April 2006 - Boost-users - lists.boost.org

Re: [Boost-users] Delayed write file handling
by James Hughes 10 Apr '06

10 Apr '06

> -----Original Message----- > From: me22 [mailto:me22.ca@gmail.com] > Sent: 10 April 2006 16:24 > To: boost-users(a)lists.boost.org > Subject: Re: [Boost-users] Delayed write file handling > > > On 4/10/06, James Hughes <JHughes(a)linx.co.uk> wrote: > > My question is this - does Boost provide anything that does > this already (or > > does anyone know of anything that does), or am I looking at > this the wrong > > way? Should I look at boost filesystem, and provide extra > bits to implement > > what we need? Alternatives? > > > > I'm not certain that you need Boost for this. > > streambufs are already designed to buffer information, so simply > providing a large enough buffer for the amount of information > generated on average in the time span you wish between writes ought to > limit writes enough for your application. > > The main issue I foresee would be insuring that std::endl isn't used > when you don't want the flush and insuring that the implementations of > your streambufs actually use the buffer as you're expecting. > > ~ SWMc I can see what you are getting at, but I am not sure that it would give us everything we need. For example, if our machine power fails, we have very limited amount of battery backup to store everything cached to flash before we run out of watts, so whatever scheme I implement needs to be able to be told to flush immediately - can I do that with streambufs? Hmm. I will look in to it though, as I like the concept. James

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Delayed write file handling
by James Hughes 10 Apr '06

10 Apr '06

Hello all, not sure if this is the right place to ask, but there maybe something in boost that can help. I would like to use/write some file handling classes that implement something like delayed writing. The reason is that on our embedded device, we need to log information to flash memory. This information may be generated very frequently (every second or more). However, since our devices are in the field for long periods of time (8-10 years) this means that to continually be writing to flash memory, we can reach the limit of the capability of the flash re-write cycle (10k to 1mill erases before failure depending on quality), even when using a load levelling fs like Jffs2 (running on Linux). By delaying to write of the logging information so that the classes 'cache' data until either told to write by the system, or a time limit is reached, we can dramatically reduce the amount of flash erasing done. I would like to do this transparently to the end user, by perhaps writing a iostream class that handles it all (with some extra bits to handle urgent shutdowns and user requested flushes). My question is this - does Boost provide anything that does this already (or does anyone know of anything that does), or am I looking at this the wrong way? Should I look at boost filesystem, and provide extra bits to implement what we need? Alternatives? James Hughes

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[string] HowTo: equiv of printf("%.5s\n", myBuf10);
by Lynn Allan 10 Apr '06

10 Apr '06

I've been puzzled about doing the equivalent of: printf("%.5s\n", myBuf10); and also: printf("%.*s\n", 5, myBuf10); std::string myBuf10("0123456789"); const char* pBuf = myBuf10.c_str(); printf("%.5s\n", pBuf); printf("%.*s\n", 5, pBuf); cout << myBuf10.substr(0, 5) << endl; // works ok -> 01234 cout << setw(5) << myBuf10 << endl; // 0123456789 I've used: myBuf10.substr(0, 5); and also ostringstream but it seems like there might be a simpler way. setw(5) doesn't work to accomplish truncation (at least for me). It seems more for padding. Am I unaware of some iomanip functionality to accomplish this? I suppose I could do something like: char hack = myBuf[5]; myBuf[5] = '\0'; .... do something myBuf[5] = hack; (may or may not be portable?) Note: newbie with std::string ... using Microsoft vc7.1 compiler Thanks.

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thread safe dynamic array and matrix
by Lars Schouw 10 Apr '06

10 Apr '06

Does boost provide me with a thread-safe dynamic array and/or matrix? The container should be thread safe for write from many thread at the same time for chaning information in the elements stored in the array. Regards Lars Schouw --------------------------------- Talk is cheap. Use Yahoo! Messenger to make PC-to-Phone calls. Great rates starting at 1¢/min.

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Program Options: What to do with positional_options_description?
by Olaf van der Spek 09 Apr '06

09 Apr '06

Hi, The example says: positional_options_description pd; pd.add("output-file", 2).add_optional("input-file", -1); But what do I do with pd after this?

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boost::bind and std::copy for member functions
by Leon Mergen 09 Apr '06

