Checking if a file opened successfully with ifstre

问题:

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;
}

回答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?

回答2:

Your error is because you are using ios::fail() as a static method when it is actually a member method.

if (input_stream.fail())
{
    ...
}

回答3:

you can also use std::ifstream::is_open. Returns true if a file is open and associated with this stream object.

// ifstream::is_open
#include <iostream>     // std::cout
#include <fstream>      // std::ifstream

int main () {
  std::ifstream ifs ("test.txt");

  if (ifs.is_open()) {
    // print file:
    char c = ifs.get();
    while (ifs.good()) {
      std::cout << c;
      c = ifs.get();
    }
  }
  else {
    // show message:
    std::cout << "Error opening file";
  }

  return 0;
}

http://www.cplusplus.com/reference/fstream/ifstream/is_open/

回答4:

You have to call fail() on the stream object. A more idiomatic way of doing this is:

input_stream.open(_file_name.c_str(), ios::in);

if( ! input_stream ) {
    return -1;
}