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I have extended a library class 'LibraryBase' which I cannot modify. 'LibraryBase' contains a const
function 'ConstFunction'. In my subclass 'LibrarySub' I am overriding 'ConstFunction'. But in this
function I now need to call a non const function.

But that gives an error and as I understand also violates the const principle.

But is there no way to call a non const function from a const function? Here is a basic example:

class Test {
public:
void testConst() const {
// this is a no go!
testNonConst();
}
void testNonConst() {
}

private:
};

int main(){
Test t;
t.testConst();
return 0;
}

On 5/19/2010 2:41 PM, none wrote:
> I have extended a library class 'LibraryBase' which I cannot modify.
> 'LibraryBase' contains a const function 'ConstFunction'. In my subclass
> 'LibrarySub' I am overriding 'ConstFunction'. But in this function I now
> need to call a non const function.
>
> But that gives an error and as I understand also violates the const
> principle.
>
> But is there no way to call a non const function from a const function?
> Here is a basic example:
>
> class Test {
> public:
> void testConst() const {
> // this is a no go!
> testNonConst();
> }
> void testNonConst() {
> }
>
> private:
> };
>
> int main(){
> Test t;
> t.testConst();
> return 0;
> }

The only legal way is to do a const_cast on 'this'. And only do it if
you know that your object is non-const, which you probably can't know
since the system is going to be calling your 'ConstFunction'...

Why do you need to call the non-const function? The whole point in
having the 'const' in a function is to promise that the state of the
object is not going to change during the call. You're violating that
promise. Could it be you're overriding a wrong function?

Another possible solution is declaring your data that you need to change
as 'mutable' (RTFM). Then you can change the value of such data in a
member function declared 'const'. Not that I advocate that.

V
--
I do not respond to top-posted replies, please don't ask

On May 19, 9:41*pm, none <""mort\"@(none)"> wrote:
> I have extended a library class 'LibraryBase' which I cannot modify. 'LibraryBase' contains a const
> function 'ConstFunction'. In my subclass 'LibrarySub' I am overriding 'ConstFunction'. But in this
> function I now need to call a non const function.

Like you have already told you are developing/maintaining something on
base of something that should not compile. It may do anything. Nothing
is granted. It is *lot* worse situation than to write directly in
assembler. I would ask for budget to rewrite it and if refused, i
would turn it down. World is full of lot more hopeful job not done.

> But that gives an error and as I understand also violates the const principle.
>
> But is there no way to call a non const function from a const function? Here is a basic example:
>
> class Test {
> public:
> * *void testConst() const {
> * * *// this is a no go!
> * * *testNonConst();

You may try silly things like:

const_cast<Test*>(this)->testNonConst();

But i would prefer to run fast. There is good life elsewhere waiting
for you.

> * *}
> * *void testNonConst() {
> * *}
>
> private:
>
> };
>
> int main(){
> * *Test t;
> * *t.testConst();
> * *return 0;
> }

On May 19, 7:52 pm, Victor Bazarov <v.baza...@comcast.invalid> wrote:
> On 5/19/2010 2:41 PM, none wrote:
> > I have extended a library class 'LibraryBase' which I cannot
> > modify. 'LibraryBase' contains a const function
> > 'ConstFunction'. In my subclass 'LibrarySub' I am overriding
> > 'ConstFunction'. But in this function I now need to call
> > a non const function.

> > But that gives an error and as I understand also violates
> > the const principle.

> > But is there no way to call a non const function from a const function?
> > Here is a basic example:

> > class Test {
> > public:
> > void testConst() const {
> > // this is a no go!
> > testNonConst();
> > }
> > void testNonConst() {
> > }

> > private:
> > };

> > int main(){
> > Test t;
> > t.testConst();
> > return 0;
> > }

> The only legal way is to do a const_cast on 'this'. And only
> do it if you know that your object is non-const, which you
> probably can't know since the system is going to be calling
> your 'ConstFunction'...

There's no problem calling a non-const function on a const
object, as long as the function doesn't actually modify the
object in any way.

> Why do you need to call the non-const function?

Probably because the class in question isn't const-correct. He
says he can't modify it, which means that he probably got it
from somewhere else, where they didn't worry about const.

> The whole point in having the 'const' in a function is to
> promise that the state of the object is not going to change
> during the call. You're violating that promise. Could it be
> you're overriding a wrong function?

He's only violating the promess if the base class function
actually does modify something. The original poster, you and
I know enough to make this promess explicit with the const
keyword, but there are doubtlessly still a lot of people around
who don't know that---including, perhaps, the author of the
original class.

--
James Kanze