我经常听到有人说c++是一种上下文敏感的语言。举个例子:

a b(c);

这是变量定义还是函数声明?这取决于符号c的含义。如果c是一个变量,则ab (c);定义一个名为b的类型为a的变量。它直接用c初始化。但如果c是类型,则ab (c);声明一个名为b的函数,该函数接受c并返回a。

如果您查找上下文无关语言的定义,它基本上会告诉您,所有语法规则的左侧必须恰好包含一个非终结符。另一方面,上下文敏感语法允许在左侧使用任意的终结符和非终结符字符串。

浏览“c++程序设计语言”的附录A,我找不到一条语法规则,它的左边除了一个非终结符之外,还有其他任何东西。这意味着c++是上下文无关的。(当然,每一种与上下文无关的语言也是与上下文相关的,因为与上下文无关的语言构成了与上下文相关的语言的一个子集,但这不是重点。)

那么,c++是上下文无关的还是上下文敏感的?


当前回答

Meta-S”是Quinn Tyler Jackson开发的上下文敏感解析引擎。我没有用过,但他讲了一个令人印象深刻的故事。在comp.compilers中查看他的评论,并查看rnaparse.com/MetaS%20defined.htm - Ira Baxter 7月25日10:42

正确的环节是解析谜题

Meta-S是一家已经倒闭的tho栖公司的财产。我可以将Meta-S的免费副本发送给任何感兴趣的人,我已经在rna解析研究中使用了它。请注意,示例文件夹中包含的“伪结语法”是由一个非生物信息学的、成熟的程序员编写的,基本上不起作用。我的语法采用了一种不同的方法,而且效果很好。

其他回答

是的。下面的表达式根据解析的上下文类型有不同的操作顺序:

编辑:当实际操作顺序发生变化时,使用“常规”编译器在修饰AST(传播类型信息)之前解析未修饰的AST会变得非常困难。与此相比,提到的其他上下文敏感的事情“相当容易”(并不是说模板计算一点都不容易)。

#if FIRST_MEANING
   template<bool B>
   class foo
   { };
#else
   static const int foo = 0;
   static const int bar = 15;
#endif

紧随其后的是:

static int foobar( foo < 2 ? 1 < 1 : 0 > & bar );

这个答案说c++不是上下文无关的…有一个暗示(不是由回答者),它不能被解析,回答提供了一个困难的代码示例,如果某个常数不是质数,就会产生一个无效的c++程序。

正如其他人所观察到的,关于语言是否上下文敏感/自由的问题与关于特定语法的相同问题是不同的。

为了解决可解析性的问题,我提供了经验证据,证明c++中存在可以使用的上下文无关语法 生成一个AST,用于源文本的上下文无关的解析 实际上是通过使用现有的基于glr解析器的工具进行解析,该工具由显式语法驱动。

Yes, it succeeds by "accepting too much"; not everything it accepts is a valid C++ program, which is why it's followed up with additional checks (type checks). And yes, the type checker may run into computability problems. In practice tools don't have this problem; if people wrote programs like that, none of them would compile. (I think the standard actually puts a limit on the amount of computation you can do unfolding a template, so in fact the computation is actually finite but probably pretty big).

如果您的意思是,则确定源程序是否是成员 的集合有效的c++源程序,那么我将同意 问题就难多了。但问题并不在于解析。

The tool resolves this issue by isolating parsing from type-checking the parsed program. (Where there are multiple interpretations in the absence of context, it records an ambiguity node in the parse tree with several possible parses; the type checking decides which one is correct and eliminates the invalid subtrees). You can see a (partial) parse tree in the example below; the whole tree is too large to fit in an SO answer. Note you get a parse tree whether the value 234797 or 234799 is used.

