Ruby学习笔记(17)_冒号用法

2017-10-24 13:26:37来源:CSDN作者:slowsnowscar人点击

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What is the difference between:

1) abc:

2) :xyz

3) Abc::Xyz

4) abc: :xyz

5) abc: xyz

6) :abc => xyz

Answer:

1) You can’t use abc: alone. See 4) for reason.

2) :xyz is a symbol literal. It’s very similar to “xyz”, except that :xyz is immutable, while “xyz” is mutable, and there is always only one :xyz in the memory (maybe this is no longer true because Ruby 2.2 introduces symbol GC?)

:xyz.class  #=> Symbol:xyz.to_s  #=> "xyz""xyz".to_sym  #=> :xyza = :xyzb = :xyza.object_id == b.object_id  #=> true

3.Abc::Xyz is very common. That’s the way you refer to the inner class/module/constant Xyz of class/module Abc. :: can but should not be used to call class/module methods.

4) abc: :xyz
Before Ruby 2.0 abc: :xyz can only appear as arguments passed to method calls. As an argument, this is a hash or part of a hash. The following 4 expressions are the same:

p abc: :xyz, foo: :bar  #=> prints {:abc => :xyz, :foo => :bar}p(abc: :xyz, foo: :bar)  #=> prints {:abc => :xyz, :foo => :bar}p({abc: :xyz, foo: :bar})  #=> prints {:abc => :xyz, :foo => :bar}p({:abc => :xyz, :foo => :bar})  #=> prints {:abc => :xyz, :foo => :bar}

As arguments, the curly braces of hashes can be omitted. And when the keys of a hash are symbols, the colon can be moved behind the symbol, and the fat arrow => can be omitted. This makes hashes look more like JSON objects.

5)abc: xyz

xyz = "just a test"hash = {abc: xyz} #hash key is symbol, value is string. 

6) :abc => xyz

Same as 5)

xyz = "just a test"hash = {:abc => xyz} # same with (5), just another representation

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