-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Wednesday 22 July 2009, Pavel Shevaev wrote:

Guys, could you please clarify whether the following usage of weak_ptr is thread safe:

//thread A: shared_ptr<Foo> p = wp.lock();

//thread B: p.reset();

It's safe, presuming your "p" in thread A is a different shared_ptr object from the "p" reset in thread B. They can safely be copies sharing ownership of the same object.

I believe it's not thread safe as it looks very similar to the example 3 from the shared_ptr documentation:

// thread A p = p3; // reads p3, writes p

// thread B p3.reset(); // writes p3; undefined, simultaneous read/write

In this unsafe case, thread A and B are manipulating the same shared_ptr object. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkpnZ2gACgkQ5vihyNWuA4VUPACfXSYjZHiK9dkNnjAziUHFK5S9 KAAAnRYrm4HRILt05OBE/9iw2NaKJSqa =nfKj -----END PGP SIGNATURE-----

Pavel Shevaev wrote:

Guys, could you please clarify whether the following usage of weak_ptr is thread safe:

//thread A: shared_ptr<Foo> p = wp.lock();

//thread B: p.reset();

p is local to thread A, so B can't reset it. // thread B: wp.reset(); is not safe (A reads wp, B writes wp). // before A and B shared_ptr<X> p( new X ); weak_ptr<X> wp( p ); // A shared_ptr<X> p2 = wp.lock(); // B p.reset(); is safe (A reads wp, B writes p).