What is difference between bless and tie in perl?

Question

I was practicing some interview questions where I found this. I had a look at perldoc -f tie but couldn't get it. I know about bless and is using in my programs.

Can anyone tell me what is tie and how it is related and different from bless, and its usage? I will appreciate any simple example.

PS: I am a perl developer and using it in day to day life. My concern is not for interview, but for knowing it

Answer 1

bless is used to construct an object.

tie allows code to provide a variable for interface. e.g. It might look like you are assigning to a variable, but you are really calling a sub. tie and other forms of magic are primarily used to provide "clever" interfaces (e.g. altering %ENV alters the environment, altering %SIG sets signal handlers, $! mirrors both errno and perror, etc). It can also be used to extend the usefulness of existing code (e.g. creating something that looks like a file handle allows one to reuse code that expects to get its input from a file handle).

Answer 2

I would probably answer your interview question like this:

Tie and bless serve similar functions, in that they associate custom behaviour with a variable. The difference comes in terms of focus - tie is about adding new behaviour to a 'standard' data structure, where bless is about creating a 'new' data structure altogether.

You are therefore more limited with tie to the fundamental operations of the data type you're working with, where a blessed data structure has an almost unlimited scope of possiblity. But the tradeoff is that tieed variables can serve as drop in replacements - if you extend a 'hash' by tieing it to a file or a database, it can still function in the same way without any sort of code revision needed.

This strength is also in many ways it's weakness though - where a future programmer when confronted with an object and a method call in some code, would know they need to look to the class to understand what's going on - they might not realise that a tied hash (or scalar) is doing 'something special'.

I would therefore suggest that as a matter of style tie should be reserved for a diagnostic and testing role - it may be handy to log when a value changes in a hash, but shouldn't be used to make things behave in unexpected ways.