在原始值为234799的AST上运行工具的名称/类型解析器成功。如果值为234797,名称解析器将失败(正如预期的那样),并给出错误消息“typen不是类型”,因此该版本不是有效的c++程序。

967 tree nodes in tree.
15 ambiguity nodes in tree.
(translation_unit@Cpp~GCC5=2#6b11a20^0 Line 1 Column 1 File C:/temp/prime_with_templates.cpp
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   (declaration@Cpp~GCC5=1036#6b04980^1#6b049a0:1 Line 1 Column 1 File C:/temp/prime_with_templates.cpp
   |(template_declaration@Cpp~GCC5=2079#6b04960^1#6b04980:1 Line 1 Column 1 File C:/temp/prime_with_templates.cpp
   | (template_parameter_list@Cpp~GCC5=2082#6afbde0^1#6b04960:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp
   |  (template_parameter@Cpp~GCC5=2085#6afbd80^1#6afbde0:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp
   |   (parameter_declaration@Cpp~GCC5=1611#6afbd40^1#6afbd80:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp
   |   |(basic_decl_specifier_seq@Cpp~GCC5=1070#6afb880^1#6afbd40:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp
   |   | (decl_specifier@Cpp~GCC5=1073#6afb840^1#6afb880:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp
   |   |  (trailing_type_specifier@Cpp~GCC5=1118#6afb7e0^1#6afb840:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp
   |   |   (simple_type_specifier@Cpp~GCC5=1138#6afb7a0^1#6afb7e0:1 Line 1 Column 10 File C:/temp/prime_with_templates.cpp)simple_type_specifier
   |   |  )trailing_type_specifier#6afb7e0
   |   | )decl_specifier#6afb840
   |   |)basic_decl_specifier_seq#6afb880
   |   |(ptr_declarator@Cpp~GCC5=1417#6afbc40^1#6afbd40:2 Line 1 Column 15 File C:/temp/prime_with_templates.cpp
   |   | (noptr_declarator@Cpp~GCC5=1421#6afbba0^1#6afbc40:1 Line 1 Column 15 File C:/temp/prime_with_templates.cpp
   |   |  (declarator_id@Cpp~GCC5=1487#6afbb80^1#6afbba0:1 Line 1 Column 15 File C:/temp/prime_with_templates.cpp
   |   |   (id_expression@Cpp~GCC5=317#6afbaa0^1#6afbb80:1 Line 1 Column 15 File C:/temp/prime_with_templates.cpp
   |   |   |(unqualified_id@Cpp~GCC5=319#6afb9c0^1#6afbaa0:1 Line 1 Column 15 File C:/temp/prime_with_templates.cpp
   |   |   | (IDENTIFIER@Cpp~GCC5=3368#6afb780^1#6afb9c0:1[`V'] Line 1 Column 15 File C:/temp/prime_with_templates.cpp)IDENTIFIER
   |   |   |)unqualified_id#6afb9c0
   |   |   )id_expression#6afbaa0
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   | (declaration@Cpp~GCC5=1033#6b04940^1#6b04960:2 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |  (block_declaration@Cpp~GCC5=1050#6b04920^1#6b04940:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   (simple_declaration@Cpp~GCC5=1060#6b04900^1#6b04920:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   |(basic_decl_specifier_seq@Cpp~GCC5=1070#6b048e0^1#6b04900:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   | (decl_specifier@Cpp~GCC5=1073#6b048c0^1#6b048e0:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   |  (type_specifier@Cpp~GCC5=1110#6b048a0^1#6b048c0:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   |   (class_specifier@Cpp~GCC5=1761#6b04880^1#6b048a0:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   |   |(class_head@Cpp~GCC5=1763#6afb980^1#6b04880:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp
   |   |   | (class_key@Cpp~GCC5=1791#6afbca0^1#6afb980:1 Line 1 Column 18 File C:/temp/prime_with_templates.cpp)class_key
   |   |   | (IDENTIFIER@Cpp~GCC5=3368#6afbcc0^1#6afb980:2[`answer'] Line 1 Column 25 File C:/temp/prime_with_templates.cpp)IDENTIFIER
   |   |   | (optional_base_clause@Cpp~GCC5=1872#6afba60^1#6afb980:3 Line 1 Column 32 File C:/temp/prime_with_templates.cpp)optional_base_clause
   |   |   |)class_head#6afb980
   |   |   |(member_specification@Cpp~GCC5=1794#6b042e0^1#6b04880:2 {2} Line 1 Column 34 File C:/temp/prime_with_templates.cpp
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   |   |   |  (member_declaration@Cpp~GCC5=1822#6b04040^1#6b04060:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
   |   |   |   (function_definition@Cpp~GCC5=1632#6b04020^1#6b04040:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
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   |   |   |   | (ptr_declarator@Cpp~GCC5=1417#6afbfe0^1#6afbec0:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
