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Re: [Boost-users] A question about boost::lambda::if_ behavior
by Bjorn.Karlsson＠readsoft.com 22 Sep '05

22 Sep '05

Hello Tom, -----Original Message----- From: tom gee [mailto:rockonedge@gmail.com] Subject: [Boost-users] A question about boost::lambda::if_ behavior > Can anyone please explain this code: [snip] The problem is that the expression std::cout << constant("Bingo:<") is evaluated immediately. Why? Because it's not a lambda expression. You have two options that will help you turn your if-branch into a lambda expression; either use boost::lambda::var or boost::lambda::constant. Using boost::lambda::var: std::for_each(setFinalVal.begin(),setFinalVal.end(), (std::cout << _1 << " ", if_(_1 < 24.000001 && _1 > 23.999999) [var(std::cout) << "Bingo:<" << _1 << ">"])); Using boost::lambda::constant: std::for_each(setFinalVal.begin(),setFinalVal.end(), (std::cout << _1 << " ", if_(_1 < 24.000001 && _1 > 23.999999) [std::cout << constant("Bingo:<") << _1 << ">"])); Cheers, Bjorn Karlsson

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Boost function
by Labour, Matthieu (MLPRO SF) 22 Sep '05

22 Sep '05

Hi Is it possible to have boost function with unlimited arguments For example: boost::function <../boost/function.html> <float (int x, ...)> f; Thank you -------------------------------------------------------- If you are not an intended recipient of this e-mail, please notify the sender, delete it and do not read, act upon, print, disclose, copy, retain or redistribute it. Click here for important additional terms relating to this e-mail. http://www.ml.com/email_terms/ --------------------------------------------------------

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Re: [Boost-users] algorithms and member functions
by Alan M. Carroll 22 Sep '05

22 Sep '05

Using the example in your other e-mail, a solution is find_if(as.begin(), as.end(), boost::bind(std::bitwise_and<int>(), SOME_FLAG, boost::bind(&A::wee, _1))); Anonymous functions and classes are nice. LISP, of course, did it best :-). The closest approximation of this in modern languages is actually in Perl. In Perl, any place you need a code/function pointer, you can write an anonymous function instead of using an existing one. This anonymous function is identical to any other function, it just doesn't have a name (and in fact, you could give it a name later, if you really wanted). It is definitely a very powerful language feature and a lot of the bind / lambda work is intended to support as much of that as possible. I suspect that we're not seeing change in this area because the C++ grammar is already extremely hard to parse and rather cantankerous. Adding this kind of feature might well push it over in to unusablity. A more uniform concept of "code block" as the chunk of stuff enclosed by curly braces would help, but you still have the problem of argument declaration. Perl solves that by having only one argument style so explicit specification is not needed. LISP solved it by embedding the argument declarations inside the code block. C++, unfortunately, puts argument declarations outside the code block while not having a keyword to indicate a function declaration (perhaps, finally, a use for "entry"?). At 11:04 AM 9/21/2005, you wrote: >Hello all, > > >What interests me is the anonymous methods / class style syntax that Java >offers. I've played with it myself, and it seems nice, and one of the >proponents in the afore mentioned flame war advocates it. > >What is the thinking behind not introducing such syntax into C++?

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A question about boost::lambda::if_ behavior
by tom gee 22 Sep '05

22 Sep '05

Can anyone please explain this code: for_each( setFinalVal.begin(), setFinalVal.end(), ( cout<<_1<<" ", if_(_1 < 24.000001 && _1 > 23.999999)[ cout <<"Bingo:<"<< _1<<">"]) ); where setFinalVal is defined as set<double>. I expect it to result in: -32 -24 -16 -12 -11 -10 -9 -8 -7 -5 -4 -3 -2 0 4 5 6 7 8 9 11 12 13 14 24 Bingo:<24>32 40 However, compiled with VC++2005 express, it results in: Bingo:<-32 -24 -16 -12 -11 -10 -9 -8 -7 -5 -4 -3 -2 0 4 5 6 7 8 9 11 12 13 14 24 24>32 40 Why?

