Re: Problem with copy constructor (a bit long story...)

=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <>
Tue, 24 Apr 2007 17:40:50 GMT
On 2007-04-24 10:49, Jeroen wrote:

Hi all,

I'm trying to implement a certain class but I have problems regarding
the copy ctor. I'll try to explain this as good as possible and show
what I tried thusfar. Because it's not about a certain code syntax but
more a 'code architecture' thing , I'll use simple example classes
(which are certainly not complete or working...) just to illustrate the
idea (and I may make some mistakes because I'm not that experienced...).

The basic idea is that I have a class which has a data vector, but
objects of that class can also share that data vector between several

class A {
   vector<double> *data; // pointer, so objects of 'A' can _share_ data.
   bool data_is_shared; // does this 'A' share data with another A?

   A(const bool do_data_init=true); // ctor
   A(const A& a); // copy constructor
   A get_shared_A(); // get an object A which shares 'this' data


A::A(bool do_data_init)
   if (do_data_init)
     data = new vector<double>;
   data_is_shared = !do_data_init;

Copy ctor, always result in an 'A' which does not share data:

A::A(const A& a)
   data = new vector<double>;
   data_is_shared = false;
   *data = *;

Get an object A which shares data with 'this':

A A::get_shared_A()
   A a(false); = data; // copy the data pointer only
   return a;

The problems focus on this last method 'get_shared_A'. It should return
an object A that shares its data with 'this'. Problem: on returning 'a',
the copy constructor _may_ be called and then an object A is returned
which has its own data vector. So this doesn't work... I cannot change
the copy ctor because that should always return an A which does not
share its data (that's a requirement for correct behaviour of class A).

In my application, 'get_shared_A' is an operator which selects a part of
the original data in 'this', and the returned result must be an object A
because it must be used in mathematical expressions just like all other A's.

So, using a 'helper'-class B I tried to change this in:

A A::get_shared_A()
   B b; = data; // copy the data pointer only
   return b;

With class B:

class B {
   vector<double> *data;

And an extra conversion ctor for A:

A::A(const& B b)
   data =;
   data_is_shared = true;

So I wanted the code to force to use this new ctor and return an object
A which shares its data. But again, after the conversion ctor is called
it is still possible that the copy ctor is also called and things do not
work correctly.

In fact, all possible solutions I came up with do not work because you
cannot be sure if the copy ctor is called when returning from
'get_shared_A', it depends on the compiler you have.

So, does anybody has any ideas to overcome this problem?

Some time a go (Qt3) I took a look at the implicit sharing mechanism of
QList, the idea is that if you do something like this:

QList a;
/* add elements to a */
QList b = a;

then b will share the elements with a until one of them makes any
changes to the list, at which point it will "detach" (which basically
means that it will create a copy of the list and make the changes to the

If I understood your problem correctly it's quite similar but the other
way around, you want explicit sharing, but the idea should still be the
same and if you understand how they did it in Qt you should be able to
make it work for you (unless you need it to be thread-safe, at which
point it becomes a bit more tricky).

Erik Wikstr?m

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