Re: Const/non-const pointer returning method

From: (Jens Thoms Toerring)
25 May 2010 19:47:08 GMT
Kai-Uwe Bux <> wrote:

Jens Thoms Toerring wrote:

  I am rather new to C++ and have run into a problem where I
haven't found an answer yet by searching (probably didn't find
the right search terms). I Have this simple program:

#include <iostream>

class A
    A( int i ) : m_ip( new int[ i ] ) { }
    int const * ip( ) const { std::cout << "const\n"; return m_ip; }

Sorry, that was a mistake just before copy-and-paste...

    int * ip( ) const { std::cerr << "non-const\n"; return m_ip; }

    int * m_ip;

int main( )
    A a( 10 );
    int const * ip = a.ip( );
    std::cout << ip[ 2 ] << '\n';

Remark: as an illustration for the problem of which member function is
called, the code is fine. Considered on its own, however, class A leaves
room for improvement, e.g., with regard to memory management and

Yes, of course, this wasn't meant to be production quality code
but just a bare-bones example, so it leaks memory etc.

My exectation was that when calling ip() to get a const pointer
the compiler would be able to figure out I want it to use the
first ip() method that returns a const pointer. But it turns out
that always the second one is invoked.

You will have to adjust your expectations (if you have not already done so).
The object a was not declared const. Hence any method call a.method() will
always invoke the non-const version.

Well, I am in the process of adjusting my expectations all the
time;-) That's part of the fun of learning a new language...

I'm not sure why and if
there's a way that I can make it pick the second one (short of
using different names for the methods)?

Yes, you could do:

  A a ( 10 );
  A const & b ( a ); // be is a const alias for the object a.
  int const * ip = b.ip();

Alternatively, some trickery using casts would do.

BTW: why would you want the const method invoked?

That's a bit longer story: The array in the class will be an
array of pointers to rather large amounts of data. And I will
need lots of copies of that class. In the copies typically
only small subsets of the data will have to be changed. Thus,
to keep the total amount of memory used down, my idea was to
have boost::shared_ptr's in the array (thus having a refe-
rence count and automatic deallocation) and to make a "real"
copy of an element only when it is needed, i.e. when a non-
const instance of the element is requested and the reference
count isn't 1. For that I had hoped for the function for re-
turning a const reference/pointer to be invoked when a const
reference/pointer is requested and the non-const returning
version otherwise (in which then a copy is made when neces-
sary). And then, of course, I hoped to make all that com-
pletely transparent to the user of the class, so they don't
have to think too much about what they're doing...

I also noticed the same
effect when using const versus non-const references as return
values, also there the non-const returning function is called
eben when one asks for a const reference. Does all this only
work when overloading the [] operator?

The operator[] is not different: the const version is called on const
objects and the non-const version is called on non-const objects. So it is
not clear, what you observed.

Probably my observations weren't very good. to be honest at
the moment I'm still a bit overloaded with understanding
what is happening when;-) But your explanation has hopefully
cleared up a few misconceptions.

                              Best regards, Jens
  \ Jens Thoms Toerring ___

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