Copy-on-write pointer object in C++

Question

I tried to follow this article http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Copy-on-write on how to implement copy on write pointers in C++. The problem is, it doesn't work for me.

The crux of the object is in overloading the dereference operator (*) to do background copying if it should return a non-const reference:

   const T& operator*() const
    {
        return *m_sp;
    }
    T& operator*()
    {
        detach();
        return *m_sp;
    }

Sadly, it seems that only the second version is ever run. C-outing the my pointed-to object creates a copy, even doing something like

   CowPtr<string> my_cow_ptr(new string("hello world"));
   const string& const_ref=*my_cow_ptr;

causes the detach() function to run.

Any ideas on why it's not working as advertised?

Can you show some more context? How do you declare your `my_cow_ptr`? — Andy Prowl, Apr 20 '13 at 18:27
@Andy Prowl: I expanded the second snippet for you. CowPtr id defined in the link. — Petr Skocik, Apr 20 '13 at 18:36
@Konrad Rudolph: Well, technically it's copy-on-returning-a-non-const-reference. But that's splitting hairs. — Petr Skocik, Apr 20 '13 at 18:42
Perhaps whoever advertised it had no flying idea what he was talking about. — n. m. could be an AI, Apr 20 '13 at 18:46

score 1 · Accepted Answer · answered Apr 20 '13 at 18:45

1

The const member function will be called on a const object. so:

const CowPtr<std::string> my_const_cow_ptr(new std::string("Hello, world"));
const std::string& str = *my_const_cow_ptr;

or

CowPtr<std::string> my_cow_ptr(new std::string("Hello, world"));
const std::string& str = *static_cast<const CowPtr<std::string>&>(my_cow_ptr);

answered Apr 20 '13 at 18:45

Pete Becker

But that also means that aside of const-casting, on every call, there is no way the compiler will know to call each function automatically... so the whole point of copy-on-write goes down the drain. – Born2Smile Oct 08 '16 at 13:41

1 Answers1