C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  how do I properly use a filebuf          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Jacob Oursland Guest Posted: Thu Apr 14, 2005 6:59 am    Post subject: how do I properly use a filebuf Hi, I am working on playing with an old game's data files (Descent II) and I want to build a framework for a generic file type so that I request a file and it opens one in the local directory, if not found open the game data archive and find the file and return the stream to that one. The problem I am having is that I want to build a stream class for this archive file format. Its real simple, file name, size, data, repeatedly until EOF. My plan of attack is to create a custom filebuf and seek to file start position, when data is requested from the buffer it is fed and underflow() is used to keep the buffer full and check that it hasn't exceeded the archived file's EOF. The precise problem I have is how to actually access the file. I have the buffer part pretty much handled (not much, since it just passes the data) but I haven't a clue how to open, read, and write (eventually) the file. The filebuf::open() opens and filebuf::close() closes nicely, but how do I then implement underflow() to read() or overflow() to write? Thanks -Jake [ 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: Sat Apr 16, 2005 12:03 am    Post subject: Re: how do I properly use a filebuf Jacob Oursland wrote: Quote: I am working on playing with an old game's data files (Descent II) and I want to build a framework for a generic file type so that I request a file and it opens one in the local directory, if not found open the game data archive and find the file and return the stream to that one. The problem I am having is that I want to build a stream class for this archive file format. Its real simple, file name, size, data, repeatedly until EOF. My plan of attack is to create a custom filebuf and seek to file start position, when data is requested from the buffer it is fed and underflow() is used to keep the buffer full and check that it hasn't exceeded the archived file's EOF. The precise problem I have is how to actually access the file. I have the buffer part pretty much handled (not much, since it just passes the data) but I haven't a clue how to open, read, and write (eventually) the file. The filebuf::open() opens and filebuf::close() closes nicely, but how do I then implement underflow() to read() or overflow() to write? I'm not sure I understand. What does your streambuf do that filebuf doesn't? From the description, the only thing particular about your application is the format of the files; streambuf's aren't normally concerned with format. (I suppose that if your file uses a binary format, a filtering streambuf could be used to convert it to text, but even then, I'm not sure if this is the right approach.) -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jacob Oursland Guest Posted: Sat Apr 16, 2005 5:46 pm    Post subject: Re: how do I properly use a filebuf [email]kanze (AT) gabi-soft (DOT) fr[/email] wrote: Quote: I'm not sure I understand. What does your streambuf do that filebuf doesn't? From the description, the only thing particular about your application is the format of the files; streambuf's aren't normally concerned with format. (I suppose that if your file uses a binary format, a filtering streambuf could be used to convert it to text, but even then, I'm not sure if this is the right approach.) -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 Sorry for being a bit unclear. My issue is I want to write a custom filebuf underflow() and overflow() and I don't know how to retrieve data from the file opened by the filebuf's open(). How do I read() and write() to the filebuf's file? Thanks -Jake [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Sun Apr 17, 2005 6:07 pm    Post subject: Re: how do I properly use a filebuf Jacob Oursland <oursland.nospam (AT) deleteme (DOT) fastmail.fm> wrote: Quote: Hi, I am working on playing with an old game's data files (Descent II) and I want to build a framework for a generic file type so that I request a file and it opens one in the local directory, if not found open the game data archive and find the file and return the stream to that one. The problem I am having is that I want to build a stream class for this archive file format. Its real simple, file name, size, data, repeatedly until EOF. My plan of attack is to create a custom filebuf and seek to file start position, when data is requested from the buffer it is fed and underflow() is used to keep the buffer full and check that it hasn't exceeded the archived file's EOF. The precise problem I have is how to actually access the file. I have the buffer part pretty much handled (not much, since it just passes the data) but I haven't a clue how to open, read, and write (eventually) the file. The filebuf::open() opens and filebuf::close() closes nicely, but how do I then implement underflow() to read() or overflow() to write? Thanks -Jake Do you need a custom streambuf I don' think so your archived file is an array of easy to handle blocks if you can hold the each file in the archive in memory then I would use a struct/class to represent the file using std::string,std::vector<char> for the name and the data, then writing it becomse just using filebuf and its member sputn to write the file in one call struct archived_block { std::string name; std::size_t len; std::vecctor<unsigned char> data; bool get(std::istream &in);// fill the struct from in }; bool decode_file(std::istream &in) { archived_block block; while(block.get(in)) { std::filebuf buf(in.name.c_str(),std::ios_base::binary|std::ios_base::in); if(!buf.is_open()) { // report error to cerr return false; } std::streamsize bytes_wrote =buf.sputn(&block.data[0],block.len); if(bytes_wrote != block.len) { // report error to cerr return false; } } return in.eof(); } I leave archived_block::get to the reader as it depends on details missing. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Sun Apr 17, 2005 9:04 pm    Post subject: Re: how do I properly use a filebuf Jacob Oursland wrote: Quote: kanze (AT) gabi-soft (DOT) fr wrote: I'm not sure I understand. What does your streambuf do that filebuf doesn't? From the description, the only thing particular about your application is the format of the files; streambuf's aren't normally concerned with format. (I suppose that if your file uses a binary format, a filtering streambuf could be used to convert it to text, but even then, I'm not sure if this is the right approach.) Sorry for being a bit unclear. My issue is I want to write a custom filebuf underflow() and overflow() and I don't know how to retrieve data from the file opened by the filebuf's open(). How do I read() and write() to the filebuf's file? The question remains : what are you trying to do? Filebuf is a concrete class, and you shouldn't normally derive from it. If you want a custom streambuf, you derive from streambuf, not from filebuf. If the goal is to massage the data read by the filebuf, the obvious solution is a filtering streambuf, a streambuf which uses a filebuf as its data source. But from what little you have said, I'm far from sure that this is the case. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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