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
它是上下文敏感的，如a b(c);有两个有效的解析-声明和变量。当你说“If c是一个类型”时，这就是上下文，就在那里，你已经准确地描述了c++对它的敏感程度。如果没有“c是什么?”的上下文，就无法明确地解析它。
在这里，上下文通过标记的选择来表示——如果标识符指定了类型，解析器将其读取为typename标记。这是最简单的解决方案，避免了上下文敏感的复杂性(在本例中)。
编辑:当然，还有更多的上下文敏感性问题，我只是关注你展示的那个。模板在这方面尤其糟糕。
其他回答
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。
首先，你正确地观察到c++标准末尾的语法中没有上下文敏感的规则，因此语法与上下文无关。
然而，这种语法并不能精确地描述c++语言，因为它生成的是非c++程序，例如
int m() { m++; }
or
typedef static int int;
c++语言被定义为“一组格式良好的c++程序”，它不是与上下文无关的(可以证明，仅仅要求声明变量就可以做到这一点)。理论上，您可以在模板中编写图灵完备程序，并根据其结果使程序具有病态形式，因此它甚至不是上下文敏感的。
现在，(无知的)人们(通常不是语言理论家，而是解析器设计者)通常在以下一些含义中使用“not context-free”
模棱两可的 不能用Bison解析 而不是LL(k) LR(k) LALR(k)或任何他们选择的解析器定义的语言类
标准后面的语法不满足这些类别(即它是模糊的，不是LL(k)…)，所以c++语法对他们来说“不是上下文无关的”。从某种意义上说，他们是对的，要创建一个工作的c++解析器是非常困难的。
注意，这里使用的属性只与上下文无关的语言有微弱的联系——歧义与上下文无关没有任何关系(事实上，上下文敏感的规则通常有助于消除歧义)，另外两个只是上下文无关语言的子集。解析与上下文无关的语言不是一个线性过程(尽管解析确定性语言是)。
c++不是上下文无关的。我以前在编译器课上学过。快速搜索得到了这个链接，其中“语法或语义”部分解释了为什么C和c++不是上下文无关的:
维基百科演讲:上下文无关语法
眼神, Ovanes
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.
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| | | (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
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| | | | (pp_statement_seq@Cpp~GCC5=894#6b04780^1#6b04760:1 Line 1 Column 67 File C:/temp/prime_with_templates.cpp
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| | | | |(jump_statement@Cpp~GCC5=1011#6afba80^1#6b04440:1 Line 1 Column 67 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
| | |(decl_specifier@Cpp~GCC5=1073#6b04a20^1#6b04a40:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
| | | (trailing_type_specifier@Cpp~GCC5=1118#6b04a00^1#6b04a20:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp
| | | (simple_type_specifier@Cpp~GCC5=1138#6b049e0^1#6b04a00:1 Line 3 Column 10 File C:/temp/prime_with_templates.cpp)simple_type_specifier
| | | )trailing_type_specifier#6b04a00
| | |)decl_specifier#6b04a20
| | )basic_decl_specifier_seq#6b04a40
| | (ptr_declarator@Cpp~GCC5=1417#6b04c40^1#6b04c80:2 Line 3 Column 15 File C:/temp/prime_with_templates.cpp
| | |(noptr_declarator@Cpp~GCC5=1421#6b04be0^1#6b04c40:1 Line 3 Column 15 File C:/temp/prime_with_templates.cpp
| | | (declarator_id@Cpp~GCC5=1487#6b04bc0^1#6b04be0:1 Line 3 Column 15 File C:/temp/prime_with_templates.cpp
| | | (id_expression@Cpp~GCC5=317#6b04b60^1#6b04bc0:1 Line 3 Column 15 File C:/temp/prime_with_templates.cpp
| | | (unqualified_id@Cpp~GCC5=319#6b04ac0^1#6b04b60:1 Line 3 Column 15 File C:/temp/prime_with_templates.cpp
| | | |(IDENTIFIER@Cpp~GCC5=3368#6b049c0^1#6b04ac0:1[`no'] Line 3 Column 15 File C:/temp/prime_with_templates.cpp)IDENTIFIER
| | | )unqualified_id#6b04ac0
| | | )id_expression#6b04b60
