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/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);