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   |   |   |   |   (noptr_declarator@Cpp~GCC5=1421#6afbf60^1#6afbf80:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |(declarator_id@Cpp~GCC5=1487#6afbea0^1#6afbf60:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
   |   |   |   |   | (id_expression@Cpp~GCC5=317#6afbb40^1#6afbea0:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |  (unqualified_id@Cpp~GCC5=319#6afbc80^1#6afbb40:1 Line 1 Column 34 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   (IDENTIFIER@Cpp~GCC5=3368#6afbc20^1#6afbc80:1[`answer'] Line 1 Column 34 File C:/temp/prime_with_templates.cpp)IDENTIFIER
   |   |   |   |   |  )unqualified_id#6afbc80
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   |   |   |   |   |)declarator_id#6afbea0
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   |   |   |   |   (parameter_declaration_clause@Cpp~GCC5=1559#6afbd00^1#6afbf80:2 Line 1 Column 41 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |(pp_parameter_declaration_list@Cpp~GCC5=1570#6afb940^1#6afbd00:1 Line 1 Column 41 File C:/temp/prime_with_templates.cpp
   |   |   |   |   | (pp_parameter_declaration_seq@Cpp~GCC5=1574#6afb800^1#6afb940:1 Line 1 Column 41 File C:/temp/prime_with_templates.cpp
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   |   |   |   |   |   |(decl_specifier@Cpp~GCC5=1073#6afbfa0^1#6afbf40:1 Line 1 Column 41 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   | (trailing_type_specifier@Cpp~GCC5=1118#6afbfc0^1#6afbfa0:1 Line 1 Column 41 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   |  (simple_type_specifier@Cpp~GCC5=1140#6afb860^1#6afbfc0:1 Line 1 Column 41 File C:/temp/prime_with_templates.cpp)simple_type_specifier
   |   |   |   |   |   | )trailing_type_specifier#6afbfc0
   |   |   |   |   |   |)decl_specifier#6afbfa0
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   |   |   |   |   |  )parameter_declaration#6afb9a0
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   |   |   |   |   )parameter_declaration_clause#6afbd00
   |   |   |   |   (function_qualifiers@Cpp~GCC5=1438#6afbce0^1#6afbf80:3 Line 1 Column 46 File C:/temp/prime_with_templates.cpp)function_qualifiers
   |   |   |   |  )noptr_declarator#6afbf80
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   |   |   |   |)function_head#6afbec0
   |   |   |   |(function_body@Cpp~GCC5=1680#6b04000^1#6b04020:2 Line 1 Column 46 File C:/temp/prime_with_templates.cpp
   |   |   |   | (compound_statement@Cpp~GCC5=888#6afbee0^1#6b04000:1 Line 1 Column 46 File C:/temp/prime_with_templates.cpp)compound_statement
   |   |   |   |)function_body#6b04000
   |   |   |   )function_definition#6b04020
   |   |   |  )member_declaration#6b04040
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   |   |   | (member_declaration_or_access_specifier@Cpp~GCC5=1806#6b042c0^1#6b042e0:2 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |  (member_declaration@Cpp~GCC5=1822#6b04820^1#6b042c0:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |   (function_definition@Cpp~GCC5=1632#6b04280^1#6b04820:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |   |(function_head@Cpp~GCC5=1674#6b04220^1#6b04280:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |   | (basic_decl_specifier_seq@Cpp~GCC5=1070#6b040e0^1#6b04220:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |   |  (decl_specifier@Cpp~GCC5=1073#6b040c0^1#6b040e0:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |   |   (trailing_type_specifier@Cpp~GCC5=1118#6b040a0^1#6b040c0:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |(simple_type_specifier@Cpp~GCC5=1138#6b04080^1#6b040a0:1 Line 1 Column 49 File C:/temp/prime_with_templates.cpp)simple_type_specifier
   |   |   |   |   )trailing_type_specifier#6b040a0
   |   |   |   |  )decl_specifier#6b040c0
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   |   |   |   | (ptr_declarator@Cpp~GCC5=1417#6b04200^1#6b04220:2 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |  (noptr_declarator@Cpp~GCC5=1422#6b041e0^1#6b04200:1 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |   (noptr_declarator@Cpp~GCC5=1421#6b041a0^1#6b041e0:1 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |(declarator_id@Cpp~GCC5=1487#6b04180^1#6b041a0:1 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |   | (id_expression@Cpp~GCC5=317#6b04160^1#6b04180:1 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |  (unqualified_id@Cpp~GCC5=320#6b04140^1#6b04160:1 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   (operator_function_id@Cpp~GCC5=2027#6b04120^1#6b04140:1 Line 1 Column 54 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   |(operator@Cpp~GCC5=2070#6b04100^1#6b04120:1 Line 1 Column 62 File C:/temp/prime_with_templates.cpp)operator