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[BGL] detect cycles in a directed graph
by Giulio Veronesi 22 Sep '05

22 Sep '05

Hi, This is my second post for the same problem. Excuse me! I think that the problema can be resolved using the EventVisitor Concept but I don't understand how use it. I would be extremely grateful for any assistance in the following problem: "get all the edges of a cycle". Thanks a lot in advance, Giulio

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[Boost.Signals] One-shot slot?
by Simon Buchan 22 Sep '05

22 Sep '05

What would be the simplest way to make a slot that, once signaled, would disconnect itself? I thought that boost::signals::trackable & 'delete this' would do it, but it seems to be only for boost::bind functors... I am trying: my_slot* foo = new my_slot(...); connection c = sig.connect(bind(foo)); foo->my_connection = c; (vastly simplified, that probably wouldn't compile) and have my_slot 'delete this' in it's op() and 'my_connection.disconnect()' in it's d'tor, but this seems very inelegant, particlarly as this is in a mutli-threaded environment (I know signal isn't thread- safe, but some boost::mutex's normally do the trick) Should I try using a Combiner to weed out slots marked 'once only' or something? Many thanks in advance!

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Boost.Graph can a graph that is a directedS be safely converted to a bidirectionalS?
by Elisha Berns 22 Sep '05

22 Sep '05

Hi, I have a Boost.Graph object that needs to model as closely as possible a DAG in order to be a dependency graph. As such it is an adjacency_list with a directedS policy. All works fine, except that it also needs to work as the inverse of a dependency graph: it needs to find the vertices for a given vertex V that are the 'dependents' (?) of V, that is to say that all the found vertices depend upon V, not that V depends upon them. So I assume that to search for the dependents of vertex V I need to get all the in-edges of V, and the in-edges of all those in-edges recursively? This is apparently feasible by using a bidirectionalS policy for the graph. But my questions are the following: 1) Is there some other better way to do this, such as another graph traversal algorithm? 2) What's the computational expense, in general, of using a bidirectionalS graph over a directedS graph in terms of speed and memory? 3) In the event that there is no better way and a bidirectionalS graph is much fatter and slower than a directedS graph does it make sense to construct the bidirectionalS graph only when needed? Since I only need this when its requested, not as a permanent feature of the application, I could copy the entire directedS graph to create some other graph for this purpose. But here the graph size can vary from a few hundred vertices to several tens of thousands of vertices, so if there is a way to avoid this its probably best. Thanks for any insight here, Elisha Berns e.berns(a)computer.org tel. (310) 556 - 8332 fax (310) 556 - 2839

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Boost.Tuple extension *style* question
by Alan M. Carroll 21 Sep '05

21 Sep '05

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ICU vs Boost Regex
by louis 21 Sep '05

21 Sep '05

I was just looking at the ICU code from IBM and noticed they have a regex engine. Does anyone have experience with this regex engine and how does it compare to the one in the boost library? Any feedback would be greatly appreciated. Thanks Louis

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list of optional dependencies for building boost?
by Benjamin Rutt 21 Sep '05

21 Sep '05

I am new to boost and have a question on building it from source. I built it once using the command: bjam --with-python-root=/home/rutt/python-2.4 -sPYTHON_VERSION=2.4 --prefix=/home/rutt/boost install So you can see that I at least resolved the optional dependency on Python. I also noticed that it warned me that it could not find the bzip2 or gzip packages (I can't remember which). At the end of the build, it mentioned something about a number of errors it had in building, yet it did install the boost libraries. So it seems that the build process continues on past failures (something like 'make -k'). I guess those "failures" must have been unmet optional dependencies? My intention is to build a fully capable boost library and have zero errors and zero unmet dependencies. Is there some documentation or method to obtain information about all optional dependencies boost can pick up at build time? I would like to install them all, to ensure a clean, zero-error build and know that I have a fully functional boost. Thank you, -- Benjamin Rutt

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