How to release pointer from boost::shared_ptr?

Don't. Boost's FAQ entry:

Q. Why doesn't shared_ptr provide a release() function?

A. shared_ptr cannot give away ownership unless it's unique() because the other copy will still destroy the object.


shared_ptr<int> a(new int);
shared_ptr<int> b(a); // a.use_count() == b.use_count() == 2

int * p = a.release();

// Who owns p now? b will still call delete on it in its destructor.

Furthermore, the pointer returned by release() would be difficult to deallocate reliably, as the source shared_ptr could have been created with a custom deleter.

So, this would be safe in case it's the only shared_ptr instance pointing to your object (when unique() returns true) and the object doesn't require a special deleter. I'd still question your design, if you used such a .release() function.

You could use fake deleter. Then pointers will not be deleted actually.

struct NullDeleter {template<typename T> void operator()(T*) {} };

// pp of type some_t defined somewhere
boost::shared_ptr<some_t> x(pp, NullDeleter() );

Kids, don't do this at home:

// set smarty to point to nothing
// returns old(smarty.get())
// caller is responsible for the returned pointer (careful)
template <typename T>
T* release (shared_ptr<T>& smarty) {
    // sanity check:
    assert (smarty.unique());
    // only one owner (please don't play games with weak_ptr in another thread)
    // would want to check the total count (shared+weak) here

    // save the pointer:
    T *raw = &*smarty;
    // at this point smarty owns raw, can't return it

    try {
        // an exception here would be quite unpleasant

        // now smash smarty:
        new (&smarty) shared_ptr<T> ();
        // REALLY: don't do it!
        // the behaviour is not defined!
        // in practice: at least a memory leak!
    } catch (...) {
        // there is no shared_ptr<T> in smarty zombie now
        // can't fix it at this point:
        // the only fix would be to retry, and it would probably throw again
        // sorry, can't do anything
        abort ();
    // smarty is a fresh shared_ptr<T> that doesn't own raw

    // at this point, nobody owns raw, can return it
    return raw;

Now, is there a way to check if total count of owners for the ref count is > 1?