Re: MSDN const_cast sample

From:
=?Utf-8?B?R2Vvcmdl?= <George@discussions.microsoft.com>
Newsgroups:
microsoft.public.vc.language
Date:
Thu, 20 Dec 2007 00:36:02 -0800
Message-ID:
<24241F1E-6887-4D79-A84D-26FED50EC856@microsoft.com>
Thanks Igor,

My question is almost answered. A little confusing about the cache approach.

back even without const_cast, staying fully within C++ type system. This
means that the optimizer is not, after all, allowed to cache the "const"
value in this situation, which in turn means that const_cast doesn't


How will compiler make use of the cached value in the future?

BTW: the following sample is not valid, since in the sample, no value is
changed through pointer p and function f, right?

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

int g_theRealVariable = 5;
f(&g_theRealVariable);

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


regards,
George

"Igor Tandetnik" wrote:

"George" <George@discussions.microsoft.com> wrote in message
news:B3ADBA4B-0423-4124-8C55-585C0C357A9A@microsoft.com

"Ben Voigt [C++ MVP]" wrote:

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

int g_theRealVariable = 5;
f(&g_theRealVariable);

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


What is your sample tries to prove? I am lost in your sample. :-)


It's trying to show that f() cannot assume *p doesn't change across
other function calls, even though it's declared const, and thus must
re-read *p from memory in the last statement (as opposed to, say,
reusing a value cached in a register from before mutate() call). This is
because, while *p is declared const int, it may refer to a value that is
also accessible as a non-const int (so-called aliasing). In the example,
modify() can change the value of *p through g_theRealVariable (since
both expressions refer to the same object).

Ben used to claim that casting away const and modifying the value is
illegal not only when the object was originally declared const, but also
when a non-const object is known to someone only via a const reference.
He argued that it is legal for the compiler to cache a value of the
"const" object and reuse it later, which would break if const_cast was
used to modify the "const" value behind your back.

But with this example, Ben shows that the value can change behind one's
back even without const_cast, staying fully within C++ type system. This
means that the optimizer is not, after all, allowed to cache the "const"
value in this situation, which in turn means that const_cast doesn't
make matters any worse than they already are (doesn't prevent any
optimizations that would otherwise be allowed).

So the bottom line is: using const_cast to cast away constness and
modify the underlying object is legal as long as the object was not
originally declared const:

void modify(const int* cp) {
    // Not illegal in and of itself
    int* p = const_cast<int*>(cp);
    *p = 42;
}

const int c = 1;
modify(&c); // illegal - c originally declared const

int i = 1;
modify(&i); // legal

--
With best wishes,
    Igor Tandetnik

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necessarily a good idea. It is hard to be sure where they are going to
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