return a hash or undef

odrevet has asked for the wisdom of the Perl Monks concerning the following question:

It's a simple question. I've got a function that return a hash or undef if no value is found.

my %resp = my_func();

if(defined %resp)
{
  #do something like print $resp{foo}; print{bar};
}
[download]

If the function return undef, I've got the warning "Odd number of elements in hash assignment".

That's makes sense, since the hash got only one value (undef). But I need to check if the hash has values, or the application will crash if I just assum there are values and try to use it...

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Re: return a hash or undef by moritz (Cardinal) on Aug 03, 2009 at 20:05 UTC
This is why the book "Perl Best Practices" recommends an empty `return;` over `return undef;` - in list context it will produce the empty list, which evaluates to false.	[reply] [d/l] [select]
Re: return a hash or undef by hobbs (Monk) on Aug 03, 2009 at 20:25 UTC
A) Don't return `undef`. Return `()` instead, and instead of checking for `if (defined %resp)`, just check `if (%resp)`. This works as long as you don't need to distinguish between a successful-but-empty return and a failure. B) Return a hashref instead of a flattened hash, and assign the function's result to a scalar instead of a hash. Then `undef`, `{}`, and `{ foo => bar }` are all distinguishable values (`!defined $resp`, `!%$resp`, and everything else, respectively.)	[reply] [d/l] [select]
Re: return a hash or undef by psini (Deacon) on Aug 03, 2009 at 19:59 UTC
Two possible solutions: Modify the sub to return a hash reference or undefined, if defined dereference it If you can't change the sub, store the result in an array and, if defined, copy it to an hash Rule One: "Do not act incautiously when confronting a little bald wrinkly smiling man."	[reply]
Re: return a hash or undef by jethro (Monsignor) on Aug 03, 2009 at 20:14 UTC
Other possibility: `if (keys %resp) {` [download]	[reply] [d/l]
Re: return a hash or undef by halfcountplus (Hermit) on Aug 03, 2009 at 20:14 UTC
If the function return undef, I've got the warning "Odd number of elements in hash assignment". Yeah, that is kind of irritating isn't it? Probably there is a good reason. You might want to investigate the use of eval...but also notice that this is allowed, even if %hash is "undef": `if (defined($hash{non-existant key})) ...` [download] the condition, of course, will be untrue, unless there really is a key called "non-existant key" and it is defined.	[reply] [d/l]