C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Use bit fields for sized integer types          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Timothy Madden Guest Posted: Wed Mar 08, 2006 4:06 am    Post subject: Use bit fields for sized integer types I hear about int32, int64, etc (int128 ?? ) in C++ Would it be better to suggest struct { int a:128; }; that is, an unnamed structure of bit-fields, to indicate a fixed-width integer type ? I think compilers should allow a number of bits at of least 2*sizeof(unsigned long)*CHAR_BIT What do you think ? --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top John Nagle Guest Posted: Wed Mar 08, 2006 3:25 pm    Post subject: Re: Use bit fields for sized integer types Timothy Madden wrote: Quote: I hear about int32, int64, etc (int128 ?? ) in C++ Would it be better to suggest struct { int a:128; }; that is, an unnamed structure of bit-fields, to indicate a fixed-width integer type ? Probably too late for that, although Pascal and Ada had such features. On a related subject, though, are the names "uint8_t", "uint16_t", etc. standardized? Various implementations use similar but not identical naming conventions for such types of specified size, and that should be standardized for portability. John Nagle Animats --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Wed Mar 08, 2006 5:43 pm    Post subject: Re: Use bit fields for sized integer types John Nagle wrote: Quote: Timothy Madden wrote: I hear about int32, int64, etc (int128 ?? ) in C++ Would it be better to suggest struct { int a:128; }; that is, an unnamed structure of bit-fields, to indicate a fixed-width integer type ? Probably too late for that, although Pascal and Ada had such features. On a related subject, though, are the names "uint8_t", "uint16_t", etc. standardized? Various implementations use similar but not identical naming conventions for such types of specified size, and that should be standardized for portability. Those names are part of the C99 standard. I believe that the committee's guidelines is to avoid unnecessary incompatibilities with C, so those names should be used for any C++ types with equivalent semantics. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Hyman Rosen Guest Posted: Thu Mar 09, 2006 5:06 am    Post subject: Re: Use bit fields for sized integer types John Nagle wrote: Quote: Probably too late for that, although Pascal and Ada had such features. Ada has this much more sensibly organized. If you need a range of values, you just declare something like a: integer range -100 to 1_000_000; and let the compiler figure out how to store it. If you need to conform to some representation constraint, you additionally say something like for a'size use 32; --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Me Guest Posted: Fri Mar 10, 2006 6:33 am    Post subject: Re: Use bit fields for sized integer types Timothy Madden wrote: Quote: I hear about int32, int64, etc (int128 ?? ) in C++ Would it be better to suggest struct { int a:128; }; that is, an unnamed structure of bit-fields, to indicate a fixed-width integer type ? There are two problems I have with this in C/C++ (ignoring how much the standards suck when it comes to plain integer types in bit-fields with your example): a. It exposes the representation. Basically, for signed integer types, you can only specify the width and the compiler determines what representation it likes to use. I can't specify a tiny negative range with a large positive range and treat values outside of that range as undefined. For a more practical purpose, I can't require it to use 2's complement and (worst of all) there really is no way to detect the min value you can store in a signed bit-field, you always have to assume 1's complement or signed-magnitude for standard conformance. b. It requires layout compatible structs to alias each other. I had an idea to abuse the register keyword: struct A { register int a; }; struct B { register int a; }: struct C { int a; }; struct D { int a; }; And make: - A isn't compatible with anything except itself, ditto for B - C and D are considered layout compatible with each other to take advantage of the ancient rule that register variables couldn't have their address taken so there would be no possible way to take the address of the variable a without evil tricks. I think this would be much more flexible and simpler than trying to get strong typedefs in the standard. Quote: I think compilers should allow a number of bits at of least 2*sizeof(unsigned long)*CHAR_BIT What do you think ? I'd find it a lot more useful if it was at least log2((UINTMAX_MAX+1)*(UINTMAX_MAX+1))+1 value bits. The log2(...) so I can multiply 2 (u)intmax_t integers, the +1 so I can also add a sign bit on top of this. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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