09 Apr '06

Hello, Ok, this is what I'm trying to do: I want to define some base class A which has a container with elements it needs to sort. I want to be flexible with the comparison function, so the base class A will define an abstract comparison function which will be implemented by derived classes. Since std::sort () expects function objects, after a lot of asking around I found out I needed to use boost::bind. However, trying almost all different types of combinations and looking up all kind of manuals, I'm pretty much clueless... Here's the test-case code: --- #include <algorithm> #include <string> #include <vector> #include <boost/bind.hpp> class A { public: virtual bool sort (std::string const * const & a, std::string const * const & b) const = 0; void func () { // std::sort ( _elements.begin(), _elements.end(), boost::mem_fn ( A::sort) ); std::sort ( _elements.begin(), _elements.end(), boost::bind ( A::sort, this, _1, _2 ) ); } private: std::vector < std::string * > _elements; }; class B : public A { public: virtual bool sort (std::string const * const & a, std::string const * const & b) const { return true; } }; class C : public A { public: virtual bool sort (std::string const * const & a, std::string const * const & b) const { return false; } }; --- I hope the code makes it a bit more clear as to what I'm trying to do... I follow examples listed at threads like http://blog.gmane.org/gmane.comp.lib.boost.user/day=20050617 , and well, this is almost a literate copy of some recommendations but it doesn't work: -- sort.cpp: In member function ‘void A::func()’: sort.cpp:13: error: no matching function for call to ‘bind(<unknown type>, A* const, boost::arg<1>&, boost::arg<2>&)’ /usr/local/include/boost/bind.hpp:1416: note: candidates are: boost::_bi::bind_t<boost::_bi::unspecified, F, typename boost::_bi::list_av_3<A1, A2, A3>::type> boost::bind(F, A1, A2, A3) [with F = bool (A::*)(const std::string* const&, const std::string* const&)const, A1 = A*, A2 = boost::arg<1>, A3 = boost::arg<2>] /usr/local/include/boost/bind/bind_mf_cc.hpp:78: note: boost::_bi::bind_t<R, boost::_mfi::cmf2<R, T, A1, A2>, typename boost::_bi::list_av_3<A1, A2, A3>::type> boost::bind(R (T::*)(B1, B2)const, A1, A2, A3) [with R = bool, T = A, B1 = const std::string* const&, B2 = const std::string* const&, A1 = A*, A2 = boost::arg<1>, A3 = boost::arg<2>] -- and well, the error is cryptic enough for me to not quite understand what I'm doing wrong... any suggestions ? Thanks in advance! Regards, Leon Mergen

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[string] Are std::string's null terminated?
by Lynn Allan 08 Apr '06

08 Apr '06

Sorry if this is off-topic. Just wanted to check .... are std::string's defined to be null terminated? I came across a web page that indicated they are, but you can find lots of misinformation on the web. http://www.codeproject.com/string/cppstringguide2.asp std::string's seem to be null terminated with the Microsoft vc7.1 and vc8 compiler, but I wanted to check if this is generally true according to the C++ standard. I'm relatively new to stl and using std::string .... mostly have used C strings and MFC CStrings (which changed from vc6 to vc7.1?) std::string strA = "ABC"; std::string strB = "DEF"; std::string strC = "GHI"; std::string strD = "JKL"; std::string strE = "MNO"; // Check if std::string's are null terminated const char* pTest = strC.c_str(); for (int i = 0; i < 80; ++i) { printf("I: %2d ch:%3d %c\n", i, pTest[i], pTest[i]); } std::string testStrings[] = { "abc", "def", "ghi", "jkl" }; const char* p = testStrings[1].c_str(); printf("\n********************\n"); for (int i = 0; i < 80; ++i) { printf("I: %2d ch:%3d %c\n", i, p[i], p[i]); } printf("\n********************\n"); TIA,

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total noob help: LAMBDA with zip_iterator
by chun ping wang 08 Apr '06

08 Apr '06

I have several questions that I asked earlier but never got an answer. So I will start one by one.. a.) How do I use lambda with zip_iterator. (assuming you have a zip_iterator of vector, list, map)? Thats my first question...

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[FYI] RCF 0.2
by Jarl Lindrud 08 Apr '06

08 Apr '06

Hi all, I just thought I would announce that I've released v. 0.2 of RCF, a portable C++ IPC framework. It makes extensive use of boost, including Boost.Threads, Boost. Serialization, and the recently accepted asio library, so maybe people on this list will find it of interest :) http://www.codeproject.com/threads/Rcf_Ipc_For_Cpp.asp /Jarl.

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regex_search? Which subexpression smatch'ed?
by Lynn Allan 08 Apr '06

08 Apr '06

<alert comment="boost and regex newbie"> I'm using regex_search to detect which subexpression matched. I adapted the "Captures" example at: http://www.boost.org/libs/regex/doc/captures.html In the snippet below, it works ok to loop thru m[num].matched, but I was wondering if there was a more direct way of getting the information about which match was "hit". (The real application is going to have about 70 possible subexpressions to match against, instead of just four.) std::string contents("String has two, three, one, and four in it. "); boost::regex reg("(one)|(two)|(three)|(four)"); >From the above, I want the output to be: <match=2>two <match=3>three <match=1>one <match=4>four void test_search() { boost::smatch m; std::string contents("String has two, three, one, and four in it. "); boost::regex reg("(one)|(two)|(three)|(four)"); std::string::const_iterator it = contents.begin(); std::string::const_iterator end = contents.end(); while (boost::regex_search(it, end, m, reg)) { for (int num = 1; num <= 4; ++num) { if (m[num].matched == true) { std::cout << "<match=" << num << ">" << m[0] << std::endl; break; } } it = m[0].second; } } </alert>

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