Re: How to ban a class copy?

From:

=??B?Smn47SBQYWxl6GVr?= <jpalecek@web.de>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 13 Jul 2008 21:01:10 CST

Message-ID:

<g5dc3c$2d5f$1@ns.felk.cvut.cz>

Chris Morley wrote:

I need to prevent a class being copied _by accident_ (so the destructor of
the copy doesn't deallocate memory etc.).

I can make a empty private constructor which takes a reference to the
class which works because then you get an private access violation error.
This gets messed up _if_ you have a friend (although a compile time assert
in the private constructor might work there).

Is this the way to solve this or is there a better one - which works even
with friends?

Yes, there are possibilities:

class CNoCopy
{
//friend class CAnother; // don't have friends though!
CNoCopy(CNoCopy& fail) {};

1) Delete the body, so the linker will complain afterwards.

2) Make the constructor templated, and make the body uncompilable.

template <class T>
struct A{};

class CNoCopy
{
   template <class T>
   CNoCopy(T& fail) { typename A<T>::type t=0; }

... or ..

template <class T>
struct B
{
   virtual void f()=0;
};

   template <class T>
   CNoCopy(T& fail) { new B<T>; }

... or even ..

   template <class T>
   CNoCopy(T& fail) { int a[-1-sizeof(T)]; };

The first possibility is, however, vulnerable to someone who provides his own definition, the second is generally
vulnerable to template specializations (although that can be dealt with).

And a side note: you should use a copy-ctor that take a const-reference, and take care of the assignment operator, too.

Regards
     Jiri Palecek

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