January 2004 - Boost - lists.boost.org

mpl functions, pow_, root_, reciprocal
by Andy Little 09 Feb '04

09 Feb '04

Hi, Following preliminary ok of "pow" mpl metafunction by Alexey Gurtovoy... Here are some basic headers for mpl functions pow_, root_ and reciprocal. useage: boost::mpl::pow_<type,exponent>::type boost::mpl::root_<type,exponent>::type boost::mpl::reciprocal<type>::type Obvious type for the type and exponent is a static_rational type. For pow_ param order is same as runtime std::pow(x,y) , "root_" follows logically from pow_ .... but obviously ... pow_ could do the job) reciprocal also useful to represent 1/ type. I don't know enough mpl yet to provide generic examples except using the pqs types. If anyone else wants to have a go...? And of course docs :-) . Otherwise will get on with it asap. Feedback welcome. regards Andy Little ---------------------------------------------------------------- headers: #ifndef BOOST_MPL_POW_HPP_INCLUDED #define BOOST_MPL_POW_HPP_INCLUDED namespace boost { namespace mpl{ template <typename Type, typename Power> struct pow_ { typedef implementation_defined type }; }} //boost::mpl #endif //----------------- #ifndef BOOST_MPL_ROOT_HPP_INCLUDED #define BOOST_MPL_ROOT_HPP_INCLUDED namespace boost { namespace mpl{ template <typename Type, typename Power> struct root_ { typedef implementation_defined type; }; }} //boost::mpl #endif //----------------------------------- #ifndef BOOST_MPL_RECIPROCAL_HPP_INCLUDED #define BOOST_MPL_RECIPROCAL_HPP_INCLUDED namespace boost { namespace mpl{ template <typename Type> struct reciprocal{ typedef implementation_defined type; }; }} //boost::mpl #endif

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function (object) return and argument metafunctions?
by David Abrahams 09 Feb '04

09 Feb '04

Hi, I know we've discussed this in the past, but did we ever settle on an interface for deducing the return and parameter types of function (object)s? Doug G.'s result_of<F(a,b)> template works for return types, but it's not very portable and is not even in Boost proper AFAIK. Many function objects and all function pointers have specific (non-templated) parameter types, and it would be extremely valuable to be able to deduce those as well. It's hard to believe that with all the functional programming libraries we have, there's no standardized facility for this sort of deduction. -- Dave Abrahams Boost Consulting www.boost-consulting.com

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Re: class static constants, mpl, and linker problems
by David Abrahams 03 Feb '04

03 Feb '04

Daniel Krügler <dsp(a)bdal.de> writes: > - Assumption one: Boost does not provide the definitions of > potential static constants hidden by the BOOST_STATIC_CONSTANT > definition (I used only the headers of the library, so my potential > testing bug would be that I had to include the corresponding > translation units of boost providing the constant's definitions, > but sorry - I did not find them!). In general BOOST_STATIC_CONSTANT is used within templates, so the definition of a corresponding member may be given in the header file: template <int N> struct int_ { BOOST_STATIC_CONSTANT(int, value = N); }; #ifndef BOOST_NO_INCLASS_MEMBER_INITIALIZATION template <int N> int const int_<N>::value; #endif > As I said: If this assumption is false, please blame me straight > for incomplete testing and for trying to spread bad rumors on this > (really!) great library. It's probably not false in most cases. > - Assumption two: Even if Boost does provide these definitions, > there may occur problems due to the VC7.1 flags /Za and /Ze (aka > disabling and enabling language extensions), which I will explain > in the following. The same point applies as to assumptions one, if > I did incomplete testing or testing with rather old boost libraries. > > The nasty thing concerning the /Ze flag is that it effectively does > not "extend" the language in every case. A small testing case is the > following program: > > a.h: ------------------------- > #ifndef A_H_INC > #define A_H_INC > struct A > { > static const int val = 42; // Declaration of a static member variable > }; > #endif > ------------------------- > > a.cpp -------------------- > #include "a.h" > const int A::val; // Definition might be needed, if the program accesses > // the adress of A::val > ------------------------- > > main.cpp -------------------- > #include "a.h" > int main() > { > } > ------------------------- > > Although this is a C++ Standard compliant program, it will not > successfully link with the /Ze flag enabled, which is to my opion a > real compiler bug (which I already posted to m.p.v.language, but got > no reasonable response yet). The obtained error message is: > > error LNK2005: "private: static int const A::val" (?val@A@@0HB) already > defined in a.obj > error LNK2005: "private: static int const A::val" (?val@A@@0HB) already > defined in main.obj Looks like a bug to me. > Question: Is this special deficiency of the VC7.1 compiler taken > account in the current boost library? I doubt it. How would we do that? > I think, it is impossible to demand, that users are enforced to use > either only /Ze or only /Za, because each of these choices has some > considerable drawbacks (at least on Windows systems...) Well, it seems to me that we can only respond to actual problems. Do you have an actual case of using a Boost library where these issues cause you trouble? -- Dave Abrahams Boost Consulting www.boost-consulting.com

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[filesystem] On POSIX, any reliable way to find file size?
by Beman Dawes 03 Feb '04

