From 44787b1a2c2a3a0873cb8afac5760ee877191883 Mon Sep 17 00:00:00 2001
From: Dmitriy Okoneshnikov <magicwinnie@yandex.ru>
Date: Sat, 1 Feb 2025 16:20:26 +0300
Subject: [PATCH] [ #493 ] C++: fixed broken `parse_error` exception

---
 source/CHANGELOG.md                     |  1 +
 source/src/BNFC/Backend/C/CFtoBisonC.hs | 42 ++++++++++++++++++-------
 2 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/source/CHANGELOG.md b/source/CHANGELOG.md
index 7fa13f6d..d4d65cca 100644
--- a/source/CHANGELOG.md
+++ b/source/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 * C: preserve case in constructors (union): e.g. label `EInt` now is union member `eInt_` rather than `eint_`
   [[#479](https://github.com/BNFC/bnfc/issues/479)]
+* C++: repair broken `parse_error` exception throw [[#493](https://github.com/BNFC/bnfc/issues/493)]
 
 # 2.9.5
 
diff --git a/source/src/BNFC/Backend/C/CFtoBisonC.hs b/source/src/BNFC/Backend/C/CFtoBisonC.hs
index 3e6d0fb8..158b15ea 100644
--- a/source/src/BNFC/Backend/C/CFtoBisonC.hs
+++ b/source/src/BNFC/Backend/C/CFtoBisonC.hs
@@ -118,6 +118,7 @@ header mode cf = unlines $ concat
     -- Fixing regression introduced in 2.9.2.
   , when (stlParser mode)
     [ "#include <algorithm> /* for std::reverse */"  -- mandatory e.g. with GNU C++ 11
+    , "#include \"" ++ ("ParserError" <.> h) ++ "\""  -- for throwing parser_error in C++
     ]
   , [ "#include <stdio.h>"
     , "#include <stdlib.h>"
@@ -160,23 +161,42 @@ header mode cf = unlines $ concat
 unionDependentCode :: ParserMode -> String
 unionDependentCode mode = unlines
   [ "%{"
-  , errorHandler name
+  , errorHandler mode
   , "int yyparse(yyscan_t scanner, YYSTYPE *result);"
   , ""
   , "extern int yylex(YYSTYPE *lvalp, YYLTYPE *llocp, yyscan_t scanner);"
   , "%}"
   ]
-  where
-  name = parserName mode
 
-errorHandler :: String -> String
-errorHandler name = unlines
-  [ "void yyerror(YYLTYPE *loc, yyscan_t scanner, YYSTYPE *result, const char *msg)"
-  , "{"
-  , "  fprintf(stderr, \"error: %d,%d: %s at %s\\n\","
-  , "    loc->first_line, loc->first_column, msg, " ++ name ++ "get_text(scanner));"
-  , "}"
-  ]
+errorHandler :: ParserMode -> String
+errorHandler mode = case mode of
+  CParser _ _ ->
+    -- This generates error handler for C with fprintf
+    unlines
+      [ "void yyerror(YYLTYPE *loc, yyscan_t scanner, YYSTYPE *result, const char *msg)"
+      , "{"
+      , "  fprintf(stderr, \"error: %d,%d: %s at %s\\n\","
+      , "    loc->first_line, loc->first_column, msg, " ++ name ++ "get_text(scanner));"
+      , "}"
+      ]
+  CppParser _ _ ->
+    --  This generates error handler for C++ with throw parse_error
+    unlines
+      [ "void yyerror(YYLTYPE *loc, yyscan_t scanner, YYSTYPE *result, const char *msg)"
+      , "{"
+      , "  std::string error_msg = msg;"
+      , "  if (loc) {"
+      , "    error_msg += \" at line \" + std::to_string(loc->first_line) +"
+      , "                 \", column \" + std::to_string(loc->first_column);"
+      , "  }"
+      , "  if (scanner) {"
+      , "    error_msg += \": '\" + std::string(" ++ name ++ "get_text(scanner)) + \"'\";"
+      , "  }"
+      , "  throw " ++ name ++ "::parse_error(loc ? loc->first_line : -1, error_msg);"
+      , "}"
+      ]
+  where
+    name = parserName mode
 
 -- | Parser entry point code.
 --