Question

Possible Duplicate:
Is const_cast safe?

Obviously I’d never write this code, but it’s a very much simpler example of something that came up in a real program.

#include <iostream>

void change(const int& data)
{
    int& data2 = const_cast<int&>(data);
    data2 = 100;
}

int main()
{
    int thing = 123;
    change(thing);

    std::cout << thing << "\n";
}

Is it well defined behavior that this alters the data referred to, or is the compiler allowed to assume that because it’s passing a const int& that the function can’t change the value passed in and generate code accordingly?

edit: All the compilers I tried it on output the changed value, 100.

This appears to be a duplicate of Can C++ compiler assume a const bool & value will not change? so I’m happy to close this one.

Answer 1

This is not well defined behavior unless you know if the original value was constant. It will work in the exact example you have given. However, if you passed in a const variable it could cause a memory access violation. This would be because the compiler put the const variable on a read-only memory page.