Thorsten's Weblog

After fixing a self-assignment blunder in basegfx/source/matrix/b2dhommatrix.cxx, which happened in the copy-on-write-related parts of operator=, I began to wonder why on earth everybody and his grandma was doing cow (copy-on-write) by hand (a search for mnRefCount and m_nRefCount (though certainly only a subset) should give you an idea how abundant this theme is. BTW: I generally agree with Herb Sutter, that cow has disadvantages in a multi-threaded world — but, alas, giant objects like bitmaps or polygons should better still provide a cow implementation in the back).

So, I dug at the usual places, and in the corners of boost and ASL, I indeed found a cow_ptr and a copy_on_write wrapper. Nice. Only that I found cow_ptr too ambitious (all the cloning stuff should be irrelevant for OOo’s cow usage), and copy_on_write lacked some of boost::shared_ptr commodities I got used to (checked_delete, glue for boost::mem_fn, operator forwarding, standard-compatible swap, you get the idea).

Consequently, I cobbled together a cow_wrapper, tailored to the usual, pimpled use of cow in OOo:

class Impl;

class MyClass
{
public:
   MyClass();

   void set( int v );
   int  get() const;

private:
   cow_wrapper mpImpl;
};

where the get() and set() methods simply forward to the Impl methods:

void MyClass::set( int v )
{
    mpImpl->set(v);
}
int MyClass::get() const
{
    return mpImpl->get();
}

And the trick is: cow_wrapper has operator-> overloaded for const and non-const access. For non-const access, the potentially shared copy of the Impl object gets copied (i.e. made unique).

Except that cow_wrapper doesn’t yet have all wanted features (checked_delete support still missing), and possibly needs some testing, I think this holds up for quite painless cow implementations (yes, I intend to move at least basegfx to cow_wrapper, eventually).