Add support for merge keys

Merge keys are specified here[1] for YAML 1.1. While not part of
the YAML 1.2 specification, they're very useful and are supported
in other implementations[2][3][4] that target 1.2.

Support for merge keys is optional and disabled by default. It can
be enabled by defining YAML_CPP_SUPPORT_MERGE_KEYS, either directly
or by setting the CMake option YAML_CPP_SUPPORT_MERGE_KEYS=ON.

[1]: http://yaml.org/type/merge.html
[2]: https://github.com/go-yaml/yaml
[3]: https://github.com/ruby/psych
[4]: https://bitbucket.org/ruamel/yaml

.travis.yml

@@ -5,6 +5,9 @@ os:
 compiler:
   - clang
   - gcc
+env:
+  - YAML_CPP_SUPPORT_MERGE_KEYS=ON
+  - YAML_CPP_SUPPORT_MERGE_KEYS=OFF
 before_install:
   - |
     if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
@@ -17,7 +20,7 @@ before_install:
 before_script:
   - mkdir build
   - cd build
-  - cmake ..
+  - cmake .. -DYAML_CPP_SUPPORT_MERGE_KEYS=$YAML_CPP_SUPPORT_MERGE_KEYS
 script:
   - make
   - test/run-tests

CMakeLists.txt

@@ -41,6 +41,7 @@ enable_testing()
 option(YAML_CPP_BUILD_TESTS "Enable testing" ON)
 option(YAML_CPP_BUILD_TOOLS "Enable parse tools" ON)
 option(YAML_CPP_BUILD_CONTRIB "Enable contrib stuff in library" ON)
+option(YAML_CPP_SUPPORT_MERGE_KEYS "Support YAML merge keys ('<<') in yaml-cpp's executable targets. Use '#define YAML_CPP_SUPPORT_MERGE_KEYS' instead when linking from another project." OFF)
 
 ## Build options
 # --> General
@@ -97,6 +98,10 @@ else()
 	add_definitions(-DYAML_CPP_NO_CONTRIB)
 endif()
 
+if (YAML_CPP_SUPPORT_MERGE_KEYS)
+    add_definitions(-DYAML_CPP_SUPPORT_MERGE_KEYS)
+endif()
+
 set(library_sources
   ${sources}
   ${public_headers}

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

@@ -122,12 +122,23 @@ class node {
     // NOTE: this returns a non-const node so that the top-level Node can wrap
     // it, and returns a pointer so that it can be NULL (if there is no such
     // key).
-    return static_cast<const node_ref&>(*m_pRef).get(key, pMemory);
+    node* value = static_cast<const node_ref&>(*m_pRef).get(key, pMemory);
+#ifdef YAML_CPP_SUPPORT_MERGE_KEYS
+    if (!value || value->type() == NodeType::Undefined) {
+      return get_value_from_merge_key(key, value, pMemory);
+    }
+#endif
+    return value;
   }
   template <typename Key>
   node& get(const Key& key, shared_memory_holder pMemory) {
     node& value = m_pRef->get(key, pMemory);
     value.add_dependency(*this);
+#ifdef YAML_CPP_SUPPORT_MERGE_KEYS
+    if (value.type() == NodeType::Undefined) {
+      return *get_value_from_merge_key(key, &value, pMemory);
+    }
+#endif
     return value;
   }
   template <typename Key>
@@ -159,6 +170,33 @@ class node {
   }
 
  private:
+#ifdef YAML_CPP_SUPPORT_MERGE_KEYS
+  template <typename Key>
+  inline node* get_value_from_merge_key(const Key& key, node* currentValue,
+                                        shared_memory_holder pMemory) const {
+    node* mergeValue =
+      static_cast<const node_ref&>(*m_pRef).get(std::string("<<"), pMemory);
+    if (!mergeValue) {
+      return currentValue;
+    }
+    if (mergeValue->type() == NodeType::Map) {
+      return &mergeValue->get(key, pMemory);
+    }
+    if (mergeValue->type() == NodeType::Sequence) {
+      for (const_node_iterator it = mergeValue->begin();
+            it != mergeValue->end(); ++it) {
+        if (it->pNode && it->pNode->type() == NodeType::Map) {
+          node* value = it->pNode->get(key, pMemory);
+          if (value && value->type() != NodeType::Undefined) {
+            return value;
+          }
+        }
+      }
+    }
+    return currentValue;
+  }
+#endif
+
   shared_node_ref m_pRef;
   typedef std::set<node*> nodes;
   nodes m_dependencies;

test/integration/load_node_test.cpp

@@ -185,6 +185,39 @@ TEST(LoadNodeTest, DereferenceIteratorError) {
   EXPECT_THROW(node.begin()->begin()->Type(), InvalidNode);
 }
 
+#ifdef YAML_CPP_SUPPORT_MERGE_KEYS
+TEST(NodeTest, MergeKeyScalarSupport) {
+  Node node = Load("{<<: {a: 1}}");
+  ASSERT_FALSE(!node["a"]);
+  EXPECT_EQ(1, node["a"].as<int>());
+}
+
+TEST(NodeTest, MergeKeyExistingKey) {
+  Node node = Load("{a: 1, <<: {a: 2}}");
+  ASSERT_FALSE(!node["a"]);
+  EXPECT_EQ(1, node["a"].as<int>());
+}
+
+TEST(NodeTest, MergeKeySequenceSupport) {
+  Node node = Load("<<: [{a: 1}, {a: 2, b: 3}]");
+  ASSERT_FALSE(!node["a"]);
+  ASSERT_FALSE(!node["b"]);
+  EXPECT_EQ(1, node["a"].as<int>());
+  EXPECT_EQ(3, node["b"].as<int>());
+}
+
+TEST(NodeTest, NestedMergeKeys) {
+  Node node = Load("{<<: {<<: {a: 1}}}");
+  ASSERT_FALSE(!node["a"]);
+  EXPECT_EQ(1, node["a"].as<int>());
+}
+#else
+TEST(NodeTest, MergeKeySupport) {
+  Node node = Load("{<<: {a: 1}}");
+  ASSERT_FALSE(node["a"]);
+}
+#endif
+
 TEST(NodeTest, EmitEmptyNode) {
   Node node;
   Emitter emitter;