c++ - std::ifstream closed for some reason? -


i'm trying open ifstreams contain file data, can read later on. i'm trying pass vector of ifstreams constructor, reason when loop through vector, references closed. but, other variables (data stream , meta stream) remain open.

what missing here? i'm coming heavy java background, i'm still learning c++

filestore.h

// // created tom on 8/16/2017. //  #ifndef cfs_filestore_h #define cfs_filestore_h  #include <fstream> #include <list> #include "bytebuffer.h"  class filestore {  private:     std::ifstream *data_stream;     std::vector<std::ifstream *> index_streams;     std::ifstream *meta_stream;  public:     filestore(std::ifstream *data, std::vector<std::ifstream *> indexes, std::ifstream *meta);     ~filestore() = default;      int get_type_count();     bytebuffer read(int type, int id);     int get_file_count(int type);      static filestore open(std::string &root); };   #endif //cfs_filestore_h

filestore.cpp

// // created tom on 8/16/2017. //  #include "filestore.h"  filestore::filestore(std::ifstream *data, std::vector<std::ifstream *> indexes, std::ifstream *meta)         : data_stream(data), index_streams(std::move(indexes)), meta_stream(meta) {      std::cout << std::boolalpha << "data open : " << data_stream->is_open() << std::endl;     std::cout  << "meta open : " << meta_stream->is_open() << std::endl;      (auto v : index_streams) {         std::cout << "index open : " << v->is_open() << std::endl;     } }  int filestore::get_type_count() {     return 0; }  bytebuffer filestore::read(int type, int id) {     return bytebuffer(); }  int filestore::get_file_count(int type) {     return 0; }  filestore filestore::open(std::string &root) {     std::ifstream data(root + "main_file_cache.dat2");     if (!data.good())         throw std::runtime_error("data file not exist.");      std::vector<std::ifstream *> indexes;     (int = 0; < 254; i++) {         std::ifstream index(root + "main_file_cache.idx" + std::to_string(i));         if (!index.good())             break;          indexes.push_back(&index);     }      std::ifstream meta(root + "main_file_cache.idx255");     if (!meta.good())         throw std::runtime_error("meta file not exist.");      return filestore(&data, indexes, &meta); }

output

data open : true meta open : true index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false index open : false 

    std::ifstream index(root + "main_file_cache.idx" + std::to_string(i));

this constructs std::ifstream object. gets declared automatic scope.

    indexes.push_back(&index); }

this pushes pointer std::ifstream indexes vector, because index's automatic scope ends afterwards, index object gets destroyed, corresponding file being closed. declared std::ifstream object in automatic scope, inside inner loop. such, object gets destroyed @ end of loop.

the subsequent code attempts dereference these pointers, stored in indexes vector, dangling pointers. results in undefined behavior.

additionally, indexes vector gets passed value, results in vector getting copied, adds confusion.

you need re-read following chapters in c++ book:

  • the chapter explains how automatic scope, , how dynamic scope works in c++.

  • the chapter explains difference between passing function arguments reference , passing them value, , means.


Comments

Post a Comment

Popular posts from this blog

Is there a better way to structure post methods in Class Based Views -

i'm using class based views in django , never specify url in ajax calls , omit action parameter in form because know these requests go through post method of class based view. notice view - profile page make many requests depending on user updating on page - post method become convoluted , if/else mess example: def post(self, request, *args, **kwargs): # handle user changing available date if request.post.get('availabledate') != none: self.updateavailabledate(request) return jsonresponse({'result':'success'}) if request.post.get('newprojectname') != none: creative_user = creativeuserprofile.objects.get(id = request.user.id) project = project.create_project(creative_user, request.post['newprojectname']) project.save() return jsonresponse({'projectid': project.id }) if request.post.get('projectimage') != none , request.files['file']: proj
Read more

performance - Why is XCHG reg, reg a 3 micro-op instruction on modern Intel architectures? -

i'm doing micro-optimization on performance critical part of code , came across sequence of instructions (in at&t syntax): add %rax, %rbx mov %rdx, %rax mov %rbx, %rdx i thought had use case xchg allow me shave instruction , write: add %rbx, %rax xchg %rax, %rdx however, dimay found agner fog's instruction tables , xchg 3 micro-op instruction 2 cycle latency on sandy bridge, ivy bridge, broadwell, haswell , skylake. 3 whole micro-ops , 2 cycles of latency! 3 micro-ops throws off 4-1-1-1 cadence , 2 cycle latency makes worse original in best case since last 2 instructions in original might execute in parallel. now... cpu might breaking instruction micro-ops equivalent to: mov %rax, %tmp mov %rdx, %rax mov %tmp, %rdx where tmp anonymous internal register , suppose last 2 micro-ops run in parallel latency 2 cycles. given register renaming occurs on these micro-architectures, though, doesn't make sense me done way. why wouldn't register renamer
Read more

jquery - Responsive Navbar with Sub Navbar -

good day, forgive me nav vocabulary not best. if question confusing apologize. i'm working on navigation bar needs centered on page @ 768px , above. items in nav open (or perhaps toggle) subnav lives directly below. these nav should appear centered on page. i have been able work through responsive portion of nav bar, having main items align vertically , sub nav menu of each item roll out below, , gave them indent separation. working well. however, issue not responsive side, behavior @ width greater 768px. happens toggling menu's sub navigation pushes other main items down page below sub nav. here's code: $(document).ready(function () { //stick in fixed 100% height behind navbar don't wrap $('#slide-nav.navbar-inverse').after($('<div class="inverse" id="navbar-height-col"></div>')); $('#slide-nav.navbar-default').after($('<div id="navbar-height-col"></d
Read more