| | | )declarator_id#6b04bc0
| | |)noptr_declarator#6b04be0
| | )ptr_declarator#6b04c40
| | )parameter_declaration#6b04c80
| | )template_parameter#6b04ca0
| |)template_parameter_list#6b04cc0
| |(template_parameter@Cpp~GCC5=2085#6b04ec0^1#6b04ee0:2 Line 3 Column 19 File C:/temp/prime_with_templates.cpp
| | (parameter_declaration@Cpp~GCC5=1611#6b04ea0^1#6b04ec0:1 Line 3 Column 19 File C:/temp/prime_with_templates.cpp
| | (basic_decl_specifier_seq@Cpp~GCC5=1070#6b04b40^1#6b04ea0:1 Line 3 Column 19 File C:/temp/prime_with_templates.cpp
| | (decl_specifier@Cpp~GCC5=1073#6b04ba0^1#6b04b40:1 Line 3 Column 19 File C:/temp/prime_with_templates.cpp
| | |(trailing_type_specifier@Cpp~GCC5=1118#6b04c60^1#6b04ba0:1 Line 3 Column 19 File C:/temp/prime_with_templates.cpp
| | | (simple_type_specifier@Cpp~GCC5=1138#6b04580^1#6b04c60:1 Line 3 Column 19 File C:/temp/prime_with_templates.cpp)simple_type_specifier
| | |)trailing_type_specifier#6b04c60
| | )decl_specifier#6b04ba0
| | )basic_decl_specifier_seq#6b04b40
| | (ptr_declarator@Cpp~GCC5=1417#6b04e60^1#6b04ea0:2 Line 3 Column 24 File C:/temp/prime_with_templates.cpp
| | (noptr_declarator@Cpp~GCC5=1421#6b04e40^1#6b04e60:1 Line 3 Column 24 File C:/temp/prime_with_templates.cpp
| | |(declarator_id@Cpp~GCC5=1487#6b04de0^1#6b04e40:1 Line 3 Column 24 File C:/temp/prime_with_templates.cpp
| | | (id_expression@Cpp~GCC5=317#6b04d80^1#6b04de0:1 Line 3 Column 24 File C:/temp/prime_with_templates.cpp
| | | (unqualified_id@Cpp~GCC5=319#6b04ce0^1#6b04d80:1 Line 3 Column 24 File C:/temp/prime_with_templates.cpp
| | | (IDENTIFIER@Cpp~GCC5=3368#6b04560^1#6b04ce0:1[`yes'] Line 3 Column 24 File C:/temp/prime_with_templates.cpp)IDENTIFIER
| | | )unqualified_id#6b04ce0
| | | )id_expression#6b04d80
| | |)declarator_id#6b04de0
| | )noptr_declarator#6b04e40
| | )ptr_declarator#6b04e60
| | )parameter_declaration#6b04ea0
| |)template_parameter#6b04ec0
| )template_parameter_list#6b04ee0
| (template_parameter@Cpp~GCC5=2085#6b05120^1#6b05140:2 Line 3 Column 29 File C:/temp/prime_with_templates.cpp
| |(parameter_declaration@Cpp~GCC5=1611#6b05100^1#6b05120:1 Line 3 Column 29 File C:/temp/prime_with_templates.cpp
| | (basic_decl_specifier_seq@Cpp~GCC5=1070#6b04d20^1#6b05100:1 Line 3 Column 29 File C:/temp/prime_with_templates.cpp
| | (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
| | |(simple_type_specifier@Cpp~GCC5=1140#6b046e0^1#6b04e80:1 Line 3 Column 29 File C:/temp/prime_with_templates.cpp)simple_type_specifier
| | )trailing_type_specifier#6b04e80
| | )decl_specifier#6b04dc0
| | )basic_decl_specifier_seq#6b04d20
| | (ptr_declarator@Cpp~GCC5=1417#6b05080^1#6b05100:2 Line 3 Column 33 File C:/temp/prime_with_templates.cpp
