Improve error messages on operator[] or as<> (#656)

Invalid access via operator[] or as<> will now print the offending key, if possible.

For example:

a:
  x: 1
  y: 2

node["a"]["z"].as<int>()

will say that the key "z" was invalid.

include/yaml-cpp/exceptions.h

@@ -114,6 +114,35 @@ inline const std::string KEY_NOT_FOUND_WITH_KEY(
   stream << KEY_NOT_FOUND << ": " << key;
   return stream.str();
 }
+
+template <typename T>
+inline const std::string BAD_SUBSCRIPT_WITH_KEY(
+    const T&, typename disable_if<is_numeric<T>>::type* = 0) {
+  return BAD_SUBSCRIPT;
+}
+
+inline const std::string BAD_SUBSCRIPT_WITH_KEY(const std::string& key) {
+  std::stringstream stream;
+  stream << BAD_SUBSCRIPT << " (key: \"" << key << "\")";
+  return stream.str();
+}
+
+template <typename T>
+inline const std::string BAD_SUBSCRIPT_WITH_KEY(
+    const T& key, typename enable_if<is_numeric<T>>::type* = 0) {
+  std::stringstream stream;
+  stream << BAD_SUBSCRIPT << " (key: \"" << key << "\")";
+  return stream.str();
+}
+
+inline const std::string INVALID_NODE_WITH_KEY(const std::string& key) {
+  std::stringstream stream;
+  if (key.empty()) {
+    return INVALID_NODE;
+  }
+  stream << "invalid node; first invalid key: \"" << key << "\"";
+  return stream.str();
+}
 }
 
 class YAML_CPP_API Exception : public std::runtime_error {
@@ -194,8 +223,9 @@ inline TypedKeyNotFound<T> MakeTypedKeyNotFound(const Mark& mark,
 
 class YAML_CPP_API InvalidNode : public RepresentationException {
  public:
-  InvalidNode()
-      : RepresentationException(Mark::null_mark(), ErrorMsg::INVALID_NODE) {}
+  InvalidNode(std::string key)
+      : RepresentationException(Mark::null_mark(),
+                                ErrorMsg::INVALID_NODE_WITH_KEY(key)) {}
   InvalidNode(const InvalidNode&) = default;
   virtual ~InvalidNode() YAML_CPP_NOEXCEPT;
 };
@@ -224,8 +254,10 @@ class YAML_CPP_API BadDereference : public RepresentationException {
 
 class YAML_CPP_API BadSubscript : public RepresentationException {
  public:
-  BadSubscript()
-      : RepresentationException(Mark::null_mark(), ErrorMsg::BAD_SUBSCRIPT) {}
+  template <typename Key>
+  BadSubscript(const Key& key)
+      : RepresentationException(Mark::null_mark(),
+                                ErrorMsg::BAD_SUBSCRIPT_WITH_KEY(key)) {}
   BadSubscript(const BadSubscript&) = default;
   virtual ~BadSubscript() YAML_CPP_NOEXCEPT;
 };

include/yaml-cpp/node/detail/impl.h

@@ -115,7 +115,7 @@ inline node* node_data::get(const Key& key,
         return pNode;
       return NULL;
     case NodeType::Scalar:
-      throw BadSubscript();
+      throw BadSubscript(key);
   }
 
   for (node_map::const_iterator it = m_map.begin(); it != m_map.end(); ++it) {
@@ -143,7 +143,7 @@ inline node& node_data::get(const Key& key, shared_memory_holder pMemory) {
       convert_to_map(pMemory);
       break;
     case NodeType::Scalar:
-      throw BadSubscript();
+      throw BadSubscript(key);
   }
 
   for (node_map::const_iterator it = m_map.begin(); it != m_map.end(); ++it) {

include/yaml-cpp/node/impl.h

@@ -12,13 +12,16 @@
 #include "yaml-cpp/node/detail/node.h"
 #include "yaml-cpp/node/iterator.h"
 #include "yaml-cpp/node/node.h"
+#include <sstream>
 #include <string>
 
 namespace YAML {
-inline Node::Node() : m_isValid(true), m_pMemory(nullptr), m_pNode(nullptr) {}
+inline Node::Node()
+    : m_isValid(true), m_invalidKey{}, m_pMemory(nullptr), m_pNode(nullptr) {}
 
