Checking if a file opened successfully with ifstream

Question

I have the following that will open a file for reading. However, I want to check to make sure that the file was open successfully, so I am using the fail to see if the flags have been set. However, I keep getting the following error:

I am new to C++, as I am coming from C. So not sure I understand this error:

cannot call member function ‘bool std::basic_ios<_CharT, _Traits>::fail() const [with _CharT = char, _Traits = std::char_traits]’ without object

Code:

int devices::open_file(std::string _file_name) {     ifstream input_stream;      input_stream.open(_file_name.c_str(), ios::in);      if(ios::fail() == true) {         return -1;     }      file_name = _file_name;      return 0; } 

Answer 1

You can simply do this:

int devices::open_file(std::string _file_name) {     ifstream input_stream;         input_stream.open(_file_name.c_str(), ios::in);     if(!input_stream)     {         return -1;     }      file_name = _file_name;     return 0; } 

fail() is not a static method, you must call it on an instance not a type, so if you want to use fail(), replace !input_stream with input_stream.fail() in my code above.

I do have to wonder what you're trying to achieve here. You're opening the file and immediately close it again. Are you simply trying to check if the file exists?