c++ - std::ifstream setting fail() even when no error -

using gcc 4.7.3 on cygwin 1.7.24. compiler options include: -std=gnu++11 -wall -wextra

i working on command line application , needed able load , save set of strings wrote quick wrapper class around std::set add load , save methods.

// keyset.h  #ifndef keyset_h #define keyset_h  #include <cstdlib> #include <sys/stat.h> #include <cerrno> #include <cstring>  #include <string> #include <set> #include <iostream> #include <fstream>  inline bool file_exists (const std::string& filename) { /*     utility routine check existance of file.  returns true or false,     prints error , exits status 2 on error. */     struct  stat buffer;     int     error = stat(filename.c_str(), &buffer);     if (error == 0) return true;     if (errno == enoent) return false;     std::cerr << "error while checking '" << filename << "': " << strerror(errno) << std::endl;     exit (2); }  class keyset { private:     std::string             filename;     std::set<std::string>   keys;  public:     keyset() {}     keyset(const std::string pfilename) : filename(pfilename) {}      void set_filename (const std::string pfilename) {filename = pfilename;}     std::string get_filename () {return filename;}     auto size () -> decltype(keys.size()) {return keys.size();}     auto cbegin() -> decltype(keys.cbegin()) {return keys.cbegin();}     auto cend() -> decltype(keys.cend()) {return keys.cend();}     auto insert(const std::string key) -> decltype(keys.insert(key)) {return keys.insert(key);}     void load ();     void save (); };  void keyset::load () {     if (file_exists(filename)) {         errno = 0;         std::ifstream   in (filename, std::ios_base::in);          if (in.fail()) {             std::cerr << "error opening '" << filename << "' reading: " << strerror(errno) << std::endl;             exit (2);         }          std::string     token;         if (token.capacity() < 32) token.reserve(32);          while (in >> token) keys.insert(token);          if (!in.eof()) {             std::cerr << "error reading '" << filename << "': " << strerror(errno) << std::endl;             exit (2);         }          in.clear(); // need clear flags before calling close         in.close();         if (in.fail()) {             std::cerr << "error closing '" << filename << "': " << strerror(errno) << std::endl;              exit (2);         }     } }  void keyset::save () {     errno = 0;     std::ofstream   out (filename, std::ios_base::out);      if (out.fail()) {         std::cerr << "error opening '" << filename << "' writing: " << strerror(errno) << std::endl;         exit (2);     }      (auto key = keys.cbegin(), end = keys.cend(); key != end; ++key) {         out << *key << std::endl;     }      out.close();     if (out.fail()) {         std::cerr << "error writing '" << filename << "': " << strerror(errno) << std::endl;         exit (2);     } }  #endif  // 

here's quick program test load method.

// ks_test.cpp  #include "keyset.h"  int main() {     keyset          test;     std::string     filename = "foo.keys.txt";      test.set_filename(filename);      test.load();      (auto key = test.cbegin(), end = test.cend(); key != end; ++key) {         std::cout << *key << std::endl;     } } 

the data file has "one 2 three" in it.

when go run test program, following error test program:

$ ./ks_test error closing 'foo.keys.txt': no error 

both cppreference.com , cplusplus.com close method should set fail bit on error. save method works fine, , load method works correctly if comment out error check after close. should work or have misunderstood how close supposed work? in advance.

edited clarify, fix typo's , adjust code per joachim pileborg's , konrad rudolph's comments.

edited add solution code.

you have 2 errors here: first how reading, more loop reading. eof flag not set until after tried read , read failed. instead should this:

while (in >> token) { ... } 

otherwise loop 1 time many , try read beyond end of file.

the second problem 1 notice, , depends on the first problem. since try read beyond end of file, stream set failbit causing in.fail() return true though there no real error.