关于使用ifstream确定C ++中是否存在文件的问题

Question

I found a c++ code which uses ifstream to decide whether a file exists. That section is,

if (!ifstream(trajectory_file_name_)) {
    cerr << "ERROR: The trajectory file:";
    cerr << "\033[1;34m";
    cerr << trajectory_file_name_;
    cerr << "\033[0m";
    cerr << " does not exist.";
    cerr << endl;
    exit(1);
}

what I have learned from the textbook (like "c++ primer plus") is something like, when judging whether a file exist via ifstream,

ifstream inFile(filename-);
if(!inFile.is_open())
{
  return;
}

I have searched many website but didn't find any information on the first one. So I wonder if anyone can give me some explanation on the usage of ifstream of the first one. Thanks!

Answer 1

This comes down to how streams convert to bool (or in this case it's due to operator! , but same principle).

It's designed so that, for short, you can check for openness ( and error flags ) like:

std::ifstream ifs("path");
if (!ifs)
{
   // ...
}

(Failure to open sets the failbit ).

Your version just skips the declaration and does the same thing with a temporary.

Note that there are other reasons a file may not be openable, eg permissions. This is not a reliable way to check whether it exists and, even if it does, you have no way of guaranteeing that it will still exist in a moment when you try to do something with it.