c++ - Why only last thread executing? -

i need create console application counts files in folders. each folder executes parallel. directories paths .txt file , put them threads.

i'm using std::thread , boost::filesystem.

it works fine 1 directory, crashes or returns wrong results many. interesting last thread gets correct result, before wrong.

here code:

dirhandler.h

#include <iostream> #include <fstream> #include <string> #include <thread> #include <windows.h> #include <boost/filesystem.hpp> using namespace std; using namespace boost::filesystem;  class dirhandler{  public: dirhandler();  void getpaths(ifstream file); void output(); static void run_thread(pair<string, int> * dir, string cur_path, void *args) {     dirhandler *prunnable = static_cast<dirhandler*>(args);     prunnable->some_counting(dir, cur_path); }  private:  vector<thread> threads; vector<pair<string, int>> paths; // current directory name , files amount  void some_counting(pair<string, int> * dir, string cur_path); void escape(); }; 

dirhandler.cpp

void dirhandler::getpaths(ifstream file) { // parse pathes , create separate thread each string line; if (file.is_open()) {     while (getline(file, line))     {         cout << line << endl;         // add thread path         pair<string, int> dir = make_pair(line, 0);         paths.push_back(dir);         threads.push_back(thread(&dirhandler::run_thread, &paths.back(), line, this));     }     (auto& thr : threads){         thr.join();     }      file.close(); } }  void dirhandler::some_counting(pair<string, int> * dir, string cur_path){...} 

main.cpp

#include <iostream> #include <windows.h> #include "dirhandler.h"  int main(int argc, char* argv[]) {      dirhandler dirhandler = dirhandler();     dirhandler.getpaths(ifstream("strings.txt")); //ifstream(argv[1])      dirhandler.output();        return 0; } 

note : while debugging i've found streams before last have id=0 in end

also i've read problem can caused reference single object. (in case it's vector<pair<string, int>> paths)

so question how few threads work correctly ?

the problem pointers paths vector, , if vector resized previous pointers (and iterators) elements in vector become invalid. using invalid pointer leads undefined behavior leads crashes.

and 1 common way of causing vector adding new elements it, in loop.

one solution either pass reference vector , index of element thread (the index not change on reallocation). solution have 2 loops, 1 add elements vector , second create threads.

there other solutions too, not passing pointer thread, instead pass pair by value. solution recommend.

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after checking code little, see since use std::thread don't need static member function wrapper @ all. instead can call non-static actual thread function directly:

threads.emplace_back(&dirhandler::some_counting, this, paths.back(), line); 

_{[note use of this second argument, change emplace_back threads vector]}