Seeking Permission or Begging Forgiveness?
A Go Brain Teaser
What do you think the following program will print?
This program will print: 4.
When you use iota in a const group, the values of the constants will advance according to the first operation. The << operator is left shift — it moves the bits of a number to the left, basically multiplying it by power of 2.
For example, if we have the number 5, which (on byte, which is 8 bits) is 00000101. When you write 5 << 1 all the bits are shifted one place to the left. Making it 00001010 which is 10=5*2. When you write 5 << 2 the bits are shifted two places to the left, making it 00010100 or 20=5*4.
The first value, Read is 1<<0 which is 1. The next value, Write is 1<<1 which is 2. Finally, Execute is 1<<2 which is 4.
iota is Go's version of enumeration, if you're coming from another language, you're probably used to enumerated types being printed as strings. In Go, you can implement that with fmt.Stringer.
Once you do that, the code above will print execute.
Note that FilePerm can be any uint16 value — the reason we have the final clause in the String method. Why do you think the comment says "don't use %s or %v"?
Using values that are a power of 2 enables you to use bitwise operations or bitmasks. For example, you can set file permission and check them using the following code:
Bitmasks are a compact way of representing flags or options. If instead of using a bitmask, you’ll use a struct like this:
Then it’ll be 3 times as big:
A 2 byte difference might not seem a lot, but as Dave Cheney says: “A few bytes here, a few there, pretty soon you’re talking real memory.”
I encourage you to try out bitmasks the next time you write some code, I’m sure you’ll find them useful.
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