April 2005 - Boost-users - lists.boost.org

enable_if and constructors
by Angus Leeming 05 Apr '05

05 Apr '05

I see examples of enable_if used to specify a return type to a function: template <class T> typename enable_if_c<boost::is_arithmetic<T>::value, T>::type foo(T t) { return t; } Is it possible to use enable_if with a constructor? For example, I'd like to define a constructor that allows only certain values of an enum: #include <boost/utility/enable_if.hpp> class foo { public: enum state { state1, state2, state3 }; // This should compile only when passed (s == state2 or s == state3) template<int ID> foo(boost::enable_if_c<some_boolean_condition, state>::type s) : status_(s) {} private: state status_; }; Even if this turns out to be impossible, I'd like to go on and try to define some_boolean_condition if I may. I see that Aleksey plans a contains_c metafunction for 1.33, http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?MPL_TODO_List Equivalent URL: http://tinyurl.com/4toly so my limited understanding leads me to some_boolean_condition is equivalent to: mpl::contains_c< mpl::vector_c< int, state2, state3 >, ID >::value Does that look correct? In the absence of contains_c, I've been trying to use mpl::find_if but my predicate compiles for both "valid" and "invalid" values, so I must be doing something wrong. I'd be grateful if someone could put me back on track. Regards, Angus #include <boost/type_traits/is_same.hpp> #include <boost/mpl/assert.hpp> #include <boost/mpl/end.hpp> #include <boost/mpl/equal_to.hpp> #include <boost/mpl/find_if.hpp> #include <boost/mpl/lambda.hpp> #include <boost/mpl/integral_c.hpp> #include <boost/mpl/not.hpp> #include <boost/mpl/vector_c.hpp> class foo { public: enum state { state1, state2, state3 }; }; int main() { // Should lead to a compile-time failure. #define TESTED_VALUE foo::state1 // Should compile. // #define TESTED_VALUE foo::state2 namespace mpl = boost::mpl; typedef mpl::vector_c< int, foo::state2, foo::state3 > allowable_values; typedef mpl::find_if< allowable_values, mpl::equal_to< mpl::_1, mpl::integral_c<allowable_values::value_type, TESTED_VALUE> > >::type iter; BOOST_MPL_ASSERT(( mpl::not_< boost::is_same< iter::type, mpl::end<allowable_values>::type > > )); return 0; }

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Re: [Boost-users] Filesystem woes
by Sean DeNigris 05 Apr '05

05 Apr '05

I got the posted code to pass with VC7.1, /MDd and /Zc:wchar_t.

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boost::bind() compiler error in MS VC++6.0
by Vivid Yang 05 Apr '05

05 Apr '05

Hi, I have experienced an unexpected compiler error when I tried to use boost::bind(): include\boost\bind\arg.hpp(26) : fatal error C1001: INTERNAL COMPILER ERROR (compiler file 'msc1.cpp', line 1794) Please choose the Technical Support command on the Visual C++ Help menu, or open the Technical Support help file for more information The code has been successfully comipled, linked and run under linux. But it failed to compile with MS VC++6.0 under WinXP SP2. I have installed MS VC++ service pack 4, 5 and 6. The project is set to "Debug MultiThreaded". The project option is "/nologo /MTd /W3 /Gm /GX /ZI /Od" . Turning on/off precompiled header file does not help neither. I have googled the problem and tried several ways to solve it. Still no luck so far. Could anybody help? Thanks a lot Vivid

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unordered_map has no lower_bound?
by Martin Wartens 05 Apr '05

05 Apr '05

Hi, I am already using the unordered_map implementation from the file vault. I have got some problems with unordered_map, because it offers no "lower_bound" (it is not a problem of the implementation, it is simply not in the specs). My question is, why is that so and what can I do about it? The use case is an efficient insert when I have to take special actions when the key is already in the container. For a map this would work like: iter = lower_bound(key); which gets the first element greater or equal to key Now I can check if the key is already in the map and execute some action to handle this case. Then I can insert the new element in the map with the "hint"-version of insert: mymap.insert(iter, newelement); The hint-version starts searching at the hint for the correct insertion point, so we don't have to search through the map twice. It is strange that such a hint-insert also exists for the unordered_map. But what hint could I give it, if I don't have lower_bound? Thanks, Martin PS: there is a small problem in hash_table.hpp causing a warning in VC7.1 line 278 should be: return (*prev_ptr)!=0; instead of return *prev_ptr;