 inline Node::Node(NodeType::value type)
     : m_isValid(true),
+      m_invalidKey{},
       m_pMemory(new detail::memory_holder),
       m_pNode(&m_pMemory->create_node()) {
   m_pNode->set_type(type);
@@ -27,6 +30,7 @@ inline Node::Node(NodeType::value type)
 template <typename T>
 inline Node::Node(const T& rhs)
     : m_isValid(true),
+      m_invalidKey{},
       m_pMemory(new detail::memory_holder),
       m_pNode(&m_pMemory->create_node()) {
   Assign(rhs);
@@ -34,24 +38,30 @@ inline Node::Node(const T& rhs)
 
 inline Node::Node(const detail::iterator_value& rhs)
     : m_isValid(rhs.m_isValid),
+      m_invalidKey(rhs.m_invalidKey),
       m_pMemory(rhs.m_pMemory),
       m_pNode(rhs.m_pNode) {}
 
 inline Node::Node(const Node& rhs)
     : m_isValid(rhs.m_isValid),
+      m_invalidKey(rhs.m_invalidKey),
       m_pMemory(rhs.m_pMemory),
       m_pNode(rhs.m_pNode) {}
 
-inline Node::Node(Zombie) : m_isValid(false), m_pMemory{}, m_pNode(nullptr) {}
+inline Node::Node(Zombie)
+    : m_isValid(false), m_invalidKey{}, m_pMemory{}, m_pNode(nullptr) {}
+
+inline Node::Node(Zombie, const std::string& key)
+    : m_isValid(false), m_invalidKey(key), m_pMemory{}, m_pNode(NULL) {}
 
 inline Node::Node(detail::node& node, detail::shared_memory_holder pMemory)
-    : m_isValid(true), m_pMemory(pMemory), m_pNode(&node) {}
+    : m_isValid(true), m_invalidKey{}, m_pMemory(pMemory), m_pNode(&node) {}
 
 inline Node::~Node() {}
 
 inline void Node::EnsureNodeExists() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   if (!m_pNode) {
     m_pMemory.reset(new detail::memory_holder);
     m_pNode = &m_pMemory->create_node();
@@ -68,14 +78,14 @@ inline bool Node::IsDefined() const {
 
 inline Mark Node::Mark() const {
   if (!m_isValid) {
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   }
   return m_pNode ? m_pNode->mark() : Mark::null_mark();
 }
 
 inline NodeType::value Node::Type() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   return m_pNode ? m_pNode->type() : NodeType::Null;
 }
 
@@ -142,7 +152,7 @@ struct as_if<std::string, void> {
 template <typename T>
 inline T Node::as() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   return as_if<T, void>(*this)();
 }
 
@@ -155,32 +165,32 @@ inline T Node::as(const S& fallback) const {
 
 inline const std::string& Node::Scalar() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   return m_pNode ? m_pNode->scalar() : detail::node_data::empty_scalar();
 }
 
 inline const std::string& Node::Tag() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   return m_pNode ? m_pNode->tag() : detail::node_data::empty_scalar();
 }
 
 inline void Node::SetTag(const std::string& tag) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   m_pNode->set_tag(tag);
 }
 
 inline EmitterStyle::value Node::Style() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   return m_pNode ? m_pNode->style() : EmitterStyle::Default;
 }
 
 inline void Node::SetStyle(EmitterStyle::value style) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   m_pNode->set_style(style);
 }
@@ -188,7 +198,7 @@ inline void Node::SetStyle(EmitterStyle::value style) {
 // assignment
 inline bool Node::is(const Node& rhs) const {
   if (!m_isValid || !rhs.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   if (!m_pNode || !rhs.m_pNode)
     return false;
   return m_pNode->is(*rhs.m_pNode);
@@ -197,14 +207,14 @@ inline bool Node::is(const Node& rhs) const {
 template <typename T>
 inline Node& Node::operator=(const T& rhs) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   Assign(rhs);
   return *this;
 }
 
 inline Node& Node::operator=(const Node& rhs) {
   if (!m_isValid || !rhs.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   if (is(rhs))
     return *this;
   AssignNode(rhs);
@@ -213,7 +223,7 @@ inline Node& Node::operator=(const Node& rhs) {
 
 inline void Node::reset(const YAML::Node& rhs) {
   if (!m_isValid || !rhs.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   m_pMemory = rhs.m_pMemory;
   m_pNode = rhs.m_pNode;
 }
@@ -221,35 +231,35 @@ inline void Node::reset(const YAML::Node& rhs) {
 template <typename T>
 inline void Node::Assign(const T& rhs) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   AssignData(convert<T>::encode(rhs));
 }
 
 template <>
 inline void Node::Assign(const std::string& rhs) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   m_pNode->set_scalar(rhs);
 }
 
 inline void Node::Assign(const char* rhs) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   m_pNode->set_scalar(rhs);
 }
 
 inline void Node::Assign(char* rhs) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   m_pNode->set_scalar(rhs);
 }
 
 inline void Node::AssignData(const Node& rhs) {
   if (!m_isValid || !rhs.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   rhs.EnsureNodeExists();
 
