Elixir中的String、binaries和bitstring

http://elixir-lang.github.io/getting-started/binaries-strings-and-char-lists.html

https://elixirforum.com/t/bitstring-and-binary/2351/4

字符串是怎麼保存到計算機中的

iex>?a
97
iex>?吳
21556

a的code point是97，也就是110 0001；吳的code point是21556，也就是101 0100 0011 0100。這是unicode標準規定的。

可是計算機中，咱們是以byte爲單位來存放的，那麼110 0001只須要一個字節，填滿8位就是0110 0001；101 0100 0011 0100須要兩個字節，填滿16位就是0101 0100 0011 0100。這個過程就是所謂的utf-8編碼過程。

他們之間的關係

A bitstring is a type that stores arbitrary number of bits, you can have a 5bit bitstring whereas binary stores arbitrary number of bytes

Here is some code that should make things clearer:

# bitstring
bs = << 3 :: size(2) >>      # => 2 bits 11
IO.inspect bs                # => <<3::size(2)>>
IO.inspect is_bitstring(bs)  # => true
IO.inspect is_binary(bs)     # => false

# binary
bin = << 3 >>                # => 8 bits or 1 byte
IO.inspect bin               # => <<3>>
IO.inspect is_bitstring(bin) # => true
IO.inspect is_binary(bin)    # => true

A binary is just a collection of bytes, so it has to have a number of bits that is divisible by 8 (i.e. a byte). So you can have a 8 bit binary, 16 bit binary and so on. If your binary is not divisible by 8, e.g. 7bits, 15bits, 14 bits, 23bits, you have a bitstring. And since a bitstring can have any number of bits even a binary is a bitstring. However, the inverse is not true.

殺手圖來了

So a subset of bitstrings are binaries, and a subset of binaries are strings. Like this: