c++ const member function

Question

I am reading a book called 'Effective C++, Second Edition' and its talking about const member functions and how you have bitwise const-ness and conceptual const-ness.

It says most compilers will go with bitwise const-ness, which is that you cannot alter data members of an object inside a const member function.

Then there's an example of a member function that doesn't seem to act bitwise in the const test.

It goes like this:

#include "stdafx.h"
#include <string>
#include <iostream.h>

using namespace std;

class mystring
{

public:
    mystring(const char* value);

    operator char *() const { return data; }

private:
    char * data;
};

mystring::mystring(const char * value)
{

    mystring::data = const_cast<char*>(value);
}


int main(int argc, char* argv[])
{
    const mystring s = "Hello";

    char * nasty = s;

    *nasty = 'M';

    printf("s: %c", s);

    return 0;
}

When this is run, it says in my book it should allow you to change the value of s, even though its const. This because char* data is pointing to the same as const char* value is pointing. *data in this case is not const.

However trying to run this in MS VC++ 6.0, it throws an access violation at line *nasty = 'M';

Can someone explain what is going on? I think I've missed something?

To me it seems that because we have a const mystring s, we should not be able to change it, but then what it says in the books seems awkward.

Answer 1

The access violation is because you try to change a string literal. Your code is equivalent to:

char * p = "Hello";
* p = 'M';

which is illegal in both C and C++ - nothing to do with const member functions.

Answer 2

You get access violation only because the char* pointer is to a string literal. Changing a string literal is undefined behavior (AV in your case), it has nothing to do with const-correctness.