C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  streambuf for reading from existing memory block          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Dylan Nicholson Guest Posted: Mon Feb 23, 2004 11:40 am    Post subject: streambuf for reading from existing memory block {Might be better asked in comp.std.c++. -mod} In order to implement a streambuf for reading/writing from/to an existing block of memory, AFACIT, it's as simple as (ignoring templates for now): struct membuf : std::streambuf { membuf(char* p, size_t sz) { setg(p, p, p + sz); setp(p, p + sz); } }; Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? Dylan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Philipp Bachmann Guest Posted: Mon Feb 23, 2004 7:09 pm    Post subject: Re: streambuf for reading from existing memory block Quote: In order to implement a streambuf for reading/writing from/to an existing block of memory, AFACIT, it's as simple as (ignoring templates for now): struct membuf : std::streambuf { membuf(char* p, size_t sz) { setg(p, p, p + sz); setp(p, p + sz); } }; Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? Hello Dylan, "streambuf"s are either both of fixed size and the stream is related to some external resource it can "sync()" its "streambuf" to - e.g. the "filebuf" gets flushed into a file by the "ofstream" both explicitly and if it has been filled up -, or the "streambuf" has no upper bound as the "stringbuf". Because your "membuf" points to memory of finite size, in your case the question is: What is "sync()" supposed to do? I.m.o. you have only one choice here (because you didn't mention a sink like a network connection, a binary file etc.): Growing your buffer, and the method to do so should be supplied by means of an "allocator" to free yourself from one specific allocation policy. Another consideration: The C++ standard library only provides formatting streams. Therefore any "basic_*buf<>"s has a template parameter indicating a character type. Writing into raw memory only makes sense in my opinion, if you establish a second hierarchy of streams with binary output like Dietmar Kuehl's "XDRStream", now part of the "Berlin" project. In short, why don't you go for "stringstream" / "stringbuf"? Cheers, Philipp. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Dill Guest Posted: Mon Feb 23, 2004 7:27 pm    Post subject: Re: streambuf for reading from existing memory block Quote: In order to implement a streambuf for reading/writing from/to an existing block of memory, AFACIT, it's as simple as (ignoring templates for now): struct membuf : std::streambuf { membuf(char* p, size_t sz) { setg(p, p, p + sz); setp(p, p + sz); } }; Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? One thing you're missing is seek abilities. The default streambuf implementation for seekpos and seekoff is to return an error. I would expect a standard memory streambuf to implement these. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top P.J. Plauger Guest Posted: Mon Feb 23, 2004 7:33 pm    Post subject: Re: streambuf for reading from existing memory block "Dylan Nicholson" <wizofaus (AT) hotmail (DOT) com> wrote Quote: Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? Triviality is not generally a good criterion for including something in the C++ Standard. P.J. Plauger Dinkumware, Ltd. http://www.dinkumware.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tom_usenet Guest Posted: Mon Feb 23, 2004 7:43 pm    Post subject: Re: streambuf for reading from existing memory block On 23 Feb 2004 06:40:03 -0500, [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote: Quote: {Might be better asked in comp.std.c++. -mod} In order to implement a streambuf for reading/writing from/to an existing block of memory, AFACIT, it's as simple as (ignoring templates for now): struct membuf : std::streambuf { membuf(char* p, size_t sz) { setg(p, p, p + sz); setp(p, p + sz); } }; Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? Something with equivalent functionality is supplied as part of the standard: std::strstreambuf. It even switches between read-write and read-only based on whether the passed pointer pointed to const or not. It shouldn't be deprecated IMHO any more than string.c_str() should - it has legitimate uses when, e.g., interacting with C api that use char*. Tom C++ FAQ: http://www.parashift.com/c++-faq-lite/ C FAQ: http://www.eskimo.com/~scs/C-faq/top.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jonathan Turkanis Guest Posted: Tue Feb 24, 2004 3:24 am    Post subject: Re: streambuf for reading from existing memory block "Dylan Nicholson" <wizofaus (AT) hotmail (DOT) com> wrote Quote: {Might be better asked in comp.std.c++. -mod} In order to implement a streambuf for reading/writing from/to an existing block of memory, AFACIT, it's as simple as (ignoring templates for now): struct membuf : std::streambuf { membuf(char* p, size_t sz) { setg(p, p, p + sz); setp(p, p + sz); } }; There is a bit more to it than that, if you want a full-featured stream buffer. Daryle Walker has written a basic_array_streambuf template (submitted to Boost) which does what you want. (See http://groups.yahoo.com/group/boost/files/more_io_3b.zip; you have to sign up for the Boost developers list to access this) His documentation states that the template is 'fairly uninteresting, meant mainly as a test case ', so I doubt you'll find much support for standardization. I've written a library (also submitted to Boost) which provides a toolkit for quickly defining new streams and stream buffers (plus adding chains of filters). It also makes it easy to define such a stream buffer, although by default it would provide buffered access to the array. (See http://www.xmission.com/~turkanis/iostreams/) Jonathan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dylan Nicholson Guest Posted: Tue Feb 24, 2004 3:32 am    Post subject: Re: streambuf for reading from existing memory block tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote Quote: On 23 Feb 2004 06:40:03 -0500, [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote: Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? Something