Problem returning const pointer

[kk_oop@yahoo.com]

Hi. I just wrote a function that returns a const pointer:

MyClass
{
....
public:
virtual ReturnClass * const getReturnClass( );
....
}

I then wrote some code that calls the function to make sure the
pointer was really const:

MyClass myClass1;
ReturnClass * myReturnClass = myClass1.getRet urnClass( );

MyClass myClass2;
myReturnClass = myClass2.getRet urnClass( );


I anticipated getting a compiler error when I attempted to reset
myReturnClass, but this did not happen. The compiler let me change
the value. What did I do wrong?

Thanks!

Ken

[Pete Becker]

Re: Problem returning const pointer

On 2008-10-22 18:05:27 -0400, kk_oop@yahoo.co m said:

Hi. I just wrote a function that returns a const pointer:
>
MyClass
{
...
public:
virtual ReturnClass * const getReturnClass( );
...
}
>
I then wrote some code that calls the function to make sure the
pointer was really const:
>
MyClass myClass1;
ReturnClass * myReturnClass = myClass1.getRet urnClass( );

myReturnClass is not a const pointer. Without seeing the definition of
myClass1 it's not possible to say what's going on. Which is why the FAQ
recommends including a minimal code sample that shows the problem,
rather than a summary that omits the key parts.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[Thomas J. Gritzan]

Re: Problem returning const pointer

kk_oop@yahoo.co m schrieb:

Hi. I just wrote a function that returns a const pointer:
>
MyClass
{
...
public:
virtual ReturnClass * const getReturnClass( );
...
}

Top-level const on return values is quite useless, since you make a copy
of the returned value anyway.

What do you want to accomplish by that?

I then wrote some code that calls the function to make sure the
pointer was really const:
>
MyClass myClass1;
ReturnClass * myReturnClass = myClass1.getRet urnClass( );
>
MyClass myClass2;
myReturnClass = myClass2.getRet urnClass( );

myReturnClass is not const.

It's like this:

const int i_const = 5;
int i2 = i_const;
i2 = 7; // i2 is not const!

i2 contains a copy of i_const, but is not const itself.

I anticipated getting a compiler error when I attempted to reset
myReturnClass, but this did not happen. The compiler let me change
the value. What did I do wrong?

Consider using references more and pointers less.

--
Thomas

[James Kanze]

Re: Problem returning const pointer

On Oct 23, 12:05 am, kk_...@yahoo.co m wrote:

Hi. I just wrote a function that returns a const pointer:

MyClass
{
...
public:
virtual ReturnClass * const getReturnClass( );
...

}

I then wrote some code that calls the function to make sure the
pointer was really const:

MyClass myClass1;
ReturnClass * myReturnClass = myClass1.getRet urnClass( );

MyClass myClass2;
myReturnClass = myClass2.getRet urnClass( );

I anticipated getting a compiler error when I attempted to
reset myReturnClass, but this did not happen. The compiler
let me change the value. What did I do wrong?

Why would you anticipate a compiler error? myReturnClass isn't
const, so there's no reason you shouldn't be able to modify it.
How is this any different from:

int const c = 43 ;
int i = c ;

For non-class types, the const keyword is ignored for rvalues;
an rvalue of a non-class type is never "cv-qualified". (For
class types, it is relevant, since you can call member functions
on an rvalue; if the rvalue is const, you can only call const
member functions on it.) The return value of a function is an
rvalue, so there's really no point in declaring it const. (Note
that except for calling a member function on it, there is no
possible way to modify an rvalue.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
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