基于辅助数组集中的元素数量对数组进行排序，作为PHP中的值(Sort array based on the number of elements in the secondary array set as the value in PHP)

 C ++中的标识符名称规则是： 
 
 
 
 它不能以数字开头 
 
 
 可以由字母，数字，下划线，通用字符名称1和实现定义的字符组成 
 
 
 不能是关键字 。 
 
 
 涵盖此C ++草案标准的部分是2.11 标识符 ，其中包括以下语法： 
 
identifier:
  identifier-nondigit            <- Can only start with a non-digit
  identifier identifier-nondigit <- Next two rules allows for subsequent 
  identifier digit               <-  characters to be those outlined in 2 above
identifier-nondigit:
  nondigit                       <- a-z, A-Z and _ 
  universal-character-name
  other implementation-defined characters
[...]
 
 和2.12 关键词解释了保留用作关键字的所有标识符 。 
 
 最后，还保留下列名称： 
 
 
 
 包含双下划线__名称，或者在任何范围内以下划线开头，后面跟着大写字母（如_Apple ） 
 
 
 以全局名称空间中的下划线开头的名称（如全局名称空间中的_apple ）将被保留。 
 
 
 本标准草案涵盖的部分是17.6.4.3.2 。 我们可以找到一个理由，说明为什么这些是从国际标准编程语言理论C中保留的，它说： 
 
 
 
 [...]这为编写大量的幕后非外部宏和函数库提供了一个名称空间，以便图书馆正确地完成其工作[...] 
 
 
 在C ++中，这也适用于名称修改，如本例所示。 
 
 
 脚注 
 
 
 
 1.允许通用字符 
 
 
 Annex E.1涵盖了允许的通用字符： 
 
 
 
 E.1允许字符的范围[charname.allowed] 
 
 
 00A8，00AA，00AD， 
 
 
 00AF，00B2-00B5,00B7-00BA，00BC-00BE，00C0-00D6,00D8-00F6,00F8-00FF 
 
 
 0100-167F，1681-180D，180F-1FFF 200B-200D，202A-202E，203F-2040,2054， 
 
 
 2060-206F 2070-218F，2460-24FF，2776-2793,2C00-2DFF，2E80-2FFF 
 
 
 3004-3007,3021-302F，3031-303F 
 
 
 3040-D7FF F900-FD3D，FD40-FDCF， 
 
 
 FDF0-FE44，FE47-FFFD 
 
 
 10000-1FFFD，20000-2FFFD，30000-3FFFD，40000-4FFFD，50000-5FFFD，60000-6FFFD，70000-7FFFD，80000-8FFFD，90000-9FFFD，A0000-AFFFD，B0000-BFFFD，C0000-CFFFD，D0000- DFFFD，E0000-EFFFD 
 

Rules for identifier names in C++ are:
 
 
 
It can not start with a number
 
 
Can be composed of letters, numbers, underscore, universal character names1 and implementation defined characters
 
 
Can not be a keyword.
 
 
The sections in the C++ draft standard that cover this are 2.11 Identifiers which includes the following grammar:
 
identifier:
  identifier-nondigit            <- Can only start with a non-digit
  identifier identifier-nondigit <- Next two rules allows for subsequent 
  identifier digit               <-  characters to be those outlined in 2 above
identifier-nondigit:
  nondigit                       <- a-z, A-Z and _ 
  universal-character-name
  other implementation-defined characters
[...]
 
and 2.12 Keywords explains all the identifier reserved for use as keywords.
 
Finally, the following names are also reserved:
 
 
 
Names that contain a double underscore __, or start with either an underscore followed by an uppercase letter (like _Apple) in any scope, 
 
 
Names that start with an underscore in the global namespace (like _apple in the global namespace) are reserved.
 
 
The section that covers this in the draft standard is 17.6.4.3.2. We can find a rationale for why these are reserved from Rationale for International Standard—Programming Languages—C which says:
 
 
 
[...]This gives a name space for writing the numerous behind-the-scenes non-external macros and functions a library needs to do its job properly[...]
 
 
In C++ this also applies to name mangling as this example shows.
 
 
Footnotes
 
 
 
1. Allowed universal characters
 
 
The universal characters that are allowed is covered in Annex E.1:
 
 
 
E.1 Ranges of characters allowed [charname.allowed]
 
 
00A8, 00AA, 00AD,
 
 
00AF, 00B2-00B5, 00B7-00BA, 00BC-00BE, 00C0-00D6, 00D8-00F6, 00F8-00FF
 
 
0100-167F, 1681-180D, 180F-1FFF 200B-200D, 202A-202E, 203F-2040, 2054,
 
 
2060-206F 2070-218F, 2460-24FF, 2776-2793, 2C00-2DFF, 2E80-2FFF
 
 
3004-3007, 3021-302F, 3031-303F
 
 
3040-D7FF F900-FD3D, FD40-FDCF,
 
 
FDF0-FE44, FE47-FFFD 
 
 
10000-1FFFD, 20000-2FFFD, 30000-3FFFD, 40000-4FFFD, 50000-5FFFD, 60000-6FFFD, 70000-7FFFD, 80000-8FFFD, 90000-9FFFD, A0000-AFFFD, B0000-BFFFD, C0000-CFFFD, D0000-DFFFD, E0000-EFFFD