   |   |   |   |   |   )operator_function_id#6b04120
   |   |   |   |   |  )unqualified_id#6b04140
   |   |   |   |   | )id_expression#6b04160
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   |   |   |   |   )noptr_declarator#6b041a0
   |   |   |   |   (parameter_declaration_clause@Cpp~GCC5=1558#6afba40^1#6b041e0:2 Line 1 Column 65 File C:/temp/prime_with_templates.cpp)parameter_declaration_clause
   |   |   |   |   (function_qualifiers@Cpp~GCC5=1438#6b041c0^1#6b041e0:3 Line 1 Column 66 File C:/temp/prime_with_templates.cpp)function_qualifiers
   |   |   |   |  )noptr_declarator#6b041e0
   |   |   |   | )ptr_declarator#6b04200
   |   |   |   |)function_head#6b04220
   |   |   |   |(function_body@Cpp~GCC5=1680#6b04300^1#6b04280:2 Line 1 Column 66 File C:/temp/prime_with_templates.cpp
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   |   |   |   |   (statement@Cpp~GCC5=857#6b04440^1#6b04780:1 Line 1 Column 67 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |(jump_statement@Cpp~GCC5=1011#6afba80^1#6b04440:1 Line 1 Column 67 File C:/temp/prime_with_templates.cpp
   |   |   |   |   | (pm_expression@Cpp~GCC5=551#6b04380^1#6afba80:1 Line 1 Column 74 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |  (cast_expression@Cpp~GCC5=543#6b04360^1#6b04380:1 Line 1 Column 74 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   (unary_expression@Cpp~GCC5=465#6b04340^1#6b04360:1 Line 1 Column 74 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   |(primary_expression@Cpp~GCC5=307#6b04320^1#6b04340:1 Line 1 Column 74 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   | (id_expression@Cpp~GCC5=317#6b042a0^1#6b04320:1 Line 1 Column 74 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   |  (unqualified_id@Cpp~GCC5=319#6b04260^1#6b042a0:1 Line 1 Column 74 File C:/temp/prime_with_templates.cpp
   |   |   |   |   |   |   (IDENTIFIER@Cpp~GCC5=3368#6b04240^1#6b04260:1[`V'] Line 1 Column 74 File C:/temp/prime_with_templates.cpp)IDENTIFIER
   |   |   |   |   |   |  )unqualified_id#6b04260
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   |   |   |   |   |   )unary_expression#6b04340
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   |(template_declaration@Cpp~GCC5=2079#6b065e0^1#6b06600:1 Line 3 Column 1 File C:/temp/prime_with_templates.cpp
   | (template_parameter_list@Cpp~GCC5=2083#6b05460^1#6b065e0:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
   |  (template_parameter_list@Cpp~GCC5=2083#6b05140^1#6b05460:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
   |   (template_parameter_list@Cpp~GCC5=2083#6b04ee0^1#6b05140:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
   |   |(template_parameter_list@Cpp~GCC5=2082#6b04cc0^1#6b04ee0:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
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   |   |  (parameter_declaration@Cpp~GCC5=1611#6b04c80^1#6b04ca0:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
   |   |   (basic_decl_specifier_seq@Cpp~GCC5=1070#6b04a40^1#6b04c80:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
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   |   |(template_parameter@Cpp~GCC5=2085#6b04ec0^1#6b04ee0:2 Line 3 Column 19 File C:/temp/prime_with_templates.cpp
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   |   (template_parameter@Cpp~GCC5=2085#6b05120^1#6b05140:2 Line 3 Column 29 File C:/temp/prime_with_templates.cpp
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   |   |  (decl_specifier@Cpp~GCC5=1073#6b04dc0^1#6b04d20:1 Line 3 Column 29 File C:/temp/prime_with_templates.cpp
   |   |   (trailing_type_specifier@Cpp~GCC5=1118#6b04e80^1#6b04dc0:1 Line 3 Column 29 File C:/temp/prime_with_templates.cpp
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Meta-S”是Quinn Tyler Jackson开发的上下文敏感解析引擎。我没有用过,但他讲了一个令人印象深刻的故事。在comp.compilers中查看他的评论,并查看rnaparse.com/MetaS%20defined.htm - Ira Baxter 7月25日10:42

