I am seeing problems with msvc when compiling a simple code snippet that instantiates a dynamic_bitset with a uint64_t. The problem occurs when instantiating a template specialization for the resize function for uint64_t. There was a change made to the block_width_type typedef from an int in 1_36_0 to block_type in 1_37_0. This change seems to have broken support for uint64_t data-types on 32-bit windows systems or any other types which are bigger than size_t on a given platform. Code snippet: #include <boost/dynamic_bitset.hpp> #include <iostream> #if defined (__GNUC__) #if !defined (SUNOS_OS) typedef unsigned long long uint64_t; #endif #else typedef unsigned __int64 uint64_t; #endif typedef uint64_t myType; boost::dynamic_bitset<myType>::size_type foo() { unsigned int blockHandles(10); return (boost::dynamic_bitset<myType>::size_type )(blockHandles*sizeof(myType)*8); } int main(int argc,char**argv) { boost::dynamic_bitset<myType>::size_type t = foo(); boost::dynamic_bitset<myType> m_myBitSet; m_myBitSet.resize(12,false); } Compilation error: Compiling... TestBitSet.cpp c:\p3_dev_boost\common\lib\boost_1_39_0\boost\dynamic_bitset\dynamic_bitset.hpp(673) : error C2220: warning treated as error - no 'object' file generated c:\p3_dev_boost\common\lib\boost_1_39_0\boost\dynamic_bitset\dynamic_bitset.hpp(649) : while compiling class template member function 'void boost::dynamic_bitset<Block>::resize(boost::dynamic_bitset<Block>::size_type,bool)' with [ Block=myType ] c:\documents and settings\abhat\my documents\aniket\projects\dynamic_bitset\testbitset\testbitset.cpp(17) : see reference to class template instantiation 'boost::dynamic_bitset<Block>' being compiled with [ Block=myType ] c:\p3_dev_boost\common\lib\boost_1_39_0\boost\dynamic_bitset\dynamic_bitset.hpp(673) : warning C4244: 'initializing' : conversion from 'unsigned __int64' to 'const boost::dynamic_bitset<Block>::size_type', possible loss of data with [ Block=myType ] The resize function (dynamic_bitset.hpp:673) calls count_extra_bits() which returns a size_type instead of block_width_type resulting in possible loss of data. At all other places this function is called within the header, it returns a block_width_type. I am not sure if it has to return a size_type in this particular case for a specific reason? Thanks, Aniket.