a working formulation integrating a new API for non-default constructable custom types, while seamlessly supporting the old API; see Tutorial.mp for details

docs/Tutorial.md

@@ -198,5 +198,50 @@ Then you could use `Vec3` wherever you could use any other type:
 ```cpp
 YAML::Node node = YAML::Load("start: [1, 3, 0]");
 Vec3 v = node["start"].as<Vec3>();
-node["end"] = Vec3(2, -1, 0);
-```
+node["end"] = Vec3{2, -1, 0};
+```
+
+Observe that in the above example the custom type is, decalred as 
+a struct, explicit default constructable and all its members are
+exposed. For non default constructable types like
+
+```cpp
+class NonDefCtorVec3 : public Vec3 {
+  using Vec3::x;
+  using Vec3::y;
+  using Vec3::z;
+ public:
+  NonDefCtorVec3(double x, double y, double z)
+      : Vec3() { this->x=x; this->y=y; this->z=z;
+  };
+};
+```
+a new API is available, that freshens up the signature of the 'convert<T>::decode'
+method and introduces the abortion of the deserialization process by throwing
+an `DecodeException`.
+
+```cpp
+namespace YAML {
+template <>
+struct convert<NonDefCtorVec3> {
+  static Node encode(const NonDefCtorVec3& rhs) {
+    return convert<Vec3>::encode(rhs);
+  }
+
+  static NonDefCtorVec3 decode(const Node& node) {
+    if (!node.IsSequence() || node.size() != 3) {
+      throw YAML::conversion::DecodeException();
+    }
+    return {node[0].as<double>(), node[1].as<double>(), node[2].as<double>()};
+  }
+};
+}
+```
+
+The behavior is exactly the same
+
+```cpp
+YAML::Node node = YAML::Load("start: [1, 3, 0]");
+NonDefCtorVec3 v = node["start"].as<NonDefCtorVec3>();
+node["end"] = NonDefCtorVec3(2, -1, 0);
+```

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

@@ -96,30 +96,84 @@ struct remove_idx<Key,
   }
 };
 
+
+//shim to emulate constexpr-if, which is a feature of c++17
+#if __cplusplus < 201703L || (defined(_MSVC_LANG) && _MSVC_LANG < 201703L)
+#define PRE_CPP17_SHIM
+#endif
+
+#ifdef PRE_CPP17_SHIM
+template <bool AorB>
+struct static_switch;
+
+template<> //new api
+struct static_switch<true> {
+  template<class T>
+  static T call(const Node& node) {
+    return convert<T>::decode(node);
+  }
+};
+
+template<>  //old api
+struct static_switch<false> {
+  template<class T>
+  static T call(const Node& node) {
+    T t;
+    if (convert<T>::decode(node, t))
+      return t;
+    throw conversion::DecodeException();
+  }
+};
+#endif
+
+//detect the method of the new api
+template <typename>
+std::false_type has_decode_new_api(long);
+
+template <typename T>
+auto has_decode_new_api(int)
+    -> decltype( T::decode(std::declval<const Node&>()), std::true_type{});
+
+
+
 template <typename T>
 inline bool node::equals(const T& rhs, shared_memory_holder pMemory) {
+
   try {
-    const auto rslt = convert<T>::decode(Node(*this, pMemory));
-    return rslt == rhs;
-  } catch (const conversion::DecodeException& e) {
-    return false;
-  } catch(...) {
-    std::rethrow_exception(std::current_exception());
+#ifdef PRE_CPP17_SHIM
+    return static_switch<decltype(has_decode_new_api<convert<T>>(
+        0))::value>::template call<T>(Node(*this, pMemory)) == rhs;
+#else
+    if constexpr (decltype(has_decode_new_api<convert<T>>(0))::value >)
+      return convert<T>::decode(Node(*this, pMemory)) == rhs;
+    else {
+      T lhs;
+      if (convert<T>::decode(Node(*this, pMemory), lhs))
+        return lhs == rhs;
+      throw conversion::DecodeException();
+    }
+#endif
+  } catch(const conversion::DecodeException& e) {
+    //throw; //prefer to throw over returning just the inability to deserialize
+    return false; //not doing this breaks upstream functionality
