Question

Howard Hinnant explained that unique_ptr can also use a custom storage type. He mentions as an example “shared memory“.

He only gives the rough idea (which is fine for a quick intro). But can anyone complete his example for a “custom storage type” (be it shared memory or not)?

To support placing unique_ptr into shared memory, the custom deleter can contain a custom pointer type (typically not a real pointer in shared memory applications). One simply places a nested type called pointer which emulates pointer behavior within your deleter, publicly accessible:

template <class T>
class MyDeleter
{
public:
    class pointer
    {
     public:
        friend bool operator==(pointer x, pointer y);
        friend bool operator!=(pointer x, pointer y);
        // ...
    };

    void operator()(pointer p);
};

void test()
{
    unique_ptr<int, MyDeleter<int> > p;
    MyDeleter<int>::pointer p2 = p.get();  // A custom pointer type used for storage
}

I suspect that // ... must be extended, and test() will probably do some additional things in a real “custom storage type” example.

Can someone tell me where he/she

• has actually already used this customization,
• and in that context,
• which what customization code, (at //... probably)
• and how client code uses it? (at test() probably)

Answer 1

You might be interested in boost::offset_ptr which served as the motivating use case for this customization point in unique_ptr. offset_ptr is a fully developed pointer type which could be installed into a custom deleter using a simple typedef. Its use case is to put unique_ptr into shared memory.