@@ -259,7 +269,7 @@ inline void Node::AssignData(const Node& rhs) {
 
 inline void Node::AssignNode(const Node& rhs) {
   if (!m_isValid || !rhs.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   rhs.EnsureNodeExists();
 
   if (!m_pNode) {
@@ -276,7 +286,7 @@ inline void Node::AssignNode(const Node& rhs) {
 // size/iterator
 inline std::size_t Node::size() const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   return m_pNode ? m_pNode->size() : 0;
 }
 
@@ -309,13 +319,13 @@ inline iterator Node::end() {
 template <typename T>
 inline void Node::push_back(const T& rhs) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   push_back(Node(rhs));
 }
 
 inline void Node::push_back(const Node& rhs) {
   if (!m_isValid || !rhs.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   rhs.EnsureNodeExists();
 
@@ -368,16 +378,21 @@ inline typename to_value_t<T>::return_type to_value(const T& t) {
 }
 }  // namespace detail
 
+template<typename Key>
+std::string key_to_string(const Key& key) {
+  return streamable_to_string<Key, is_streamable<std::stringstream, Key>::value>().impl(key);
+}
+
 // indexing
 template <typename Key>
 inline const Node Node::operator[](const Key& key) const {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   detail::node* value = static_cast<const detail::node&>(*m_pNode).get(
       detail::to_value(key), m_pMemory);
   if (!value) {
-    return Node(ZombieNode);
+    return Node(ZombieNode, key_to_string(key));
   }
   return Node(*value, m_pMemory);
 }
@@ -385,7 +400,7 @@ inline const Node Node::operator[](const Key& key) const {
 template <typename Key>
 inline Node Node::operator[](const Key& key) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   detail::node& value = m_pNode->get(detail::to_value(key), m_pMemory);
   return Node(value, m_pMemory);
@@ -394,28 +409,28 @@ inline Node Node::operator[](const Key& key) {
 template <typename Key>
 inline bool Node::remove(const Key& key) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   return m_pNode->remove(detail::to_value(key), m_pMemory);
 }
 
 inline const Node Node::operator[](const Node& key) const {
   if (!m_isValid || !key.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   key.EnsureNodeExists();
   m_pMemory->merge(*key.m_pMemory);
   detail::node* value =
       static_cast<const detail::node&>(*m_pNode).get(*key.m_pNode, m_pMemory);
   if (!value) {
-    return Node(ZombieNode);
+    return Node(ZombieNode, key_to_string(key));
   }
   return Node(*value, m_pMemory);
 }
 
 inline Node Node::operator[](const Node& key) {
   if (!m_isValid || !key.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   key.EnsureNodeExists();
   m_pMemory->merge(*key.m_pMemory);
@@ -425,7 +440,7 @@ inline Node Node::operator[](const Node& key) {
 
 inline bool Node::remove(const Node& key) {
   if (!m_isValid || !key.m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   key.EnsureNodeExists();
   return m_pNode->remove(*key.m_pNode, m_pMemory);
@@ -435,7 +450,7 @@ inline bool Node::remove(const Node& key) {
 template <typename Key, typename Value>
 inline void Node::force_insert(const Key& key, const Value& value) {
   if (!m_isValid)
-    throw InvalidNode();
+    throw InvalidNode(m_invalidKey);
   EnsureNodeExists();
   m_pNode->force_insert(detail::to_value(key), detail::to_value(value),
                         m_pMemory);

include/yaml-cpp/node/node.h

@@ -8,6 +8,7 @@
 #endif
 
 #include <stdexcept>
+#include <string>
 
 #include "yaml-cpp/dll.h"
 #include "yaml-cpp/emitterstyle.h"
@@ -116,6 +117,7 @@ class YAML_CPP_API Node {
  private:
   enum Zombie { ZombieNode };
   explicit Node(Zombie);
+  explicit Node(Zombie, const std::string&);
   explicit Node(detail::node& node, detail::shared_memory_holder pMemory);
 
   void EnsureNodeExists() const;
@@ -130,6 +132,8 @@ class YAML_CPP_API Node {
 
  private:
   bool m_isValid;
+  // String representation of invalid key, if the node is invalid.
+  std::string m_invalidKey;
   mutable detail::shared_memory_holder m_pMemory;
   mutable detail::node* m_pNode;
 };

include/yaml-cpp/traits.h

@@ -7,6 +7,11 @@
 #pragma once
 #endif
 
+#include <type_traits>
+#include <utility>
+#include <string>
+#include <sstream>
+
 namespace YAML {
 template <typename>
 struct is_numeric {
@@ -100,4 +105,31 @@ template <class Cond, class T = void>
 struct disable_if : public disable_if_c<Cond::value, T> {};
 }
 
