Ulrich Eckhardt wrote:
Greetings! Firstly, this is not a regression, the behaviour was present in 1.30, too. I have attached a simple program that should demonstrate the behaviour, but the core is these lines, with foo being derived from enable_shared_from_this:
// 1 std::auto_ptr<foo> g(new foo); boost::shared_ptr<foo> f(g); // 2 std::auto_ptr<foo> g(new foo); boost::shared_ptr<foo const> f(g); // 3 std::auto_ptr<foo const> g(new foo); boost::shared_ptr<foo const> f(g);
In all three cases, I'd expect to be able to call shared_from_this(). However, only the first two cases really work. I think it boils down to the function detail::sp_enable_shared_from_this() in the ctor of shared_ptr<> that takes an auto_ptr<>.
This is, well, "by design". enable_shared_from_this requires that at least one shared_ptr "owns" the plain address of the object (without any cv qualifiers.) It does not matter what the static type of the shared_ptr is, i.e. what the T is in shared_ptr<T>; the actual pointer type needs to be "foo*". shared_ptr<void const volatile> p( new foo ); is enough for shared_from_this() to work (provided that you find a way to call it, of course.) The reason for this restriction is that enable_shared_from_this<foo> contains a weak_ptr<foo>, and in order to initialize it from a shared_ptr that stores a pointer to const foo, the implementation would need to const_cast the constness away twice, once in order to access the weak_ptr member, a second time to initialize a weak_ptr<foo> from a shared_ptr<foo const>.
BTW: I believe that a conversion from auto_ptr<T> to shared_ptr<T const> doesn't compile on VC6, but I can't test that ATM(I'm not at work). If any kind soul wants to, they could add such a conversion to the regression-tests. Same goes for volatile probably.
I see multiple auto_ptr<T> to shared_ptr<T const> conversions in shared_ptr_test.cpp, have you looked at it?