Why is it not a good idea to dynamically create a lot of symbols in ruby (for versions before 2.2)?
What is the function of symbol in ruby? what's difference between string and symbol? Why is it not a good idea to dynamically create a lot of symbols?
Answer
Symbols are like strings but they are immutable - they can't be modified.
They are only put into memory once, making them very efficient to use for things like keys in hashes but they stay in memory until the program exits. This makes them a memory hog if you misuse them.
If you dynamically create lots of symbols, you are allocating a lot of memory that can't be freed until your program ends. You should only dynamically create symbols (using string.to_sym) if you know you will:
- need to repeatedly access the symbol
- not need to modify them
As I said earlier, they are useful for things like hashes - where you care more about the identity of the variable than its value. Symbols, when correctly used, are a readable and efficient way to pass around identity.
I will explain what I mean about the immutability of symbols RE your comment.
Strings are like arrays; they can be modified in place:
12:17:44 ~$ irb
irb(main):001:0> string = "Hello World!"
=> "Hello World!"
irb(main):002:0> string[5] = 'z'
=> "z"
irb(main):003:0> string
=> "HellozWorld!"
irb(main):004:0>
Symbols are more like numbers; they can't be edited in place:
irb(main):011:0> symbol = :Hello_World
=> :Hello_World
irb(main):012:0> symbol[5] = 'z'
NoMethodError: undefined method `[]=' for :Hello_World:Symbol
from (irb):12
from :0