Avoid double deletion when NoViableAltException is thrown

When ownership of the configurations set is passed to a NoViableAltException,
it get deleted more than once if the exception happens to get copied.

contributors.txt

@@ -213,3 +213,4 @@ YYYY/MM/DD, github id, Full name, email
 2018/12/20, WalterCouto, Walter Couto, WalterCouto@users.noreply.github.com
 2018/12/23, youkaichao, Kaichao You, youkaichao@gmail.com
 2019/02/06, ralucado, Cristina Raluca Vijulie, ralucris.v[at]gmail[dot]com
+2019/02/23, gedimitr, Gerasimos Dimitriadis, gedimitr@gmail.com

runtime/Cpp/runtime/src/NoViableAltException.cpp

@@ -9,6 +9,20 @@
 
 using namespace antlr4;
 
+namespace {
+
+// Create a normal shared pointer if the configurations are to be deleted. If not, then
+// the shared pointer is created with a deleter that does nothing.
+Ref<atn::ATNConfigSet> buildConfigsRef(atn::ATNConfigSet *configs, bool deleteConfigs) {
+  if (deleteConfigs) {
+    return Ref<atn::ATNConfigSet>(configs);
+  } else {
+    return Ref<atn::ATNConfigSet>(configs, [](atn::ATNConfigSet *){});
+  }
+}
+
+}
+
 NoViableAltException::NoViableAltException(Parser *recognizer)
   : NoViableAltException(recognizer, recognizer->getTokenStream(), recognizer->getCurrentToken(),
                          recognizer->getCurrentToken(), nullptr, recognizer->getContext(), false) {
@@ -17,12 +31,10 @@ NoViableAltException::NoViableAltException(Parser *recognizer)
 NoViableAltException::NoViableAltException(Parser *recognizer, TokenStream *input,Token *startToken,
   Token *offendingToken, atn::ATNConfigSet *deadEndConfigs, ParserRuleContext *ctx, bool deleteConfigs)
   : RecognitionException("No viable alternative", recognizer, input, ctx, offendingToken),
-    _deadEndConfigs(deadEndConfigs), _startToken(startToken), _deleteConfigs(deleteConfigs) {
+    _deadEndConfigs(buildConfigsRef(deadEndConfigs, deleteConfigs)), _startToken(startToken) {
 }
 
 NoViableAltException::~NoViableAltException() {
-  if (_deleteConfigs)
-    delete _deadEndConfigs;
 }
 
 Token* NoViableAltException::getStartToken() const {
@@ -30,5 +42,5 @@ Token* NoViableAltException::getStartToken() const {
 }
 
 atn::ATNConfigSet* NoViableAltException::getDeadEndConfigs() const {
-  return _deadEndConfigs;
+  return _deadEndConfigs.get();
 }

runtime/Cpp/runtime/src/NoViableAltException.h

@@ -27,10 +27,9 @@ namespace antlr4 {
 
   private:
     /// Which configurations did we try at input.index() that couldn't match input.LT(1)?
-    atn::ATNConfigSet* _deadEndConfigs;
+    /// Shared pointer that conditionally deletes the configurations (based on flag
-
+    /// passed during construction)
-    // Flag that indicates if we own the dead end config set and have to delete it on destruction.
+    Ref<atn::ATNConfigSet> _deadEndConfigs;
-    bool _deleteConfigs;
 
     /// The token object at the start index; the input stream might
     /// not be buffering tokens so get a reference to it. (At the