本文介绍了BNF语法为一个简单的c ++程序示例的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 29岁程序员，3月因学历无情被辞！ 所以我试图为一个简单的c ++程序编写语法。 这是语法现在的样子： PDefs。 Program :: = [Def]; terminator Def; comment//; comment/ ** /; comment＃; DFun。 Def :: = Type Id（[Arg]）{[Stm]}; separator Arg，; terminator Stm; ADecl。 Arg :: = Type Id; SExp。 Stm :: = Exp; ; SDecl。 Stm :: = Type Id; ; SDecls。 Stm :: = Type Id，[Id]; ; SInit。 Stm :: = Type Id=Exp; ; SReturn。 Stm :: =returnExp; ; SWhile。 Stm :: =while（Exp）Stm; SBlock。 Stm :: ={[Stm]}; SIfElse。 Stm :: =if（Exp）StmelseStm; EInt。 Exp15 :: = Integer; EDouble。 Exp15 :: = Double; ETrue。 Exp15 :: =true; EFalse。 Exp15 :: =false; EId。 Exp15 :: = Id; EApp。 Exp15 :: = Id（[Exp]）; EPIncr。 Exp14 :: = Exp15++; EPDecr。 Exp14 :: = Exp15 - ; EIncr。 Exp13 :: =++Exp14; EDecr。 Exp13 :: = - Exp14; ETimes。 Exp12 :: = Exp12*Exp13; EDiv。 Exp12 :: = Exp12/Exp13; EPlus。 Exp11 :: = Exp11+Exp12; EMinus。 Exp11 :: = Exp11 - Exp12; ELt。 Exp9 :: = Exp9< Exp10; EGt。 Exp9 :: = Exp9> Exp10; ELtEq。 Exp9 :: = Exp9< =Exp10; EGtWq。 Exp9 :: = Exp9> =Exp10; EEq。 Exp8 :: = Exp8==Exp9; ENEq。 Exp8 :: = Exp8！=Exp9; EAnd。 Exp4 :: = Exp4&& Exp5; EOr。 Exp3 :: = Exp3|| Exp4; EAss。 Exp2 :: = Exp3=Exp2; coercions Exp 15; separator Exp，; separator Id，; Tbool。 Type :: =bool; Tdouble。 Type :: =double; Tint。 Type :: =int; Tvoid。 Type :: =void; 令牌ID（字母（数字|数字|'_'）*）; 这是需要解析的简单c ++程序 //一个小的C ++程序 #include< iostream> int main（） { std :: cout< 你好，世界！ << std :: endl; return 0; } 所以当我试图解析它我得到错误在第6行意味着std :: cout行。因为我是新的bnf我不知道如何想解决这个。如果有人能给你一个例子，你将如何去解决这样的情况将是巨大的。 谢谢！解决方案它失败的行不能被解析，因为你缺少一些规则： 您需要一个规则来解析合格的ID 。 $ $ $ 限定的ID是一种特殊类型的标识符，可以（为您的目的）在与（不合格）标识符相同的情况下。 std :: cout 和 std :: endl 是合格的id，并且它们的（简化）规则可以看起来像这样： < qualified_id> :: =< nested_name_specifier> < unqualified_id> < nested_name_specifier> :: =< namespace_name> ::< nested_name_specifier> ;? < unqualified_id> 和 的 您需要一个规则来解析表达式。 $ c><< 运算符。 此附加类型表达式的（简化）规则可能如下所示： < shift_left_expression> :: =< other_expression> < shift_left_expression> :: =< shift_left_expression> << < other_expression> 其中（为您的目的）< other_expression> 。字符串字面值（string literal） 是一种文字类型，可以作为表达式的一部分（例如标识符）使用（用于您的目的）。 Hello，world！是一个字符串文字，一个（简化的）规则可以看起来像这样： < string_literal> :: =\< s_char_sequence> ;?\< s_char_sequence> :: =< s_char> < s_char_sequence> :: =< s_char_sequence> < s_char> 其中< s_char> 是您要允许的任何字符在一个字符串字面量（为了保持简单，不允许字符，例如。）。 So i am trying to write grammar for a simple c++ program.this is how the grammar looks like right now:PDefs. Program ::= [Def] ;terminator Def "" ;comment "//" ;comment "/*" "*/" ;comment "#" ;DFun. Def ::= Type Id "(" [Arg] ")" "{" [Stm] "}" ;separator Arg "," ;terminator Stm "" ;ADecl. Arg ::= Type Id ;SExp. Stm ::= Exp ";" ;SDecl. Stm ::= Type Id ";" ;SDecls. Stm ::= Type Id "," [Id] ";" ;SInit. Stm ::= Type Id "=" Exp ";" ;SReturn. Stm ::= "return" Exp ";" ;SWhile. Stm ::= "while" "(" Exp ")" Stm ;SBlock. Stm ::= "{" [Stm] "}" ;SIfElse. Stm ::= "if" "(" Exp ")" Stm "else" Stm ;EInt. Exp15 ::= Integer ;EDouble. Exp15 ::= Double ;ETrue. Exp15 ::= "true" ;EFalse. Exp15 ::= "false" ;EId. Exp15 ::= Id ;EApp. Exp15 ::= Id "(" [Exp] ")" ;EPIncr. Exp14 ::= Exp15 "++" ;EPDecr. Exp14 ::= Exp15 "--" ;EIncr. Exp13 ::= "++" Exp14 ;EDecr. Exp13 ::= "--" Exp14 ;ETimes. Exp12 ::= Exp12 "*" Exp13 ;EDiv. Exp12 ::= Exp12 "/" Exp13 ;EPlus. Exp11 ::= Exp11 "+" Exp12 ;EMinus. Exp11 ::= Exp11 "-" Exp12 ;ELt. Exp9 ::= Exp9 "<" Exp10 ;EGt. Exp9 ::= Exp9 ">" Exp10 ;ELtEq. Exp9 ::= Exp9 "<=" Exp10 ;EGtWq. Exp9 ::= Exp9 ">=" Exp10 ;EEq. Exp8 ::= Exp8 "==" Exp9 ;ENEq. Exp8 ::= Exp8 "!=" Exp9 ;EAnd. Exp4 ::= Exp4 "&&" Exp5 ;EOr. Exp3 ::= Exp3 "||" Exp4 ;EAss. Exp2 ::= Exp3 "=" Exp2 ;coercions Exp 15 ;separator Exp "," ;separator Id "," ;Tbool. Type ::= "bool" ;Tdouble. Type ::= "double" ;Tint. Type ::= "int" ;Tvoid. Type ::= "void" ;token Id (letter (letter | digit | '_')*) ;and this is the simple c++ program that needs to be parsed// a small C++ program#include <iostream>int main(){ std::cout << "Hello, world!" << std::endl; return 0;}so when i try to parse it i get the error in line 6 meaning the std::cout line. Since i am new to bnf i do not know how to "think" to solve this. If someone could give an example of how you would go to solve a situation like this would be great.!Thank you! 解决方案 The line on which it fails, cannot be parsed because you are missing some rules :You need a rule for parsing qualified ids.A qualified id is a special type of identifier, and can (for your purposes) be used in the same situations as an (unqualified) identifier.std::cout and std::endl are qualified ids, and a (simplified) rule for them could look something like this :<qualified_id> ::= <nested_name_specifier> <unqualified_id><nested_name_specifier> ::= <namespace_name> "::" <nested_name_specifier>?in which (for your purposes), <unqualified_id> and <namespace_name> can be treated as identifiers.You need a rule for parsing an expression with the << operator.A (simplified) rule for this additional type of expression could look something like this :<shift_left_expression> ::= <other_expression><shift_left_expression> ::= <shift_left_expression> "<<" <other_expression>in which (for your purposes) <other_expression> stands for any other type of expression.You need a rule for parsing string literals.A string literal is a type of literal, and it can be used (for your purposes) as part of an expression, like an identifier."Hello, world!" is a string literal, and a (simplified) rule for them could look something like this :<string_literal> ::= "\"" <s_char_sequence>? "\""<s_char_sequence> ::= <s_char><s_char_sequence> ::= <s_char_sequence> <s_char>in which <s_char> is any character that you want to allow inside a string literal (to keep it simple, don't allow the " character in there eg.). 这篇关于BNF语法为一个简单的c ++程序示例的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！
