问题
If I want to safely try to open a file in D, is the preferred way to either
- try to open it, catch exception (and optionally figure out why) if it fails or
- check if it exists, is readable and only then open it
I'm guessing the second alternative results in more IO and is more complex right?
回答1:
Generally, it's better to check whether the file exists first, because it's often very likely that the file doesn't exist, and simply letting it fail when you try and open it is a case of using exceptions for flow control. It's also inefficient in the case where the file doesn't exist, because exceptions are quite expensive in D (though the cost of the I/O may still outweigh the cost of the exception given how expensive I/O is).
It's generally considered bad practice to use exceptions in cases where the exception is likely to be thrown. In those cases, it's far better to return whether the operation succeeded or to check whether the operation is likely to succeed prior to attempting the operation. In the case of opening files, you'd likely do the latter. So, the cleanest way to do what you're trying to do would be to do something like
if(filename.exists)
{
auto file = File(filename);
...
}
or if you want to read the whole file in as a string in one go, you'd do
if(filename.exists)
{
auto fileContents = readText(filename);
...
}
exists and readText are in std.file, and File is in std.stdio.
If you're dealing with a case where it's highly likely that the file will exist and that therefore it's very unlikely that an exception will be thrown, then skipping the check and just trying to open the file is fine. But what you want to avoid is relying on the exception being thrown when it's not unlikely that the operation will fail. You want exceptions to be thrown rarely, so you check that operations will succeed before attempting them if it's likely that they will fail and throw an exception. Otherwise, you end up using exceptions for flow control and harm the efficiency (and maintainability) of your program.
And it's often the case that a file won't be there when you try and open it, so it's usually the case that you should check that a file exists before trying to open it (but it does ultimately depend on your particular use case).
回答2:
If the file is expected to be there according to normal program operation and the given user input, then use 1 - just try to open the file and rely on exception handling to handle the exceptional situation that the file is not there.
For example:
/// If the user has a local configuration file in his home directory, open that.
/// Otherwise, open the global configuration file that is a part of the program,
/// and should be installed on all systems where the program is running.
File configFile;
if ("~/.transmogrifier.conf".expandTilde.exists)
configFile.open("~/.transmogrifier.conf".expandTilde);
else
configFile.open("/etc/transmogrifier.conf");
Note that using 2 might lead to security issues in your program. For example, if the file is present at the moment when your program checks if the file exists, but is gone when it tries to open it, your program may behave in an unexpected way. If you use 2, make sure that your program still behaves in a desirable way if opening the file fails even though your program just checked that the file exists and is readable.
回答3:
I'd say you need to be prepared for an exception to be thrown anyway, otherwise you have a race condition (another process may delete the file between the test and the open etc). So it's best just to go ahead and open, then deal with the contingency.
来源：https://stackoverflow.com/questions/18888354/open-a-file-in-d