正确的环节是解析谜题

Meta-S是一家已经倒闭的tho栖公司的财产。我可以将Meta-S的免费副本发送给任何感兴趣的人,我已经在rna解析研究中使用了它。请注意,示例文件夹中包含的“伪结语法”是由一个非生物信息学的、成熟的程序员编写的,基本上不起作用。我的语法采用了一种不同的方法,而且效果很好。

是的,c++是上下文敏感的,非常上下文敏感。您不能通过使用上下文无关的解析器简单地解析文件来构建语法树,因为在某些情况下,您需要从以前的知识中了解符号来决定(例如。在解析时构建一个符号表)。

第一个例子:

A*B;

这是一个乘法表达式吗?

OR

这是B变量作为a类型指针的声明吗?

如果A是一个变量,那么它就是一个表达式,如果A是类型,它就是一个指针声明。

第二个例子:

A B(bar);

这是一个函数原型接受一个条形参数吗?

OR

这是声明A类型的变量B,并调用A的构造函数和bar常量作为初始化式吗?

您需要再次了解bar是符号表中的变量还是类型。

第三个例子:

class Foo
{
public:
    void fn(){x*y;}
    int x, y;
};

当解析时构建符号表没有帮助时,就是这种情况,因为x和y的声明在函数定义之后。因此,您需要首先浏览类定义,然后在第二步查看方法定义,以确定x*y是一个表达式,而不是指针声明或其他东西。

c++模板已经被证明是图灵强大的。虽然不是正式的参考资料,但从这方面来看,我们可以从以下几个方面入手:

http://cpptruths.blogspot.com/2005/11/c-templates-are-turing-complete.html

我大胆地猜测一下(古老的民间和简明的CACM证明表明,60年代的ALGOL不能用CFG表示),并说c++因此不能仅用CFG正确地解析。cfg与各种TP机制结合在一起,无论是在树通道还是在减少事件中——这是另一个故事。一般来说,由于停止问题,存在一些c++程序不能被证明是正确的/不正确的,但仍然是正确的/不正确的。

{PS-作为Meta-S的作者(上面有几个人提到过),我可以很肯定地说Thothic并没有消失,也不是免费的软件。也许我这样回答是为了不被删除或被投票降至-3。