03 Feb '04

There have been requests for a Boost.Filesystem function which returns file size. But I don't want to provide such a function if it will be unreliable. On POSIX compliant 32-bit systems, is there any reliable way to find the size of a file? It looks to me as if stat::st_size's type of off_t isn't large enough on 32-bit systems to cope with large file sizes. Am I wrong, or missing something? How big is the largest possible file on, say, Linux? Isn't it larger than the 2 gigs a 32-bit signed integer can represent? --Beman

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value_initialized, deep const, and template instantiation
by Jim Apple 02 Feb '04

02 Feb '04

value_initialized has non-deep (shallow?) const semantics. const value_initialized<T> is supposed to be const. Unfortunately, due to compiler errors in VC++ and Borland, it has operator T &() const instead of operator T&() operator const T&() const which breaks const-correctness in a big way. I'd like for there to be a config macro that identifies this error. I would make one, as well as a test for it, myself, but I only have access to icc and gcc on linux, both of which pass the test, so it's hard trying to identify the extent of the overloading selection failure. After creating this macro, we could use it to make a const correct value_initialized class for users with more advanced compilers. For those compilers, I think there would be value to adding operator-> for the invocation of member functions, like boost::optional. As an alternative, deep-copy would break const expectations in generic functions that take a const T &, but it would preserve it elsewhere, so it may be worth some more thought. Finally, I have come up with a similar class (which I wrote with deep const sematics, although I am open to argument about shallow const) for solving the following problem: ---------------------------------------------------------------------- #include <iostream> template<typename T> struct wrap { static const T joe; }; template<typename T> const T wrap<T>::joe; struct call_out { int num; call_out() : num(77) {} }; template<typename T> struct check_call { check_call() { std::cout << wrap<T>::joe.num << std::endl; } }; check_call<call_out> x; int main(){} ----------------------------------------------------------------------- This outputs 0, not 77, on gcc 3.3.2, intel 8, and vc6. I don't know if that's correct, 14.7.1/1 says: "... in particular, the initialization (and any associated side effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist." If this is std-conforming behavior, or not, i have a workaround that allows: ------------------------------------------------------------------------ #include <iostream> #include "can_init.hpp" template<typename T> struct wrap { static can_init<const T> joe; }; template<typename T> can_init<const T> wrap<T>::joe; struct call_out { int num; call_out() : num(77) {} }; template<typename T> struct check_call { check_call() { wrap<T>::joe.ensure_init(); std::cout << wrap<T>::joe->num << std::endl; } }; check_call<call_out> x; int main(){} ----------------------------------------------------------------------- which obviously uses deep const. Is there any interest in including this in boost/utility/? Should it have shallow or deep const semantics? Jim

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compile-time array count
by Gennaro Prota 02 Feb '04

02 Feb '04

Having a look to some old list messages I've seen there has been some discussion about discovering the number of elements of an array in compile-time. Sorry if I have simply misunderstood something here (I'm rapidly catching up with old threads), but isn't this enough? http://article.gmane.org/gmane.comp.lib.boost.devel/3216 That's for expressions, of course, not type-ids. Genny.

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Re: [boost] Re: Boost Digest, Vol 622, Issue 5
by John Maddock 02 Feb '04

02 Feb '04

> How about using Boost.Thread's call_once function? I realize it requires > extra code, but something along these lines seems like it should both > avoid too much locking and still work: > Aha, good thinking, yes that's a good idea. John.

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ICE with is_abstract on VC 7.0
by Robert Ramey 01 Feb '04

01 Feb '04

When I compile anything that includes is_abstract.hpp I get a compiler ICE with VC 7.0 since is_abstract.hpp is included by type_traits.hpp it means that a large part of boost 1.31 release candidate 2 is now uncompilable on this platform. is_abstract was known to test the limits of many platforms. It is incredible to me that this could be dropped into "release candidate # 2" without having been part of the development tree for a reasonable amount of time. When can we expect 1.31 release candidate # 3? Robert Ramey

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1.31.0 Release candidate 2 available
by Beman Dawes 01 Feb '04

01 Feb '04

Release candidate 2 is available at http://www.esva.net/~beman/boost_1_31_0_rc2.zip Feedback appreciated! Boost developers should limit changes to the RC_1_31_0 branch to fixes for showstoppers. Anything less should go in the next release. --Beman

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problem with new iterators in 1.31 Release candidate # 2
by Robert Ramey 01 Feb '04

01 Feb '04

I have a problem with the latest version of the new iterators. This is illustrated in the following example. #include <strstream> #include <iterator> #include <boost/iterator/counting_iterator.hpp> main(){ std::istrstream is("abcdefg"); typedef boost::counting_iterator<std::istream_iterator<char> > ci; const ci end = std::istream_iterator<char>(); // note the const !!! ci begin = std::istream_iterator<char>(is); unsigned int size; // the following should fail at compilation ? // in fact it compiles are returns a value of -7 !!! size = begin - end; // the following should compile and return a value of 7 // in fact, it compiles but goes into an infinite loop size = end - begin; } Note that this problem did not appear with the first release candidate. Also, if one follows the links from the main documentation page, one is directed to the documentation of the old iterators. This is extremely confusing. This has been pointed out before. This above example is using 1.31. "release candidate" #2 compiled with VC 7.0 Robert Ramey

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