January 2005 - Boost - lists.preview.boost.org

boost.filesystem - rationale for copy_file
by larsbj＠gullik.net 30 Jan '05

30 Jan '05

What is the rationale for having copy_file fail if the target file exists? This is not how any cp/copy in any shell I know of works, actually quite surprising. And with boost.filesystem there is no way to avoid this, short of rewriting the function to allow it. The other way around would have been easy: fs::exists(target) || fs::copy_file(source, target) (One option is to delete the target before doing the copy, but then it is impossible to keep the same inode as before, and f.x. for backup files this is something that you want.) -- Lgb

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Re: [boost] [iostreams] [regression] iostreams added to CVS
by JOAQUIN LOPEZ MU?Z 30 Jan '05

30 Jan '05

Hi, I think the note on the first page of the docs is no longer needed: Boost.Iostreams is now part of Boost. Also, don't forget to add an entry in libs/libraries.htm and in /index.htm (under the "Version 1.33.0" paragraph). Joaquín M López Muñoz Telefónica, Investigación y Desarrollo ----- Mensaje original ----- De: Jonathan Turkanis <technews(a)kangaroologic.com> Fecha: Domingo, Enero 30, 2005 0:29 am Asunto: [boost] [iostreams] [regression] iostreams added to CVS > Dear all, > > The iostreams library has been added to CVS. > > I've also added a line to boost-root/status/Jamfile. Is this > enough to have the > library included in the regression tests, or is something more > required? > Jonathan > > > > _______________________________________________ > Unsubscribe & other changes: > http://lists.boost.org/mailman/listinfo.cgi/boost

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Composites and Visitors
by Dave Handley 30 Jan '05

30 Jan '05

Since my last post concerning my implementation of a composite and visitor framework, I have been working on a memory management policy for the composite. The new version of the code is available in the sandbox. I would like to highlight the key changes, along with a few comments from a very useful offline discussion I had with Pavel Vozenilek: 1) A few bugs have been fixed - specifically to do with memory management. I haven't written a full test suite yet (but intend to do so in the near future) so I can't guarantee any degree of robustness in the code as yet. 2) I have put in 3 memory management policies. These enable you to ignore memory management, allow the composite to be responsible for its own memory with conventional pointers, or allow the composite to be responsible for its own memory with boost smart pointers. As yet I have been unable to sort out deep copying of the tree with the smart pointer policy. One option on this may be to implement shallow copies of the tree in the copy constuctor and equals operator, and implement a separate deep_copy function. What are the general views on this? Are there any tricks with boost shared_ptrs that enable you to do deep copies of trees from the constructor? 3) I am intending to put in a policy that will allow you to select the type of container used for internal storage. Thanks to Pavel for this suggestion. 4) I know that this is not strictly a composite in the traditional GoF sense. I actually created this particular version of a composite due to the fact that it is easier to write deep iterators when every node in a composite tree knows that it is part of the tree (unlike the GoF model where only the group nodes actually know about the tree structure). My design goals for this code were: a) Create a tree where individual nodes know about their location in the tree. b) Provide various iterators for this tree. c) Provide tight integration with the visitor pattern. These design goals were specifically designed to try and reduce virtual function bloat that, in my experience, is common with composites. In discussion with Pavel, I have come up with a design that also succeeds in meeting these criteria but ensures that leaf nodes do not carry around an empty list. I am working on this at present, but it is worthy of note that the API will be slightly more complex. The files in the sandbox are now: composite_visitor.zip - contains only the composite and visitor headers and a quick instruction document. composite_visitor_tests_win32.zip - contains the win32 test program. I haven't included the vcproj or sln files, but they should be trivial to create. Dave Handley

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shared_ptr doubts
by Don G 29 Jan '05

