because you would modify an element of your class.

Answer 2

First of all, don't call explicitely the destructor of your class, this will be called when the variable goes out of scope automatically.

darray.~DArray();

What you are promising with const in the method is that the member variables will not be modified. The variable int* tab is a pointer to an int. With your setValue function you are not changing the address of the pointer (which is promised not to be changed by the final const in the signature of your method) but the int value pointed by it. This is fine.

However, if you change the pointer address, for example with tab = nullptr , you will see a compiler error like:

error: assignment of member 'DArray::tab' in read-only object

Answer 3

Why can I modify the class with a const function in C++11?

It is possible to modify mutable state of the object. It is technically also possible to modify the non-mutable state using const_cast , but it would be a bad idea to do so because if the object itself is const, then behaviour would be undefined.

That's however not what you do in this code.

why the setValue() const works meanwhile it's a const.

Because it doesn't modify any member of this . It modifies an array that is pointed by a pointer to non-const . Constness of a member function, or constness of the object is not transfer to an indirectly pointed object.