The changes made to enable_shared_from_this since 1.35 cause classes to be larger than they were before. I created a small test that I ran against 1.35 and the current trunk. These are the results for VC9 SP1 release mode. BOOST_VERSION = 103500 sizeof(SharedFromThisEnabled) = 8 sizeof(MultipleInheritanceDerived) = 12 sizeof(ChainedBaseDerived) = 12 BOOST_VERSION = 103600 sizeof(SharedFromThisEnabled) = 12 sizeof(MultipleInheritanceDerived) = 16 sizeof(ChainedBaseDerived) = 12 Did the changes made offer functionality worth the extra space? The MultipleInheritanceDerived pattern is recommended in most of the Boost.Asio examples. If the changes made truly offer some value perhaps a base class chaining version of enabled_shared_from_this should be provided. Thanks, Michael Marcin #include <iostream> #include <boost/enable_shared_from_this.hpp> #include <boost/version.hpp> class SharedFromThisEnabled : public boost::enable_shared_from_this<SharedFromThisEnabled> { public: }; class AbstractBase { public: virtual void Foo() = 0; }; class MultipleInheritanceDerived : public AbstractBase , public boost::enable_shared_from_this<MultipleInheritanceDerived> { public: virtual void Foo() {} }; #if (BOOST_VERSION==103500) template< class T, class Base > class enable_shared_from_this_chained_base : public Base { public: typedef T _internal_element_type; // for bcc 5.5.1 mutable boost::weak_ptr<_internal_element_type> _internal_weak_this; }; #elif (BOOST_VERSION==103600) template< class T, class Base > class enable_shared_from_this_chained_base : public Base { protected: // virtual destructor because we need a vtable for dynamic_cast from base to derived to work virtual ~enable_shared_from_this_chained_base() { } private: mutable boost::detail::weak_count _weak_count; mutable boost::detail::shared_count _shared_count; }; #else #error "Update enable_shared_from_this_chained_base implementation and version number" #endif class ChainedBaseDerived : public enable_shared_from_this_chained_base<ChainedBaseDerived,AbstractBase> { public: virtual void Foo() {} }; int main() { std::cout << "BOOST_VERSION = " << BOOST_VERSION << std::endl << "sizeof(SharedFromThisEnabled) = " << sizeof(SharedFromThisEnabled) << std::endl << "sizeof(MultipleInheritanceDerived) = " << sizeof(MultipleInheritanceDerived) << std::endl << "sizeof(ChainedBaseDerived) = " << sizeof(ChainedBaseDerived) << std::endl ; }