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.NET Forum / Languages / Managed C++ / August 2006need some help with list::end ()
Hi all, I'm using stl's list to store some file names. Its declared as: list < char * > filenames; i enumerate the list by using: list < char * >::const_iterator filename; filename = filenames.begin(); than I simply do a filename++ to go to the next item..... I check for filenames.end() in an *if* condition somewhere in my code to check if I'm dealing with the last item of the list. The code to do this was working fine. But now due to some change in my code, some of the list items are deleted *before* the filenames.end() check. I have noticed that this check now never evaluates to true although I know in the debugger that I'm standing at the last item. I'm sure I'm missing some basic concept here. How can I check now that this is the last item of the list? I'm using vc++ 2k5 total unmanaged. Regards, -ab. > Hi all, > [quoted text clipped - 22 lines] > here. How > can I check now that this is the last item of the list? How are you deleting the items? Just doing filenames.erase(filename) invalidates the iterator to the deleted object. You should be doing something like if (some_condition) filename = filenames.erase(filename); else ++filename; This will *either* use the returned iterator to the next object, or increment the current iterator. You should do one, but not both! :-) Bo Persson Sorry for the late reply. > How are you deleting the items? Just doing filenames.erase(filename) > invalidates the iterator to the deleted object. I use the "remove" method. I have the following declaration: list<char * >::const_iterator name, prev; at some point I do: prev = name; name ++; filenames.remove ( *prev ); and this is how I remove the items. regards, -ab. > > Hi all, > > [quoted text clipped - 37 lines] > > Bo Persson > Sorry for the late reply. > [quoted text clipped - 14 lines] > > and this is how I remove the items. If you have duplicates in your list, the call to remove will remove more than one of them at a time, including possibly the one now referenced by "name". Use the loop that Bo suggested: if (some_condition) filename = filenames.erase(filename); else ++filename; that's the canonical way to iterate through a list removing some elements as you go. -cd >> Sorry for the late reply. >> [quoted text clipped - 26 lines] > that's the canonical way to iterate through a list removing some > elements as you go. Also, how are you adding the items to the list? Since you're storing raw char*'s, the list isn't managing the memory occupied by the strings - you are. You should consider changing to a std::list<std::string> instead. -cd > char*'s, the list isn't managing the memory occupied by the strings - you > are. list <char * > filenames ; WIN32_FIND_DATAA finddata; //emailblockslocation HANDLE filehandle = FindFirstFileA ( m_folderlocation , & finddata ); char * filename = NULL; do { if ( (finddata.dwFileAttributes & FILE_ATTRIBUTE_ARCHIVE )== FILE_ATTRIBUTE_ARCHIVE ) { filename = new char [500]; strcpy ( filename , finddata.cFileName ); filenames.push_back ( filename ); } } while ( FindNextFileA( filehandle, & finddata ) != 0 ); FindClose ( filehandle ); > >> Sorry for the late reply. > >> [quoted text clipped - 32 lines] > > -cd >> char*'s, the list isn't managing the memory occupied by the strings >> - you are. [quoted text clipped - 10 lines] > { > filename = new char [500]; This memory is being leaked when you remove an item from the list - nothing will call delete[] on it (unless there's yet more code that you haven't shown that does delete it). > strcpy ( filename , finddata.cFileName ); > filenames.push_back ( filename ); [quoted text clipped - 3 lines] > } while ( FindNextFileA( filehandle, & finddata ) != 0 ); > FindClose ( filehandle ); I'd recommend changing your container to std::list<std::string>, then the memory for the string will be automatically freed when an item is removed from the list. -cd > I'd recommend changing your container to std::list<std::string>, then the > memory for the string will be automatically freed when an item is removed yes I was going to write the code for the deletion of char *, but I got stuck in that filenames.end() condition which btw is still not working. This std::string idea is good and will reduce my code. -Ab. > >> char*'s, the list isn't managing the memory occupied by the strings > >> - you are. [quoted text clipped - 28 lines] > > -cd > yes I was going to write the code for the deletion of char *, but I got > stuck in that filenames.end() condition which btw is still not working. Have you tried what Bo and Carl recommended? Here's a complete example: for(std::list<std::string>::iterator filename = filenames.begin(); filename != filenames.end(); ) { if(FileNeedsToBeDeletedCondition) filename = filenames.erase(filename); else ++filename; } If have a radically different mechanism, it's probably not going to work. I strongly recommend the book Effective STL by Scott Meyers, which discusses this and dozens of other tricks with STL containers. Tom Hi, I have solved the problem by using the list::back() method. As back() represents the last item in the list, so thats what I was looking for. The problem with what I was doing was my lack of understanding with the list's "end" method. I thought that the "end" represents the last item, which is not correct. Thanks for the book recommendation! Regards, -Ab. PS: again, sorry for having replied so late,,,, I hate my replying-late habbit !!! > > yes I was going to write the code for the deletion of char *, but I got > > stuck in that filenames.end() condition which btw is still not working. [quoted text clipped - 16 lines] > > Tom Hi, Ok I'll try the *remove* method of the list and if I still cant get the begin() method to work right I'll get back to this post. Regards, -ab. > > Sorry for the late reply. > > [quoted text clipped - 28 lines] > > -cd Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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