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4.4 默认值

[开头省略：<< 3.14 >>甚至不是合法的语法。]

默认大小取决于类型。对于整数，它是 8.对于 float，它是 64.对于二进制，它是指定二进制文件的实际大小：

1> Bin = << 17/integer, 3.2/float, <<97, 98, 99>>/binary >>. 
<<17,64,9,153,153,153,153,153,154,97,98,99>>
  ^ |<-------------------------->|<------>|

  |             float=64          binary=24
integer=8

2> `size(Bin)`. % Returns the number of bytes:
12            % 8 bits + 64 bits + 3*8 bits = 96 bits => 96/8 = 12 bytes

在匹配时，只允许在模式的末尾使用没有 Size 的二进制段，默认的 Size 是匹配右侧的二进制的其余部分：

25> Bin = <<97, 98, 99>>.
<<"abc">>

26> << X/integer, Rest/binary >> = Bin.
<<"abc">>

27> X.
97

28> Rest.
<<"bc">>

模式中具有二进制类型的所有其他段必须指定大小：

12> Bin = <<97, 98, 99, 100>>.         
<<"abcd">>

13> << B:1/binary, X/integer, Rest/binary >> = Bin. %'unit' defaults to 8 for  
<<"abcd">>                    %binary type, total segment size is Size * unit  

14> B.
<<"a">>

15> X.
98

16> Rest.
<<"cd">>

17> << B2/binary, X2/integer, Rest2/binary >> = Bin. 
* 1: a binary field without size is only allowed at the end of a binary pattern