constants - C/C++ changing the value of a const

Question

I had an article, but I lost it. It showed and described a couple of C/C++ tricks that people should be careful. One of them interested me but now that I am trying to replicate it I'm not being able to put it to compile.

The concept was that it is possible to change by accident the value of a const in C/C++

It was something like this:

const int a = 3;          // I promise I won't change a
const int *ptr_to_a = &a; // I still promise I won't change a
int *ptr;
ptr = ptr_to_a;

(*ptr) = 5;               // I'm a liar; a is now 5

I wanted to show this to a friend but now I'm missing a step. Does anyone know what's missing for it to start compiling and working?

ATM I'm getting invalid conversion from 'const int*' to 'int*' but when I read the article I tried and it worked great.

Answer 1

you need to cast away the constness:

linux ~ $ cat constTest.c
#include <stdio.h>


void modA( int *x )
{
        *x = 7;
}


int main( void )
{

        const int a = 3; // I promisse i won't change a
        int *ptr;
        ptr = (int*)( &a );

        printf( "A=%d
", a );
        *ptr = 5; // I'm a liar, a is now 5
        printf( "A=%d
", a );

        *((int*)(&a)) = 6;
        printf( "A=%d
", a );

        modA( (int*)( &a ));
        printf( "A=%d
", a );

        return 0;
}
linux ~ $ gcc constTest.c -o constTest
linux ~ $ ./constTest
A=3
A=5
A=6
A=7
linux ~ $ g++ constTest.c -o constTest
linux ~ $ ./constTest
A=3
A=3
A=3
A=3

also the common answer doesn't work in g++ 4.1.2

linux ~ $ cat constTest2.cpp
#include <iostream>
using namespace std;
int main( void )
{
        const int a = 3; // I promisse i won't change a
        int *ptr;
        ptr = const_cast<int*>( &a );

        cout << "A=" << a << endl;
        *ptr = 5; // I'm a liar, a is now 5
        cout << "A=" << a << endl;

        return 0;
}
linux ~ $ g++ constTest2.cpp -o constTest2
linux ~ $ ./constTest2
A=3
A=3
linux ~ $

btw.. this is never recommended... I found that g++ doesn't allow this to happen.. so that may be the issue you are experiencing.