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multi_index and VC70 problem
by Martin 05 Apr '05

05 Apr '05

When compiling the following code on VC70 struct tTest { tTest() : x(0), y(0), z(0) int x,y,z; }; typedef multi_index_container< tTest, indexed_by< ordered_unique< composite_key< BOOST_MULTI_INDEX_MEMBER(tTest, int, x) ,BOOST_MULTI_INDEX_MEMBER(tTest, int, y) ) // replaced > with ) since gmane thinks I'm top-posting ) ) ) tTestList; void func() { tTestList u; u.insert(tTest()); } I get an error boost\multi_index\composite_key.hpp(573) : error C2100: illegal indirection boost\multi_index\ordered_index.hpp(562) : see reference to function template instantiation 'boost::multi_index::composite_key<Value,KeyFromValue0,KeyFromVal ue1,KeyFromValue2,KeyFromValue3,KeyFromValue4,KeyFromValue5,KeyFromValue6,KeyFr omValue7,KeyFromValue8,KeyFromValue9>::result_type boost::multi_index::composite_key<Value,KeyFromValue0,KeyFromValue1,KeyFromValu e2,KeyFromValue3,KeyFromValue4,KeyFromValue5,KeyFromValue6,KeyFromValue7,KeyFro mValue8,KeyFromValue9>::operator ()(const ChainedPtr &) const' being compiled with [ Value=boost::multi_index::member_offset<tTest,int,0>, KeyFromValue0=boost::multi_index::member_offset<tTest,int,4>, KeyFromValue1=boost::tuples::null_type, KeyFromValue2=boost::tuples::null_type, KeyFromValue3=boost::tuples::null_type, KeyFromValue4=boost::tuples::null_type, KeyFromValue5=boost::tuples::null_type, KeyFromValue6=boost::tuples::null_type, KeyFromValue7=boost::tuples::null_type, KeyFromValue8=boost::tuples::null_type, KeyFromValue9=boost::tuples::null_type, ChainedPtr=tTest ]

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Re: [Boost-users] Re: lambda dereferencing pointer
by Sean DeNigris 05 Apr '05

05 Apr '05

Sorry my man, include boost/lambda/bind.hpp also.

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How to represent bipartite graphs ?
by Markus Krosche 05 Apr '05

05 Apr '05

Hello together, in my software I have to represent bipartite graphs, in my case petri nets (two disjoint sets - the places and the transitions - and only links between elements of the disjoint sets). I would like to use BGL for this purpose, but I did not find a "nice" solution for this problem. I am quit new to BGL and the only way I see is representing the two types of vertices inside a vertex-property. A better way would be to have two types of vertex properties, but I think this is not possible. Is there anyone who has a solution or an idea for this kind of problem? Thanks and best regards Markus

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Re: [Boost-users] lambda dereferencing pointer
by Sean DeNigris 05 Apr '05

05 Apr '05

> std::partition( m_files.begin(), m_files.end(), > boost::bind( &boost::filesystem::is_directory, > *boost::lambda::_1 ) ); You are using bind from Boost.bind and _1 from Boost.lambda. If you change to boost::lambda::bind it will compile. See http://www.boost.org/doc/html/lambda/s08.html#id547254 for more detail. - Sean

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C++ Template Metaprogramming Answers Page (was: EX 2.1)
by Dave Gomboc 05 Apr '05

05 Apr '05

> > Low-tech solution: "scroll down for next hint" "scroll down for > > solution" > > If you're willing to give up on the goal of "one page per exercise," > and would like to go instead with a finer-grained structure, we can > accomplish any restructuring you wish through the Wiki's > administrative interface; just let us know. Show the first hint, with a link. Clicking the link shows everything the same, but the link is replaced with a second hint, and a third link appears. Repeat until out of hints -- in which case no link appears. For example: http://www.uhs-hints.com/uhsweb/hints/halflife2/8.php Dave

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lambda dereferencing pointer
by Matthias Kaeppler 05 Apr '05

05 Apr '05

Consider m_files to be a vector of pointers to boost::filesystem::pathS: std::partition( m_files.begin(), m_files.end(), boost::bind( &boost::filesystem::is_directory, *boost::lambda::_1 ) ); I want to invoke is_directory on the pointees. The error message is extremely long-winded and expands over several pages. I suppose you don't want to see it :) But maybe you can see immediately what I'm doing wrong? -- Matthias Kaeppler

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