If changing a const object is undefined behavior then how do constructors and destructors operate with write access?

Question

C++ standard says that modifying an object originally declared const is undefined behavior. But then how do constructors and destructors operate?

Answer 1

Here's a way that ignoring the standard could lead to incorrect behavior. Consider a situation like this:

class Value
{
    int value;

public: 
    value(int initial_value = 0)
        : value(initial_value)
    {
    }

    void set(int new_value)
    {
        value = new_value;
    }

    int get() const
    {
        return value;
    }
}

void cheat(const Value &v);

int doit()
{
    const Value v(5);

    cheat(v);

    return v.get();
}

If optimized, the compiler knows that v is const so could replace the call to v.get() with 5.

But let's say in a different translation unit, you've defined cheat() like this:

void cheat(const Value &cv)
{
     Value &v = const_cast(cv);
     v.set(v.get() + 2);
}

So while on most platforms this will run, the behavior could change depending on what the optimizer does.