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
In reply to Re^5: Best way to check if something is a file handle?
by davido
in thread Best way to check if something is a file handle?
by tobyink
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