Hello, I previously sent out an email to the list about a pointer plus bits like data structure and received some feedback about a pointer plus bits data structure. After taking into consideration all of the different points of view, comments and concerns voiced by the Boost community I have implemented an extension to my bitfield_tuple which allows the user to easily store parts of a pointer and use the extra bits for storage. Here is a basic overview of the bitfield_tuple: Its a tuple like data structure which stores bitfields instead of whole types inside of single tuple. This allows the user to skirt packing limitations of structs and classes in C++ and create bitfields over the storage type of their choice OR leave that decision up to the data structure (the behavior for selecting the storage type is to select the smallest possible unsigned integral type which will hold the all of the bitfields). example of basic bitfield_tuple declaration. #include<boost/integer/bitfield_tuple.hpp> struct red; struct green; struct blue; using namespace boost; using namespace boost::bitfields; typedef bitfield_tuple< member<unsigned char, red, 5>, member<char, green, 6>, member<unsigned int, blue,5>
rgb565_t;
The bitfield_tuple takes ordered named template parameters (In the case of the storage type order is ignored. IE no matter where you put the storage< type > template the ending result will be the same). the bitfield_tuple supports the following template named template parameters (all of the templates which follow are located within the boost::bitfields namespace): bit_align< std::size_t> -> increases the offset of the next field to an offset which is divisible by the supplied value unless the current index is divisible by the supplied value then there is no increase. filler<std::size_t> -> bits to be skipped. member<typename, typename, std::size_t> -> this is a basic bitfield parameters it takes are the type the bitfield is to return, a naming type and the width of a bitfield flag<typename> -> boolean version of member. storage<typename> -> uses the supplied type for internal storage. pointer<typename, typename, typename> -> NEW described below. Pointer Member Information: The pointer member takes three template parameters. 1) The type you would like to make a pointer to. 2) The name which can be used (instead of an index, index is also still available to use) to access the pointer. 3) This is an integral_constant or integral_constant like type which is used to make a mask over the value bits of the pointer. This has a default value to leave the last two bits open. You can use leading or ending bits of the pointer for storage by supplying a different mask. Reasoning behind allowing for leading storage bits. There are some systems which do low address allocations and then store values within the higher part of the address. While this is much more rare then the use case of stuffing two bits inside of the end of a pointer I felt that it would be best to provide a way to actually do this. Reasoning behind restricting the mask to an integral_constant or integral_constant like type. The intent for this is to associate a non deduced type with the mask itself, however if people feel better simply using hex to specify the mask I could change the behavior to do that. There are mask creation utilities which can be used to create the mask from a higher level the simply supplying hex. Here are some examples of how to declare a bitfield_tuple with a pointer member: #include <boost/integer/bitfield_tuple.hpp> #include <boost/assert.hpp> using namespace boost; using namespace boost::bitfields; struct ptr; struct color; struct i1; struct i2; typedef bitfield_tuple< pointer<int, ptr> > ptr_1; typedef bitfield_tuple< pointer<int, ptr>, flag<b1>, flag<b2> > ptr_plus_two_bools; typedef bitfield_tuple< pointer<int, ptr>, member<unsigned int,color,2> > ptr_plus_two_bit_color; int main() { ptr_1 p1; int i = 10; p1.get<0>() = &i; BOOST_ASSERT(*p1.get<ptr>() == 10); ptr_plus_two_bools p2; p2.get<ptr>() = &i; ptr.get<b1>() = true; ptr.get<2>() = false; ptr_plus_two_bit_color p3; p3.get<ptr>() = &i; p3.get<color>() = 3; return 0; } The pointer does NOT have to be the first parameter of the bitfield tuple one can simply place things in front of the the pointer and the value mask of the pointer will not be affected. The only thing which is different when this is done is the offset of the pointer within the bitfield_tuples storage, this only changes the location of the bitfield where the pointer is actually being stored. That being said you will always receive the value bits of the pointer in the exact same way they were taken from inside the pointer. -- thanks, Brian Bartman