return msb;
}
+// The following implements the BitUnion system of defining bitfields
+//on top of an underlying class. This is done through the extensive use of
+//both named and unnamed unions which all contain the same actual storage.
+//Since they're unioned with each other, all of these storage locations
+//overlap. This allows all of the bitfields to manipulate the same data
+//without having to know about each other. More details are provided with the
+//individual components.
+
+//This namespace is for classes which implement the backend of the BitUnion
+//stuff. Don't use any of this directly! Use the macros at the end instead.
namespace BitfieldBackend
{
+ //A base class for all bitfields. It instantiates the actual storage,
+ //and provides getBits and setBits functions for manipulating it. The
+ //Data template parameter is type of the underlying storage.
template<class Data>
class BitfieldBase
{
protected:
Data __data;
+ //This function returns a range of bits from the underlying storage.
+ //It relies on the "bits" function above. It's the user's
+ //responsibility to make sure that there is a properly overloaded
+ //version of this function for whatever type they want to overlay.
inline uint64_t
getBits(int first, int last)
{
return bits(__data, first, last);
}
+ //Similar to the above, but for settings bits with replaceBits.
inline void
setBits(int first, int last, uint64_t val)
{
}
};
+ //A class which specializes a given base so that it can only be read
+ //from. This is accomplished by only passing through the conversion
+ //operator.
template<class Type, class Base>
class _BitfieldRO : public Base
{
}
};
+ //Similar to the above, but only allows writing.
template<class Type, class Base>
class _BitfieldWO : public Base
{
}
};
+ //This class implements ordinary bitfields, that is a span of bits
+ //who's msb is "first", and who's lsb is "last".
template<class Data, int first, int last=first>
class _Bitfield : public BitfieldBase<Data>
{
}
};
+ //When a BitUnion is set up, an underlying class is created which holds
+ //the actual union. This class then inherits from it, and provids the
+ //implementations for various operators. Setting things up this way
+ //prevents having to redefine these functions in every different BitUnion
+ //type. More operators could be implemented in the future, as the need
+ //arises.
template <class Type, class Base>
class BitUnionOperators : public Base
{
};
}
+//This macro is a backend for other macros that specialize it slightly.
+//First, it creates/extends a namespace "BitfieldUnderlyingClasses" and
+//sticks the class which has the actual union in it, which
+//BitfieldOperators above inherits from. Putting these classes in a special
+//namespace ensures that there will be no collisions with other names as long
+//as the BitUnion names themselves are all distinct and nothing else uses
+//the BitfieldUnderlyingClasses namespace, which is unlikely. The class itself
+//creates a typedef of the "type" parameter called __DataType. This allows
+//the type to propagate outside of the macro itself in a controlled way.
+//Finally, the base storage is defined which BitfieldOperators will refer to
+//in the operators it defines. This macro is intended to be followed by
+//bitfield definitions which will end up inside it's union. As explained
+//above, these is overlayed the __data member in its entirety by each of the
+//bitfields which are defined in the union, creating shared storage with no
+//overhead.
#define __BitUnion(type, name) \
namespace BitfieldUnderlyingClasses \
{ \
union { \
type __data;\
+//This closes off the class and union started by the above macro. It is
+//followed by a typedef which makes "name" refer to a BitfieldOperator
+//class inheriting from the class and union just defined, which completes
+//building up the type for the user.
#define EndBitUnion(name) \
}; \
}; \
BitfieldUnderlyingClasses::name::__DataType, \
BitfieldUnderlyingClasses::name> name;
+//This sets up a bitfield which has other bitfields nested inside of it. The
+//__data member functions like the "underlying storage" of the top level
+//BitUnion. Like everything else, it overlays with the top level storage, so
+//making it a regular bitfield type makes the entire thing function as a
+//regular bitfield when referred to by itself. The operators are defined in
+//the macro itself instead of a class for technical reasons. If someone
+//determines a way to move them to one, please do so.
#define __SubBitUnion(type, name) \
union { \
type __data; \
inline const __DataType operator = (const __DataType & _data) \
{ __data = _data; }
+//This closes off the union created above and gives it a name. Unlike the top
+//level BitUnion, we're interested in creating an object instead of a type.
#define EndSubBitUnion(name) } name;
-//This is so we can send in parameters with commas
+//The preprocessor will treat everything inside of parenthesis as a single
+//argument even if it has commas in it. This is used to pass in templated
+//classes which typically have commas to seperate their parameters.
#define wrap(guts) guts
//Read only bitfields
+//This wraps another bitfield class inside a _BitfieldRO class using
+//inheritance. As explained above, the _BitfieldRO class only passes through
+//the conversion operator, so the underlying bitfield can then only be read
+//from.
#define __BitfieldRO(base) \
BitfieldBackend::_BitfieldRO<__DataType, base>
#define __SubBitUnionRO(name, base) \
__SubBitUnion(wrap(_BitfieldRO<__DataType, base>), name)
//Write only bitfields
+//Similar to above, but for making write only versions of bitfields with
+//_BitfieldWO.
#define __BitfieldWO(base) \
BitfieldBackend::_BitfieldWO<__DataType, base>
#define __SubBitUnionWO(name, base) \
__SubBitUnion(wrap(_BitfieldWO<__DataType, base>), name)
//Regular bitfields
+//This uses all of the above to define macros for read/write, read only, and
+//write only versions of regular bitfields.
#define Bitfield(first, last) \
BitfieldBackend::_Bitfield<__DataType, first, last>
#define SubBitUnion(name, first, last) \
#define SubBitUnionWO(name, first, last) \
__SubBitUnionWO(Bitfield(first, last), name)
+//Use this to define an arbitrary type overlayed with bitfields.
#define BitUnion(type, name) __BitUnion(type, name)
+//Use this to define conveniently sized values overlayed with bitfields.
#define BitUnion64(name) __BitUnion(uint64_t, name)
#define BitUnion32(name) __BitUnion(uint32_t, name)
#define BitUnion16(name) __BitUnion(uint16_t, name)
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