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Ann: Forward lookahead asserts now supported by regex lib
by John Maddock 23 Jan '02

23 Jan '02

The regex lib, now has experimental support for perl style forward lookahead assertions, using the syntax extensions: (?=exp) positive zero width forward assertion on exp. (?!exp) negative zero width forward assertion on exp. I'm currently looking for anyone willing to test these, and for real world test cases to add to the regression tests, before documenting the extensions and making them "official". Many thanks. - John Maddock http://ourworld.compuserve.com/homepages/john_maddock/

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error with thread.hpp
by thrashmodule 19 Jan '02

19 Jan '02

I got this message: c:\boost_1_26_0\boost\thread\thread.hpp(17) : fatal error C1189: #error : Thread support is unavailable! I use VC++ on Win98 so now what??????????

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Class destructor invocation - possible by hand?
by Jonathan Brownell 19 Jan '02

19 Jan '02

I have a question for all you C++ gurus. I'm writing a class that I would like to use the following technique in, and I'm not sure if it's portable or even legal, although VC++6 and VC++.NET, which I'm compiling in, work beautifully with it. Consider the following class, a very simplified example that I wrote to demonstrate: class House { public: explicit House(const int rooms) : m_rooms(rooms) { } ~House() { } private: const int m_rooms; }; Pretty straightforward. I'd like m_rooms to remain const to protect it from accidental modification by member functions, since it will *never* be changed during operation. Now, the first problem that arises is that this is illegal: House housingComplex[100]; Because, obviously, the constructor requires that the number of rooms be passed to it. (Yes, we could create it as an array of House pointers and create them dynamically, but in my case that will require inconveniences by everybody using my class.) So, we make it a default parameter and add an assignment operator: class House { public: explicit House(const int rooms = -1) : m_rooms(rooms) { } ~House() { } House& operator =(const House& source); private: const int m_rooms; }; When rooms == -1, the House is considered to be uninitialized. Now, we can do the following: House housingComplex[100]; housingComplex[0] = House(5); housingComplex[1] = House(4); (etc) The question is, how to implement the assignment operator? This is my method: House& House::operator =(const House& source) { assert(m_rooms == -1); this->~House(); new(this) House(source); return *this; } Yikes! I know this looks weird. First, assert verifies in debug mode that the object is in its uninitialized state. Then, the destructor is called manually to destruct the class and its contents. (In the actual class I'm working on, there are numerous member variables that must be appropriately destructed, and all of them are - I've checked.) Simply calling ~House(); will not compile, because the compiler assumes I'm trying to use the ~ operator illegally, so I have to call it through the 'this' pointer. Then, the placement new operator is used to create a new House in exactly the same space in memory as the old one, using the default copy constructor so that all the const variables are (re)initialized. Finally, a technicality for all assignment operators, a reference to the object is returned. (This is so C-style expressions like "var1 = var2 = var3 = 5;" can be used.) Now, this works beautifully on my VC++. But... is it legal C++?? Am I entering the realm of "undefined behavior", or is this a perfectly legitimate technique? Is there an obviously better solution to my problem? Are there caveats that I should be looking out for? Please, someone with more knowledge of the C++ standards, give me a hand here. Thanks! -Jonathan

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RE: [Boost-Users] Class destructor invocation - possible by hand ?
by Tom Matelich 17 Jan '02

17 Jan '02

> > > > Also, I would recommend that you either reconsider > > > having the constant member variable, or use a > > > vector<>. > > > > Using a vector<T> won't work in this case, because the STL uses > > T::operator = in order to copy vector contents into new > locations during > > memory reallocation. And there's no logical way to > implement operator = > > when a const data member is present, which is the cause of this > > conversation. > > I could use std::list, since it doesn't do reallocation, but clients > > using this class in containers will most likely need random access > > iterators. > > > > Any other ideas? > > Easy option: Use a vector<shared_ptr<T> >. This will give you > basically > the behavior you want, with the ability to initialize the elements > directly. The drawback is that the data will be dynamically allocated. Another drawback is the interface is now pointers, not references. If you don't mind the dynamic allocation, the vector<shared_ptr<T> > can be contained by another class, overloading operator[], etc. The Boost Iterator Adapter library works very nicely for providing vector<T>::iterator behavior from a vector<shared_ptr<T> >::iterator. I can send you our vector-like class which uses adapted iterators if you would like. Re: the moderation. Yes, this is not a boost question, but it is a question boost can provide good solutions for, so where do you draw the line?