with equivalent functionality is supplied as part of the standard: std::strstreambuf. It even switches between read-write and read-only based on whether the passed pointer pointed to const or not. Except that strstreambuf can't be used with std::iostreams! The problem was, I had some code that only worked with a std::istream<char> object, and I indeed had a std::istream<char> buffer for it to read from - unfortunately I needed to limit the number of bytes it was allowed to read from (i.e. a substream). The easiest way to accomplish this was (using my membuf class): void pass_substream_to_foo(std::istream& stream, int size) { std::vector<char> buf(size); stream.read(&buf[0], size); std::membuf mbuf(&buf[0], is.gcount()); std::istream substream(&mbuf); foo(substream); } Of course I'm sure there are other ways of solving this exact problem, but it works well enough, and seems safe. Dylan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tom_usenet Guest Posted: Tue Feb 24, 2004 1:45 pm    Post subject: Re: streambuf for reading from existing memory block On 23 Feb 2004 22:32:21 -0500, [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote: Quote: Something with equivalent functionality is supplied as part of the standard: std::strstreambuf. It even switches between read-write and read-only based on whether the passed pointer pointed to const or not. Except that strstreambuf can't be used with std::iostreams! Huh? std::strstreambuf derives from std::basic_streambuf<char>, so it certainly can. Quote: void pass_substream_to_foo(std::istream& stream, int size) { std::vector<char> buf(size); stream.read(&buf[0], size); std::strstreambuf mbuf(&buf[0], is.gcount()); Quote: std::istream substream(&mbuf); foo(substream); } Of course I'm sure there are other ways of solving this exact problem, but it works well enough, and seems safe. The above change is all that is required I think. Take a look at D.7 (page 703) in the standard. It even works with signed and unsigned char buffers. Tom C++ FAQ: http://www.parashift.com/c++-faq-lite/ C FAQ: http://www.eskimo.com/~scs/C-faq/top.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Tue Feb 24, 2004 7:27 pm    Post subject: Re: streambuf for reading from existing memory block [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote in message news:<7d428a77.0402231612.4c0126ea (AT) posting (DOT) google.com>... Quote: tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote in message news:<onak30ppm13edjfqd5r9f1j00e2393qeg1 (AT) 4ax (DOT) com>... On 23 Feb 2004 06:40:03 -0500, [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote: Surely it would make sense to supply something as trivial as this as part of the standard? Or is there a reason I can't rely on the above working? Something with equivalent functionality is supplied as part of the standard: std::strstreambuf. It even switches between read-write and read-only based on whether the passed pointer pointed to const or not. Except that strstreambuf can't be used with std::iostreams! But std::strstreambuf can. Quote: The problem was, I had some code that only worked with a std::istream<char> object, and I indeed had a std::istream<char buffer for it to read from - unfortunately I needed to limit the number of bytes it was allowed to read from (i.e. a substream). The easiest way to accomplish this was (using my membuf class): void pass_substream_to_foo(std::istream& stream, int size) { std::vector stream.read(&buf[0], size); std::membuf mbuf(&buf[0], is.gcount()); std::strstreambuf mbuf( &buf[ 0 ], is.gcount() ) ; should work. And is fully standard. Putting your own class (membur) in std:: is forbidden. Normally, you can get away with it, if there is a good reason to do so, but what is the reason here? Quote: std::istream substream(&mbuf); foo(substream); } Of course I'm sure there are other ways of solving this exact problem, but it works well enough, and seems safe. In this particular case, wouldn't a simple filtering streambuf be more appropriate. No need to copy all of the data. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dylan Nicholson Guest Posted: Wed Feb 25, 2004 3:06 pm    Post subject: Re: streambuf for reading from existing memory block tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote Quote: On 23 Feb 2004 22:32:21 -0500, [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote: Something with equivalent functionality is supplied as part of the standard: std::strstreambuf. It even switches between read-write and read-only based on whether the passed pointer pointed to const or not. Except that strstreambuf can't be used with std::iostreams! Huh? std::strstreambuf derives from std::basic_streambuf<char>, so it certainly can. Hmm, well not under VC6 it doesn't - it was part of the pre-standard iostreams library. I suspect this might be true of other compilers.. Quote: The above change is all that is required I think. Take a look at D.7 (page 703) in the standard. It even works with signed and unsigned char buffers. Well you've answered my question anyway - the standard library does supply what I needed, but unfortunately not for all the compilers I need to support. Dylan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dylan Nicholson Guest Posted: Thu Feb 26, 2004 12:21 am    Post subject: Re: streambuf for reading from existing memory block tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote Quote: On 23 Feb 2004 22:32:21 -0500, [email]wizofaus (AT) hotmail (DOT) com[/email] (Dylan Nicholson) wrote: Something with equivalent functionality is supplied as part of the standard: std::strstreambuf. It even switches between read-write and read-only based on whether the passed pointer pointed to const or not. Except that strstreambuf can't be used with std::iostreams! Huh? std::strstreambuf derives from std::basic_streambuf<char>, so it certainly can. Ok, ignore this thread completely! For some reason I got the impression that some of the compilers I were using only supported "non-std" strstreambuf, but it turned out not to be the case. std::strstreambuf<> is exactly what I was looking for. Dylan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.