Stream Checking and is_open()

The C++ file stream classes inherit a stream-state member from the ios_base class. This member, as discussed earlier, stores information that reflects the stream status: All is well, end-of-file has been reached, I/O operation failed, and so on. If all is well, the stream state is zero (no news is good news). The various other states are recorded by setting particular bits to 1. The file stream classes also inherit the ios_base methods that report about the stream state and that are summarized in Table 17.4. You can check the stream state to find whether the most recent stream operation succeeded or failed. For file streams, this includes checking the success of an attempt to open a file. For example, attempting to open a non-existent file for input sets failbit. So you could check this way:

fin.open(argv[file]);
if (fin.fail())  // open attempt failed
{
   ...
}

Or because an ifstream object, like an istream object, is converted to a bool type where a bool type is expected, you could use this:

fin.open(argv[file]);
if (!fin)  // open attempt failed
{
   ...
}

However, newer C++ implementations have a better way to check whether a file has been opened—the is_open() method:

if (!fin.is_open())  // open attempt failed
{
   ...
}

The reason this is better is that it tests for some subtle problems that the other forms miss, as discussed in the following Caution.