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regex_match doesn't match in an embedded string
by soleani 17 Jan '02

17 Jan '02

Hi there, not sure how I could write that Subject line. Sorry :-) Here's the thing, I have a file with the following: KEY1 = ${short_localhost} KEY2 = bbbf<control2.cfg:OTHER_SECTION:${long_localhost}>aaaf:<he I need to extract the value between the {} when it starts with a $. My expression works ok for KEY1 (the value part, the string to the right of the equal sign), however it doesn't match for the value for KEY2. This is what I have: static const boost::regex expression( "\\$\\{(.*?)\\}" ); boost::cmatch what; if ( boost::regex_match( input, what, expression ) ) { cerr << "GOT IT!" << endl; } else { cerr << "NOPE" << endl; } where input is a std::string with the value described above. Why does it work for KEY1 and not KEY2? Interestingly enough, if I do the following: KEY1 = dd${short_localhost} it doesn't match either. This same expression in Perl works fine though. thanks, -Anibal

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RE: [Boost-Users] boost tokenizer
by Jim.Hyslop 17 Jan '02

17 Jan '02

Peter Dimov [SMTP:pdimov@mmltd.net] wrote: > From: "hicks" <hicks(a)kgk.co.jp> > > Do you know why the compiler places precedence on that interpretation, > > rather than on one giving an instatiation of a local variable? > > The C++ standard says that a function declaration takes precedence. Don't > ask me why. :-) (At the risk of sliding seriously off topic...) I suspect it's something along these lines. Given this function: void f() { int WhatIsThis(); } Is "WhatIsThis" intended to be a declaration of a function, or is it intended to be a definition and default-initialization of an int variable? A C++ compiler cannot know, so the C++Standards committee had several choices: 1) require the compiler to flag the statement as an ambiguous statement 2) require the compiler to interpret the statement as a declaration of a function 3) require the compiler to interpret the statement as a definition of an int If we examine the same syntax in C, there is no question what it is: 'WhatIsThis' is a declaration of a function. That effectively eliminates #3 from consideration because you don't want one statement to have different meanings in C and in C++. I suspect the Committee then either flipped a coin (or some used other democratic means, such as debating and voting on the issue, thumb-wrestling, or playing paper-rock-scissors) to decide between #1 and #2, or choose to adapt the well-worn guideline "When in doubt, do as the ints do" to "When in doubt, do as C does". -- Jim

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boost tokenizer
by hicks 17 Jan '02

17 Jan '02

Dear boost users I am trying to use the boost tokenizer with Borland Builder C++ 5 compiler. This far compiles fine: boost::tokenizer<> tok(std::string(ac0)); All further references to tok such as tok.assign(std::string(ac0)) or tok.begin() result in an error message " Structure requires on the left side of . or .*" Any advice would be welcome. Craig Hicks Engineer, KGK, Tokyo Japan

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Boost Array Question
by Duane Murphy 16 Jan '02

16 Jan '02

Below is a simple example that I cannot get to compile. I would like to create a subclass of boost::array in order to give it some additional functionality (I would like a associative array like interface). When I compile this, I get an "illegal initialization" error for test. I am using Metrowerks Codewarrior 7 on Mac OS X. Is there some special incantation required? I suspect that I am in some kind of specialized area and I cant get there from here. template < class T, std::size_t N > struct test_array : public boost::array< T, N > { }; static boost::array< char,1 > array_test = {{ 'a' }}; static test_array< char, 1 > test = {{ 'a' }}; It appears that if I aggregate instead of inherit that this might work (at least it compiles :-) template < class T, std::size_t N > struct test_array { boost::array< T, N > array_; }; static boost::array< char,1 > array_test = {{ 'a' }}; static test_array< char, 1 > test = {{ 'a' }}; // The funny thing is this works also: static test_array< char, 1 > test2 = { 'a' }; TIA, ...Duane

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tokenizer
by rradh 16 Jan '02

16 Jan '02

Hi, I have string line("a_1 = b_1 ( c_2 )") tokenizer< > tok(line); for (tokenizer<>::iterator beg=tok.begin(); beg!=tok.end();++beg) { cout << *beg << endl; } gives me a 1 b 1 c 2 I want to be able to specify delimiters like in strtok so that I get a_1, b_1 and c_2 but couldn't figure out how to specify the tokenizer function. Any suggestions? Regards Rajesh

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graph: depth first search
by tobi_schuele 15 Jan '02

15 Jan '02

Hi, I'd like to use depth_first_search to check whether a particular vertex is reachable from the root vertex. Is there a way to stop the search when the vertex is found? In other words, can a visitor be used to cut the search tree? Sorry, if this is a newbie question. Thank you very much in advance for your help. Tobias

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