Re: Simple const-related question

From:
nsdevelop12@yahoo.com
Newsgroups:
comp.lang.c++
Date:
Mon, 18 Feb 2008 18:14:49 -0800 (PST)
Message-ID:
<1858a46b-25e6-497d-ae87-e100e17e066e@i12g2000prf.googlegroups.com>
On Feb 18, 9:05 pm, Jeff Schwab <j...@schwabcenter.com> wrote:

Daniel T. wrote:

On Feb 18, 5:45 pm, nsdevelo...@yahoo.com wrote:

On Feb 18, 6:30 pm, nsdevelo...@yahoo.com wrote:

Is "const Object* pA = new Object(1)" the same as "const Object A(1)"
in terms of creating const objects that should not be modified through
cast-to-non-const pointers. See the following example:
const int* createInt()
{
    return new int(1);
}
void myFunc( const int* const_param )
{
    cout << "const_param == " << *const_param << endl;
    int *non_const_param = const_cast<int*>( const_param );
    *non_const_param = 999;
    cout << "const_param == " << *const_param << endl;
}
int main(int argc, char* argv[])
{
          int a = 1;
    const int b = 1;
          int* c = new int(1);
    const int* d = new int(1);
    const int* e = createInt();
    myFunc( &a ); // ok
    myFunc( &b ); // bad
    myFunc( c ); // ok
    myFunc( d ); // bad (?)
    myFunc( e ); // bad (?)
    return 0;
}


I believe the last two examples are OK. "int const* p = new int()" is
fundamentally different from "int const i = 5", because the type of the
object pointed to may be in a read-only area of memory in the second
case, but not in the first. I'm not sure why you would ever need this,
though; if you need a non-const pointer to a dynamically allocated
object, why not just hang onto it in the first place? Unnecessary
const_casts defeat the purpose of declaring a pointer-to-const in the
first place.

I guess my real question is what does "new int(1)" or "new MyObject()"
create - is it always non-const object, or does it depend on the
context as in the following:

MyObject *p1 = new MyObject(); // p1 is non-const
const MyObject *p2 = new MyObject(); // p2 is ???

What about:

MyObject* myFunction()
{
    return new MyObject();

}

MyObject* p1 = myFunction(); // p1 is non-const
const MyObject *p2 = myFunction(); // p2 is ???


Get out of the Java habit of putting () after the class name when
calling new. I'm pretty sure that "new Object();"


Allocates a default-constructed Object.

and "new Object;" do


Allocates a potentially uninitialized object, if Object is a POD type.

http://www.research.att.com/~bs/bs_faq2.html#malloc

I don't usually use the parentheses, either, but I guess I probably
should. I also don't usually allocate objects directly with new.

different things.

In answer to your question, the object returned by new is inherently
non-const, but once placed in a const pointer, non-const functions can
no longer be called on that object through that pointer. That doesn't
mean the object is const though, note for example:

Object* o = new Object;
const Object* o2 = o;

o->non_const_func(); // perfectly OK even though the object is being
held in a pointer to const elsewhere.


Agreed.


Thanks for your help. I'm really trying to avoid the caveat described
here:

http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.13

Using the author's example, I want to make sure that "const Set *s =
new S(...)" is not the same as "const Set s". Although the author
doesn't fully explain why changing "s" in the latter case through an
explicit non-const pointer cast (e.g. "Set *ps =
const_cast<Set*>(&s)") is a problem, I'm assuming its because the
compiler was free to optimize by substituting the value of "s"
wherever it was used in code. If that's the case, then I guess the
former case works because "s" is a pointer to dynamic memory that is
allocated at runtime. But I'm no expert in compilers, so I just
wanted to be sure there were not other tricks at play (like what if
"Set" had an inline constructor or something).

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