Difference between casting ifstream to bool and using ifstream::is_open()

问题

Maybe a dummy question, but I need a clear answer to it. Is there any difference at all in the return of any of those functions

int FileExists(const std::string& filename)
{
  ifstream file(filename.c_str());
  return !!file;
}

int FileExists(const std::string& filename)
{
  ifstream file(filename.c_str());
  return file.is_open();
}

So in other words, my question is: does casting the fstream to bool give exactly the same result as fstream::is_open()?

回答1:

No. is_open checks only whether there is an associated file, while a cast to bool also checks whether the file is ready for I/O operations (e.g. the stream is in a good state) (since C++11).

is_open

Checks if the file stream has an associated file.

std::basic_ios::operator bool

Returns true if the stream has no errors occurred and is ready of I/O operations. Specifically, returns !fail().

来源：https://stackoverflow.com/questions/14920457/difference-between-casting-ifstream-to-bool-and-using-ifstreamis-open