Fix some Node::operator[] regressions from 0.5.1

"const Node Node::operator[](const Key& key) const" changed from
returning new empty node if the key was missing in 0.5.1 to returning
a shared 'zombie' node in 0.5.2 to resolve a memory leak.

(Specifically 1025f76d was where this
was introduced)

This caused some regressions where this 'zombie' object threw exceptions
in some functions where the 'empty' object would not.

This change fixes the Node::as(fallback) method (to return the
'fallback' instead of throwing an exception) and the
Node::begin()/Node::end() methods to return default-constructed
iterators (so begin() == end() in such cases) instead of another
exception.

include/yaml-cpp/node/impl.h

@@ -149,7 +149,7 @@ inline const T Node::as() const {
 template <typename T, typename S>
 inline const T Node::as(const S& fallback) const {
   if (!m_isValid)
-    throw InvalidNode();
+    return fallback;
   return as_if<T, S>(*this)(fallback);
 }
 
@@ -282,26 +282,26 @@ inline std::size_t Node::size() const {
 
 inline const_iterator Node::begin() const {
   if (!m_isValid)
-    throw InvalidNode();
+    return const_iterator();
   return m_pNode ? const_iterator(m_pNode->begin(), m_pMemory)
                  : const_iterator();
 }
 
 inline iterator Node::begin() {
   if (!m_isValid)
-    throw InvalidNode();
+    return iterator();
   return m_pNode ? iterator(m_pNode->begin(), m_pMemory) : iterator();
 }
 
 inline const_iterator Node::end() const {
   if (!m_isValid)
-    throw InvalidNode();
+    return const_iterator();
   return m_pNode ? const_iterator(m_pNode->end(), m_pMemory) : const_iterator();
 }
 
 inline iterator Node::end() {
   if (!m_isValid)
-    throw InvalidNode();
+    return iterator();
   return m_pNode ? iterator(m_pNode->end(), m_pMemory) : iterator();
 }
 

test/node/node_test.cpp

@@ -80,6 +80,12 @@ TEST(NodeTest, MapWithUndefinedValues) {
   EXPECT_EQ(2, node.size());
 }
 
+TEST(NodeTest, UndefinedConstNodeWithFallback) {
+  Node node;
+  const Node& cn = node;
+  EXPECT_EQ(cn["undefined"].as<int>(3), 3);
+}
+
 TEST(NodeTest, MapIteratorWithUndefinedValues) {
   Node node;
   node["key"] = "value";
@@ -91,6 +97,32 @@ TEST(NodeTest, MapIteratorWithUndefinedValues) {
   EXPECT_EQ(1, count);
 }
 
+TEST(NodeTest, ConstIteratorOnConstUndefinedNode) {
+  Node node;
+  const Node& cn = node;
+  const Node& undefinedCn = cn["undefined"];
+
+  std::size_t count = 0;
+  for (const_iterator it = undefinedCn.begin(); it != undefinedCn.end(); ++it) {
+    count++;
+ }
+  EXPECT_EQ(0, count);
+}
+
+TEST(NodeTest, IteratorOnConstUndefinedNode) {
+  Node node;
+  const Node& cn = node;
+  const Node& undefinedCn = cn["undefined"];
+
+  Node& nonConstUndefinedNode = const_cast<Node&>(undefinedCn);
+
+  std::size_t count = 0;
+  for (iterator it = nonConstUndefinedNode.begin(); it != nonConstUndefinedNode.end(); ++it) {
+    count++;
+  }
+  EXPECT_EQ(0, count);
+}
+
 TEST(NodeTest, SimpleSubkeys) {
   Node node;
   node["device"]["udid"] = "12345";