c++ - How to check why file is not opened? -

std::ifstream fin; fin.open("file.txt", std::ios::in); std::cout << fin.is_open(); 

this code prints false file not opened. how check why it. maybe error message fin object? guessing reason file opened writing. want open reading. possible open file both reading , writing.?

i using linux. strerror(errno) error no such file or directory file exists. maybe error because opened using other object? opened using c api, therefore fin cant open it. how can open file opened?

the standard specifies there preprocessor symbol errno, expands references (thread local) int, in system functions expected put extended error code. defines function strerror, allow recovering char const* text message, given number (but unlike errno, function not guaranteed thread safe).

the standard not place requirements on filebuf use errno, @ least not in case of open, in practice, filebuf invoke lower level functions (hopefully) use errno. (this required posix, of system calls set errno in case of error. i'm less sure windows.)

as bi-directional access: it's bad idea, but... filebuf potentially bidirectional, ifstream won't define << operators, , ofstream won't define >> operators, might want use std::fstream. if want able seek, you'll want open file in binary mode; otherwise, can seek places you've been before, , memorized position. if want read you've written, better solution close output, open reading.