+template <typename S, typename T>
+struct is_streamable {
+  template <typename SS, typename TT>
+  static auto test(int)
+      -> decltype(std::declval<SS&>() << std::declval<TT>(), std::true_type());
+
+  template <typename, typename>
+  static auto test(...) -> std::false_type;
+
+  static const bool value = decltype(test<S, T>(0))::value;
+};
+
+template<typename Key, bool Streamable>
+struct streamable_to_string {
+  static std::string impl(const Key& key) {
+    std::stringstream ss;
+    ss << key;
+    return ss.str();
+  }
+};
+
+template<typename Key>
+struct streamable_to_string<Key, false> {
+  static std::string impl(const Key&) {
+    return "";
+  }
+};
 #endif  // TRAITS_H_62B23520_7C8E_11DE_8A39_0800200C9A66

src/node_data.cpp

@@ -196,7 +196,7 @@ void node_data::insert(node& key, node& value, shared_memory_holder pMemory) {
       convert_to_map(pMemory);
       break;
     case NodeType::Scalar:
-      throw BadSubscript();
+      throw BadSubscript(key);
   }
 
   insert_map_pair(key, value);
@@ -226,7 +226,7 @@ node& node_data::get(node& key, shared_memory_holder pMemory) {
       convert_to_map(pMemory);
       break;
     case NodeType::Scalar:
-      throw BadSubscript();
+      throw BadSubscript(key);
   }
 
   for (node_map::const_iterator it = m_map.begin(); it != m_map.end(); ++it) {

test/integration/error_messages_test.cpp

@@ -0,0 +1,61 @@
+#include "yaml-cpp/yaml.h"  // IWYU pragma: keep
+
+#include "gtest/gtest.h"
+
+#define EXPECT_THROW_EXCEPTION(exception_type, statement, message) \
+  ASSERT_THROW(statement, exception_type);                         \
+  try {                                                            \
+    statement;                                                     \
+  } catch (const exception_type& e) {                              \
+    EXPECT_EQ(e.msg, message);                                     \
+  }
+
+namespace YAML {
+namespace {
+
+TEST(ErrorMessageTest, BadSubscriptErrorMessage) {
+  const char *example_yaml = "first:\n"
+                             "   second: 1\n"
+                             "   third: 2\n";
+
+  Node doc = Load(example_yaml);
+
+  // Test that printable key is part of error message
+  EXPECT_THROW_EXCEPTION(YAML::BadSubscript, doc["first"]["second"]["fourth"],
+                         "operator[] call on a scalar (key: \"fourth\")");
+  
+  EXPECT_THROW_EXCEPTION(YAML::BadSubscript, doc["first"]["second"][37],
+                         "operator[] call on a scalar (key: \"37\")");
+
+
+  // Non-printable key is not included in error message
+  EXPECT_THROW_EXCEPTION(YAML::BadSubscript,
+                         doc["first"]["second"][std::vector<int>()],
+                         "operator[] call on a scalar");
+
+  EXPECT_THROW_EXCEPTION(YAML::BadSubscript, doc["first"]["second"][Node()],
+                         "operator[] call on a scalar");
+}
+
+TEST(ErrorMessageTest, Ex9_1_InvalidNodeErrorMessage) {
+  const char *example_yaml = "first:\n"
+                             "   second: 1\n"
+                             "   third: 2\n";
+
+  const Node doc = Load(example_yaml);
+
+  // Test that printable key is part of error message
+  EXPECT_THROW_EXCEPTION(YAML::InvalidNode, doc["first"]["fourth"].as<int>(),
+                         "invalid node; first invalid key: \"fourth\"");
+  
+  EXPECT_THROW_EXCEPTION(YAML::InvalidNode, doc["first"][37].as<int>(),
+                         "invalid node; first invalid key: \"37\"");
+ 
+  // Non-printable key is not included in error message
+  EXPECT_THROW_EXCEPTION(YAML::InvalidNode,
+                         doc["first"][std::vector<int>()].as<int>(),
+                         "invalid node; this may result from using a map "
+                         "iterator as a sequence iterator, or vice-versa");
+}
+}   
+}