[C++-sig] Re: Exposing copy constructors with Boost::Python

[David Abrahams]

Christopher Dawson <dawson at physics.uq.edu.au> writes:

> Apologies if this is a common problem with a common solution, but I
> haven't been able to find it anywhere.
>
> I have a simple class which stupidly has a c-style array as a member:

Your class has a pointer as a member, not an array.

> class Test {
> public:
> 	int *a;
>
> 	Test(int N) {
> 		a = new int[N];
> 	}
>
> 	Test(const Test& T) {
> 		a = new int[T.N];
                     ^^
?? There is no N member of Test??

Your copy ctor doesn't copy the contents of the array.  Intentional?

> 	}
>
> 	Test& operator=(const Test& T) {
> 		if (N != T.N) {
> 			delete[] a;
> 			a = new int[N];
> 			for (int i=0;i<N;i++)
> 				a[i] = T.a[i];
> 		}
> 	}
> 	~Test() {
> 		if (a) 
> 			delete[] a;
> 	}
> };
>
> I have wrapped this with Boost::Python (1_31_0), and understood that
> the copy and assignment constructors would be wrapped automatically.A
>
> However copying the object from Python

What do you mean by "copying the object from Python"?

> just copies the pointer, not
> the array. I can explicitly expose the copy constructor with
>
> 	.def(init<const Test&>())
>
> so  it's possible to do 
> 	
> 	a = Test(4)
> 	b = Test(a),
>
> but I'd like to be able to to
>
> 	b = a

When you do that in Python, it *always* just rebinds the reference
that B was referring to so b and a are "pointing at" the same object.
Nothing ever gets copied.  There's no way to change that; it's just
the way Python works.

> Have I missed something about exposing copy constructors?

Forget about Python for a moment; it looks to me like you've missed
something about writing copy ctors in C++.

Don't you want this C++ to work

      Test x[2];
      x.a[0] = 1;
      x.a[1] = 2;
      Test y = x;
      assert(y.a[0] == x.a[0]);
      assert(y.a[1] == x.a[1]);

??
-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com