Make null handling YAML 1.2 compliant.

include/yaml-cpp/null.h

@@ -8,6 +8,7 @@
 #endif
 
 #include "yaml-cpp/dll.h"
+#include <string>
 
 namespace YAML {
 class Node;
@@ -17,6 +18,7 @@ inline bool operator==(const _Null&, const _Null&) { return true; }
 inline bool operator!=(const _Null&, const _Null&) { return false; }
 
 YAML_CPP_API bool IsNull(const Node& node);  // old API only
+YAML_CPP_API bool IsNullString(const std::string& str);
 
 extern YAML_CPP_API _Null Null;
 }

src/emitterutils.cpp

@@ -9,6 +9,7 @@
 #include "stringsource.h"
 #include "yaml-cpp/binary.h"  // IWYU pragma: keep
 #include "yaml-cpp/ostream_wrapper.h"
+#include "yaml-cpp/null.h"
 
 namespace YAML {
 namespace Utils {
@@ -152,12 +153,8 @@ void WriteCodePoint(ostream_wrapper& out, int codePoint) {
 
 bool IsValidPlainScalar(const std::string& str, FlowType::value flowType,
                         bool allowOnlyAscii) {
-  if (str.empty()) {
-    return false;
-  }
-
   // check against null
-  if (str == "null") {
+  if (IsNullString(str)) {
     return false;
   }
 

src/null.cpp

@@ -2,4 +2,8 @@
 
 namespace YAML {
 _Null Null;
+
+bool IsNullString(const std::string& str) {
+  return str.empty() || str == "~" || str == "null" || str == "Null" || str == "NULL";
+}
 }

src/singledocparser.cpp

@@ -11,6 +11,7 @@
 #include "yaml-cpp/eventhandler.h"
 #include "yaml-cpp/exceptions.h"  // IWYU pragma: keep
 #include "yaml-cpp/mark.h"
+#include "yaml-cpp/null.h"
 
 namespace YAML {
 SingleDocParser::SingleDocParser(Scanner& scanner, const Directives& directives)
@@ -75,7 +76,7 @@ void SingleDocParser::HandleNode(EventHandler& eventHandler) {
 
   const Token& token = m_scanner.peek();
 
-  if (token.type == Token::PLAIN_SCALAR && token.value == "null") {
+  if (token.type == Token::PLAIN_SCALAR && IsNullString(token.value)) {
     eventHandler.OnNull(mark, anchor);
     m_scanner.pop();
     return;