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metaprogramming exercise 2-0
by Chris Goller 17 Feb '05

17 Feb '05

I just finished the first exercise in the C++ meta programming book. Then I checked what had been done before on this mailing list. Here is my solution: using namespace boost; template <bool use_ref, typename T> struct add_const_ref_impl { typedef T const & type; }; template <typename T> struct add_const_ref_impl<true, T> { typedef T type; }; template<class T> struct add_const_ref { typedef typename add_const_ref_impl<is_reference<T>::value, T>::type type; }; From Bruce Trask comes: using namespace boost; template<typename T> struct add_const_ref { typedef typename add_reference<typename add_const<T>::type>::type type; }; His solution is SO much better then mine. However, when I first saw it I thought it must be wrong because I assumed that add_const on a reference would would return a const reference. In other words, the code below would be false: bool a = is_same<add_const<int &>::type, int &>::value; However! bool "a" is true. So my question is why is "a" true? Why isn't add_const<int &>::type == int const &? Thanks, Chris

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Re: [Boost-users] unit test and BOOST_CHECK_CLOSE
by Vladimir Krasovsky 17 Feb '05

17 Feb '05

Hello Thomas, Thursday, February 17, 2005, 1:35:48 PM, you wrote: Thomas> dear list, Thomas> When executing a test defined by: Thomas> BOOST_CHECK_CLOSE(t3.x(3), 0.0, epsilon); Thomas> I get the message: Thomas> difference between t3.x(3){2.34188e-17} and 0.0{0} exceeds 1e-08% Thomas> where obviously the difference does not exceeds the given threshold 1e-08... Thomas> What's going on ? As I remember, tolerance should be given in percents. See http://www.boost.org/libs/test/doc/components/test_tools/floating_point_com… for more detailed description of algorithm used in BOOST_CHECK_CLOSE. Either I do not understand something or you cannot use BOOST_CHECK_CLOSE for direct comparison with 0.0. Inequations given never satisfied in case one of arguments is zero. I think you could write BOOST_CHECK(t3.x(3) >= -epsilon && t3.x(3) <= epsilon); given that epsilon is non-negative. -- Best regards, Vladimir mailto:vkrasovsky@yandex.ru

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Interval Arithmetic Tribool
by Robert Ferguson 16 Feb '05

16 Feb '05

Dear list, I hope I am writing to the correct place, and that this isn't a frequently asked question.. but I'd like to know alittle about comparisons and the Interval library. I am using intervals in a search algorithm to take advantage of the natural inclusion function as proposed in a paper I'm trying to implement. So far so good, except that I'm a little unhappy that I have to use "try" and "catch" to detect the indeterminate case. I'd like to use the boost::logic::tribool to identify the semi-frequent indeterminant case. For example: tribool cmp = interval1 < interval2; if (!indeterminate(cmp)) { if (cmp) { // something } else { // something else } else { // something } When I do this and the indeterminate case happens, an exception triggers and the function quits. This could be an error on my behalf, but it seems like this the default behavior. Its not that I can't do the try, catch, its just a matter of style. Can I have my cake and eat it too? Best, Robert Ferguson

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Program runs OK in Cygwin, but got "Segment Fault" in Linux
by Welson Sun 16 Feb '05

16 Feb '05

Hi, The program is really simple, it creates a graph and output to a .dot file using the graphviz library in the BGL. It runs well in Cygwin, but in Linux, it got a segment fault when outputing the vertex id. Can you C++ gurus see what is going on? Thanks! Welson

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[Important] Mailing list downtime this Saturday, February 19
by Doug Gregor 16 Feb '05

16 Feb '05

The Open Systems Lab, which hosts most of the Boost mailing lists, will be migrating to new hardware this Saturday. The outage will be this Saturday, February 19 starting at noon Eastern Standard Time and should not last more than 8 hours. lists.boost.org will be unavailable during the outage as will the mailing lists themselves. Thanks, Doug Gregor

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[Boost.Test] and assert()
by Richard Jennings 16 Feb '05

16 Feb '05

When testing my program using the Unit Test Framework I find that if the program fails an assert() it is aborted with the message "Abnormal program termination" and no more tests are run. Is this the expected behaviour? The release notes for Boost.Test v1.30.0 say that a handler was added for SIGABRT (raised by abort() called by assert()) so I was expecting the test framework to catch this exception and report it in a similar way to a divide-by-zero error. I have my code compiled in debug with Borland C++ Builder 6.0 running on Windows XP. The behaviour is the same whether the command line option --catch_system_errors=yes or no. Richard

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starting to learn Boost MPL and type_trait
by Sharon Galtzur 16 Feb '05

16 Feb '05

Hello all, I am trying to start learning the Boost MPL. The internals are more interesting for me then the usage I read both Abrahams&Gurtovoy and Alexandrescu books (both excelent btw) Some things i still don't quite understand: 1. How (or rather Why) does is_class works. 2. Same for is_enum 3. Can someone explain how MPL lambda works specially how the "_" works I tried to scan some of the archives but could not find any definite answer - i guess the discussions if they were are lost. P.S. neat trick to someone who try to learn the internals is to make a program that just include the header u interested to learn and just do the "pre-compile" stage (/EP /P flags in VC7.1 iirc) - you get a nice header without the unreadable MACROS. Thanks

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Inserting elements in extensible associative sequences
by Lemings, Eric B. (Contr) 15 Feb '05

15 Feb '05

The semantics of MPL's insert metafunction for an extensible associative sequence is defined as: "For any Extensible Associative Sequence s, iterator pos in s, and arbitrary type x: typedef insert<s,x>::type r; Semantics: r is concept-identical and equivalent to s, except that at< r, key_type<s,x>::type >::type is identical to value_type<s,x>::type." What happens if the key type of x is already in s? Thanks, Eric.

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OT: std::map<>::operator[] and default constructor
by Noel Yap 15 Feb '05

15 Feb '05

Please direct me to a more appropriate list if this is too off topic: It's come to my attention that our vendor's std::map<>::operator[] /always/ calls the default constructor of the key even when the key already exists in the map. This is what they say: It turned out to be a documentation bug. Our implementation is in sync with the standard. From 23.3.1.2 [lib.map.access]: T& operator[](const key_type& x); -1- Returns: (*((insert(make_pair(x, T()))).first)).second. Notice the T() in the call to make_pair() above: that's an invocation of the default ctor of the mapped type (i.e., Aoeu in the user's test case). So the program behaves as expected. If you do not like this behavior, replacing the expression map[16] in the program with map.find (16)->second will avoid the construction of the unused temporary. I modified the test case to incorporate this change. My inclination is to say the standard is a spec, not an implementation. Allowing std::map<>::operator[] not to call the default constructor requires no more and promises no less than the spec. All this is true unless, of course, there's some order of complexity issue I'm missing. But someone else had said, "an implementation has to have the same 'observable behaviour' as the abstract machine defined in the standard. Calling the default ctor definitely counts as observable behaviour, so I don't think RW have any choice but to implement it this way". What's the best place to get an official answer to the real intent of std::map<>::operator[]? Must it /always/ call the default constructor? Thanks, Noel

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[serialization] memory leak in xml_wiarchive_impl ctor
by Sergey Skorniakov 15 Feb '05

15 Feb '05

Hello, All! If init() member of xml_wiarchive_impl throws an exception (for example, xml_archive_exception from basic_xml_grammar::init()), gimpl member is not deleted. With best regards, Sergey.

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