Re: MSDN const_cast sample

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>
Tue, 18 Dec 2007 15:39:48 -0600
"Ben Voigt [C++ MVP]" <rbv@nospam.nospam> wrote in message

All I know is that f isn't aware of the internal working of mutate, it
knows the contract, which is that mutate receives a reference to const.
Therefore f can assume that *p cannot change and therefore can cache its
value throughout the function.

Yes, that is the contract and it is true that a function that takes a
reference to const and casts away const and modifies it is bad. But
the point I am making is that if the object when declared as non-
const, is passed to such a function, it is not undefined behavior. It
is only undefined behaviour if the object passed to this function was
a const object (const declaration for creating it). I am not saying
such functions are a good programming style, its bad bad bad since f()
can not really know if the constified object is actually a const or
not (may be with some effort but it won't make it a good function).

Here is a reference -
- Note that the call f(poly) is fine. It does not result in undefined
behaviour because poly is not a const object. But cpoly is and hence
f(cpoly) is undefined behaviour.

I guess, when in f(), as soon as a const_cast is applied and the
object is non-const, the compiler has to make sure that it writes the
change at the memory where the object resides and not the cached value.

There is no const_cast applied in f. mutate could be in a different
translation unit, or even in a dynamic library updated after the compile.

I think as long as the object was created non-const, the behavior is not
"undefined" which would mean "anything goes", it is loosely defined,
meaning the compiler may or may not ever use the updated value, but it is
not for example allowed to crash the program.

I just thought about it a little more, and I've been wrong.

f has to assume that the variable could be aliased, so it has to reload the
value after calling any unknown function. This is best proved without using

void mutate(const int&);
void f(const int* const p)
    int j = *p;
    cout << *p << j << endl;

int g_theRealVariable = 5;

void mutate(const int& i)
    // modify i through a non-const alias
    g_theRealVariable = 42;

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