29 Jan '05

Not to be contentious, but IMHO it seems that shared_ptr's reach has exceeded its grasp. After looking thoroughly into shared_ptr, I have major doubts about what appear to be design-induced inefficiencies: 1. There is no support for "introducing" shared_ptr to a type T such that shared_ptr would use an intrusively managed count. This is just about required in my book for practicle use of reference counting in large applications. I see intrusive_ptr<>, but this would not help template code that wanted to share ownership of some type T; should it use shared_ptr<T> or intrusive_ptr<T>? Only type T knows. 2. The idea of weak_ptr seems to force external reference counting and a mutex (of some sort) to guard the counter(s). If there is a safe way to use lock-free counting and still get the assurances of weak_ptr::lock, I'm not clever enough to see it. Also, I can see no hope of tuning shared_ptr<T> for intrusive counting and keep weak_ptr. To me, this feature is worth no more than sizeof(ref_count) per object and no hidden locks, at least on platforms that support atomic integer operations. weak_ptr<T> would not be available if shared_ptr<T> was based on intrusion, again unless someone more clever than I could find a way to do so that wouldn't burden code that doesn't use weak_ptr. 3. The deleter feature also adds virtual function bloat as well as more per-object overhead. To me, this kind of overhead would preclude use of shared_ptr in all by the most trivial applications. The problem again is that this is not something that can be avoided by shunning the use of some features. For example, if the deleter where part of a shared_ptr<void> specialization or something. While I like the aesthetics of these features, I can see little practicle need for them given their cost. In other words, I would trade both of them for the knoweldge that I could "use the shared_ptr, Luke" without concern for such bloat. Herb Sutter advocated this position in a recent C++ User Journal article, which is what sparked my curiousity in the first place. Again, perhaps I've missed something profound and fundamental in the specification of shared_ptr that would allow for these features and a highly effecient implementation of basic reference counting. As I see it, I will have to keep my own counted_ptr which does "just the counting, ma'am." My $2e-02, Don ===== __________________________________ Do you Yahoo!? Yahoo! Mail - You care about security. So do we. http://promotions.yahoo.com/new_mail

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mpl::set problem
by Alexander Nasonov 29 Jan '05

29 Jan '05

I can't understand why derefencing begin of set<int,char> gives set2<int,char>, not char or int as one would expect. typedef set<int,char> s; typedef deref<begin<s>::type>::type e; BOOST_MPL_ASSERT(( or_<is_same<e,int>, is_same<e,char> > )); If I change s definition to typedef insert<set<int>,char>::type s; it compiles fine on gcc 3.4 and intel-linux 8.0. -- Alexander Nasonov

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StringAlgo::split issue
by Dirk Gregorius 29 Jan '05

29 Jan '05

Hi, I used the split function on the following string: vector<string> tokens; string str= "( 448 448 64 ) ( 448 0 64 ) ( 0 448 64 ) name 0 0 0 0.5 0.5 0 0 0"; split( tokens, str, is_any_of( ()), token_compress_on ); // I pass 'space' , '(' and ')' into is_any_of() I was supposed to get 18 tokens. The first nine values, the string "name" and the remaining eight digits. "split" always returns a string vector containing 19 tokens, where the first element of the vector is an empty string. Why does the function insert empty strings into the collection? Can I use split to obtain the 18 wanted tokens? -Dirk

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Boost Test - when you've got nothing else to do
by Robert Ramey 29 Jan '05

29 Jan '05

I find Boost Test very valuable and use on a regular basis. I think that it would be even more useful if it had the following additional features: a) automatic heap checking for memory leaks for libraries that can support it. b) the ability to create a temporary fstream with a unique name in a safe way. c) The ability to specify an execution time limit so that if the program goes off into space it doesn't waste an in ordinate amount of time. (perhaps this would be easier to include in bjam) d) The abtility to specify a maximum amount of memory to be allocated. When this allocation is passed, the program would terminate. e) Currently, all tests compiled with borland compilers fail immediatly when the program and library are built in release mode. I would like to see this fixed. I did spend a little time trying to track this down but was unsuccessful. f) A more tutorial type manual. Robert Ramey

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[test] CHECK_EQUAL_COLLECTION
by Jody Hagins 29 Jan '05

29 Jan '05

I have a collection of boost:shared_ptr<>, and I want to test what they point to. CHECK_EQUAL_COLLECTION does not take a predicate, so what method do you suggest? Currently, I create "copies" of the collections with "holders" that invoke the comparison on the "pointed-to-object" but this seems a bit klunky. What is the preferred method for this type of collection?

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non-member reset()
by Stefan Slapeta 28 Jan '05

28 Jan '05

Hi all, do we have a non-member reset() somewhere? Something like template <typename T> void reset(T& t) { std::swap(t, T()); } (of course to be refined...) Stefan

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Meta-math
by Jason Hise 28 Jan '05

28 Jan '05

I was just playing around, trying to compute a square root at compile time. I came up with the following: template < unsigned int N, unsigned int X = 1, unsigned int X2 = ( X + N / X ) / 2 > struct Sqrt { typedef typename Sqrt < N, X2 > :: ValueHolder ValueHolder; }; template < unsigned int N, unsigned int X > struct Sqrt < N, X, X > { struct ValueHolder { enum ValueStorage { Value = X }; }; }; This works for most values, but unfortunately some ( like 80 ) end up oscillating and X never becomes equal to X2. How could I go about correcting this? -Jason

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