Boost C++ Libraries: Ticket #2481: make_shared and allocate_shared friendship

attachment set

Thu, 06 Nov 2008 18:39:16 GMT

attachment → make_shared_access.patch

Thu, 06 Nov 2008 18:41:25 GMT

Added a brute force patch that implements this for c++03 against boost 1.36 release. I left variadic template support as a todo because I don't know the syntax and don't have a compiler for it.

Frank Mori Hess — Fri, 12 Dec 2008 17:26:41 GMT

One limitation of your solution: suppose I have private constructors and a static factory function for a class, and the factory function returns a shared_ptr. If I make make_shared_access a friend, it opens a hole where people can still create objects of the class without using the factory function by using make_shared or allocate_shared directly.

cc set

Wed, 04 Feb 2009 13:16:20 GMT

cc jwakely.boost@… added

Right, you don't even need the make_shared_access extension to open that hole, the design is compromised if you declare make_shared or allocate_shared as a friend.

Given:

class MustUseFactory {
  MustUseFactory() { }
public:
  static shared_ptr<MustUseFactory> create();
};

You can still use make_shared by using aliasing:

shared_ptr<MustUseFactory> MustUseFactory::create()
{
    typedef aligned_storage<sizeof(MustUseFactory)> Storage;
    shared_ptr<Storage> storage = make_shared<Storage>();
    MustUseFactory* p = new (storage->address()) MustUseFactory();
    return shared_ptr<MustUseFactory>(storage, p);
}

This doesn't work with enable_shared_from_this, I leave that as an exercise for the reader ;-)

status, milestone changed

Peter Dimov — Sat, 28 Feb 2009 17:07:38 GMT

status new → assigned
milestone Boost 1.38.0 → Boost 1.39.0

Peter Dimov — Sat, 07 Mar 2009 21:22:19 GMT

It still seems to me that the best course of action is to make allocate_shared use A::construct. Unfortunately, this doesn't work with C++03 allocators, whose construct method only takes a single T const& argument. C++0x allocators are OK, but changing boost::allocate_shared to call construct will make it fail for C++03 allocators.

milestone changed

Peter Dimov — Tue, 01 Dec 2009 01:47:06 GMT

milestone Boost 1.39.0 → To Be Determined

status changed

Peter Dimov — Thu, 24 Feb 2011 22:08:43 GMT

status assigned → new

jwakely.boost@… — Tue, 08 Nov 2011 17:28:52 GMT

Using std::allocator_traits<A>::construct will work with C++11 or C++03 allocators, which is what LWG 2070 suggests and what I've implemented for GCC 4.7

Of course that requires a C++11 library that provides allocator_traits

status changed; resolution set

Peter Dimov — Wed, 12 Feb 2014 00:45:48 GMT

status new → closed
resolution → fixed

boost::allocate_shared now uses std::allocator_traits<>::construct and destroy (if available), as per LWG 2070.