| | (noptr_declarator@Cpp~GCC5=1421#6b05020^1#6b05080:1 Line 3 Column 33 File C:/temp/prime_with_templates.cpp
| | (declarator_id@Cpp~GCC5=1487#6b05000^1#6b05020:1 Line 3 Column 33 File C:/temp/prime_with_templates.cpp
| | |(id_expression@Cpp~GCC5=317#6b04fa0^1#6b05000:1 Line 3 Column 33 File C:/temp/prime_with_templates.cpp
| | | (unqualified_id@Cpp~GCC5=319#6b04f00^1#6b04fa0:1 Line 3 Column 33 File C:/temp/prime_with_templates.cpp
| | | (IDENTIFIER@Cpp~GCC5=3368#6b046c0^1#6b04f00:1[`f'] Line 3 Column 33 File C:/temp/prime_with_templates.cpp)IDENTIFIER
| | | )unqualified_id#6b04f00
| | |)id_expression#6b04fa0
| | )declarator_id#6b05000
| | )noptr_declarator#6b05020
| | )ptr_declarator#6b05080
| |)parameter_declaration#6b05100
| )template_parameter#6b05120
| )template_parameter_list#6b05140
| (template_parameter@Cpp~GCC5=2085#6b05440^1#6b05460:2 Line 3 Column 36 File C:/temp/prime_with_templates.cpp
| (parameter_declaration@Cpp~GCC5=1611#6b05420^1#6b05440:1 Line 3 Column 36 File C:/temp/prime_with_templates.cpp
| |(basic_decl_specifier_seq@Cpp~GCC5=1070#6b05160^1#6b05420:1 Line 3 Column 36 File C:/temp/prime_with_templates.cpp
| | (decl_specifier@Cpp~GCC5=1073#6b04fe0^1#6b05160:1 Line 3 Column 36 File C:/temp/prime_with_templates.cpp
| | (trailing_type_specifier@Cpp~GCC5=1118#6b050e0^1#6b04fe0:1 Line 3 Column 36 File C:/temp/prime_with_templates.cpp
| | (simple_type_specifier@Cpp~GCC5=1140#6b050c0^1#6b050e0:1 Line 3 Column 36 File C:/temp/prime_with_templates.cpp)simple_type_specifier
| | )trailing_type_specifier#6b050e0
| | )decl_specifier#6b04fe0
| |)basic_decl_specifier_seq#6b05160
| |(ptr_declarator@Cpp~GCC5=1417#6b053e0^1#6b05420:2 Line 3 Column 40 File C:/temp/prime_with_templates.cpp
| | (noptr_declarator@Cpp~GCC5=1421#6b053c0^1#6b053e0:1 Line 3 Column 40 File C:/temp/prime_with_templates.cpp
| | (declarator_id@Cpp~GCC5=1487#6b05360^1#6b053c0:1 Line 3 Column 40 File C:/temp/prime_with_templates.cpp
| | (id_expression@Cpp~GCC5=317#6b05280^1#6b05360:1 Line 3 Column 40 File C:/temp/prime_with_templates.cpp
| | |(unqualified_id@Cpp~GCC5=319#6b051a0^1#6b05280:1 Line 3 Column 40 File C:/temp/prime_with_templates.cpp
| | | (IDENTIFIER@Cpp~GCC5=3368#6b046a0^1#6b051a0:1[`p'] Line 3 Column 40 File C:/temp/prime_with_templates.cpp)IDENTIFIER
| | |)unqualified_id#6b051a0
| | )id_expression#6b05280
| | )declarator_id#6b05360
| | )noptr_declarator#6b053c0
| |)ptr_declarator#6b053e0
| )parameter_declaration#6b05420
| )template_parameter#6b05440
| )template_parameter_list#6b05460
我感觉在“上下文敏感”的正式定义和“上下文敏感”的非正式使用之间存在一些混淆。前者有明确的含义。后者用于表示“为了解析输入，您需要上下文”。
这里也有一个问题: 上下文敏感性vs模糊性。
这是一个与上下文无关的语法:
<a> ::= <b> | <c>
<b> ::= "x"
<c> ::= "x"
它是模棱两可的，所以为了解析输入“x”，你需要一些上下文(或者忍受这种模棱两可，或者发出“警告:E8271 - input is ambiguous in line 115”)。但它肯定不是上下文敏感的语法。
