解析c++源代码时相互左递归错误

Question

我正在尝试解析cpp源语法的子集。以下ANTLR4解析器规则直接从c++语言规范中复制(除了连字符被下划线取代):

abstract_declarator:
    ptr_operator abstract_declarator?
  |  direct_abstract_declarator
;
direct_abstract_declarator:
    direct_abstract_declarator? '(' parameter_declaration_clause ')' cv_qualifier_seq? exception_specification?
   |   direct_abstract_declarator? '[' constant_expression? ']'
   |   '(' abstract_declarator ')'
;

但是当org.antlr.v4。工具正在解析语法:

错误(119):cppProcessor。g4:::以下规则集是相互左递归的[direct_abstract_declarator]

direct_abstract_declarator?左边的语法导致错误。我该如何改正呢?为什么ANTLR4不能支持它?

手动将规则重构为这种形式不会产生错误:

direct_abstract_declarator:
       direct_abstract_declarator '(' parameter_declaration_clause ')' cv_qualifier_seq? exception_specification?
     |  '(' parameter_declaration_clause ')' cv_qualifier_seq? exception_specification?
     | direct_abstract_declarator '[' constant_expression? ']'
     | '[' constant_expression? ']' 
     | '(' abstract_declarator ')'

那么在处理左递归规则时，ANTLR4是否有可能直接支持第一种语法?

Answer 1

ANTLR 4支持直接左递归，但不支持间接或隐藏左递归。您可以通过显式地展开可选结构来解决上述情况。

direct_abstract_declarator
   :   direct_abstract_declarator '(' parameter_declaration_clause ')' cv_qualifier_seq? exception_specification?
   |   direct_abstract_declarator '[' constant_expression? ']'
   |   '(' parameter_declaration_clause ')' cv_qualifier_seq? exception_specification?
   |   '[' constant_expression? ']'
   |   '(' abstract_declarator ')'
;