这个操作员在红宝石中做什么？ << =

Question

I have some code that I am debugging that uses this operator and I'm not sure why it is used.

It appears to be appending the object to an array. If that was all I do not know why the engineer didn't simply use the '<<' operator. What is the difference?

Thanks!

Answer 1

http://www.tutorialspoint.com/ruby/ruby_operators.htm

it looks like its a bitwise left shift operation and assignment in one.

x <<= 2

is the same as

x = x << 2

Answer 2

It is not always the case that << modifies the target: it might be the result that is of importance. Consult the API for the actual types used as to the behavior.

A bit-shift of an integer does not have a side-effect (the computation is discarded unless it is assigned/used):

a = 1
a << 2
a # => 1
a <<= 2
a # => 4

But << on an array does have a side-effect (and <<= would just perform a useless assignment ¹ that hides the side-effect nature of the operation):

b = [1]
b << 2
b # => [1,2]

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¹ In rare cases, it might be "clever" with accessors to use obj.prop <<= val for side-effecting operations as it will invoke both the getter and the setter - and the setter may contain logic. However, I use the word "clever" and not "good" here for a reason :)