Re^4: Best way to check if something is a file handle?

The strange thing is that although $fh->can('close') doesn't return true, it also doesn't complain about there being no can method, whereas $fh->might('close') does complain about a missing might method. So it is, in some manner being treated as an object that kinda half implements the UNIVERSAL interface.

Writing the empty string to a handle is quite a nice solution I suppose, but there may be some tied interfaces where printing even the empty string has side-effects. :-(

And as it happens I'm actually more interested in reading than writing.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

Comment on Re^4: Best way to check if something is a file handle? Select or Download Code

Replies are listed 'Best First'.
Re^5: Best way to check if something is a file handle? by davido (Cardinal) on Jul 09, 2012 at 15:41 UTC
`eval{}` some innocuous operation such as `$fh->format_name` and check if an error has been generated. That takes 'duck typing' to its logical end; A filehandle can "format_name", so let's "format_name" and see if we get an error. However, you have to keep in mind that if $fh contains a glob that holds a string, that string will silently try to convert itself to a filehandle. For example: `$something = 'STDOUT'; someglob = $something; $fh = \someglob; $fh->format_name;` will be accepted just fine, because `$fh` contains a reference to a glob that holds the STDOUT filehandle, even though it was just a string only two operations ago. In fact the third layer of indirection isn't even needed; `someglob` also contains a filehandle, of course. Perl would be equally happy with `someglob->format_name`. And in fact, even `$something->format_name` is fine with Perl; as long as the string looks like a known filehandle, it's fine. On the other hand, `$something = 'frobcinate'; $something->format_name;` will throw an exception unless you've opened a filehandle named `*frobcinate`. This may be a terrible oversimplification that fails to work in some important cases. Certainly it would fail if `$something` were blessed into a class that is nothing like a filehandle except that it has a method named `format_name`. Dave	[reply] [d/l] [select]
Re^6: Best way to check if something is a file handle? by tobyink (Canon) on Jul 09, 2012 at 16:25 UTC
It also fails for IO::All, but yes, `format_name` seems to be a good choice of an innocuous method for `eval`ing. I don't think there's any silver bullet. :-( `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]