+  } catch (...) {
+    throw;
   }
 }
+#undef PRE_CPP17_SHIM
+
 
 inline bool node::equals(const char* rhs, shared_memory_holder pMemory) {
-  try {
-    const auto rslt =
-        convert<std::string>::decode(Node(*this, std::move(pMemory)));
-    return rslt == rhs;
-  } catch (const conversion::DecodeException& e) {
-    return false;
-  } catch(...) {
-    std::rethrow_exception(std::current_exception());
+  std::string lhs;
+  if (convert<std::string>::decode(Node(*this, std::move(pMemory)), lhs)) {
+    return lhs == rhs;
   }
+  return false;
 }
 
+
+
+
 // indexing
 template <typename Key>
 inline node* node_data::get(const Key& key,

include/yaml-cpp/node/impl.h

@@ -14,7 +14,7 @@
 #include "yaml-cpp/node/node.h"
 #include <sstream>
 #include <string>
-
+#include <type_traits>
 
 namespace YAML {
 inline Node::Node()
@@ -88,6 +88,44 @@ inline NodeType::value Node::Type() const {
 
 // access
 
+//detect the method of the new api
+template <typename>
+std::false_type has_decode_new_api(long);
+
+template <typename T>
+auto has_decode_new_api(int)
+    -> decltype( T::decode(std::declval<const Node&>()), std::true_type{});
+
+//shim to emulate constexpr-if, which is a feature of c++17
+#if __cplusplus < 201703L || (defined(_MSVC_LANG) && _MSVC_LANG < 201703L)
+#define PRE_CPP17_SHIM
+#endif
+
+#ifdef PRE_CPP17_SHIM
+template <bool AorB>
+struct static_switch;
+
+template<> //new api
+struct static_switch<true> {
+  template<class T>
+  static T call(const Node& node) {
+    return convert<T>::decode(node);
+  }
+};
+
+template<>  //old api
+struct static_switch<false> {
+  template<class T>
+  static T call(const Node& node) {
+    T t;
+    if (convert<T>::decode(node, t))
+      return t;
+    throw conversion::DecodeException();
+  }
+};
+#endif
+
+
 // template helpers
 template <typename T, typename S>
 struct as_if {
@@ -99,13 +137,25 @@ struct as_if {
       return fallback;
 
     try {
-      return convert<T>::decode(node);
+#ifdef PRE_CPP17_SHIM
+      return static_switch<decltype(has_decode_new_api<convert<T>>(
+          0))::value>::template call<T>(node);
+#else
+      if constexpr (decltype(has_decodex<convert<T>>(0))::value >)
+        return convert<T>::decodex(node);
+      else {
+        T t;
+        if (convert<T>::decode(node, t))
+          return t;
+        throw conversion::DecodeException();
+      }
+#endif
     } catch (const conversion::DecodeException& e) {
       return fallback;
     } catch (...) {
-      std::rethrow_exception(std::current_exception());
+      throw;
     }
-  }
+  };
 };
 
 //specialize for Node
@@ -125,7 +175,7 @@ struct as_if<Node, S> {
     } catch (const conversion::DecodeException& e) {
       return fallback;
     } catch (...) {
-      std::rethrow_exception(std::current_exception());
+      throw;
     }
   }
 };
@@ -154,14 +204,27 @@ struct as_if<T, void> {
       throw TypedBadConversion<T>(node.Mark());
 
     try {
-      return convert<T>::decode(node);
+#ifdef PRE_CPP17_SHIM
+      return static_switch<decltype(has_decode_new_api<convert<T>>(
+          0))::value>::template call<T>(node);
+#else
+      if constexpr (decltype(has_decodex<convert<T>>(0))::value >)
+        return convert<T>::decodex(node);
+      else {
+        T t;
+        if (convert<T>::decode(node, t))
+          return t;
+        throw conversion::DecodeException();
+      }
+#endif
     } catch(const conversion::DecodeException& e) {
-      throw TypedBadConversion<T>(node.Mark());
+        throw TypedBadConversion<T>(node.Mark());
     } catch (...) {
-      std::rethrow_exception(std::current_exception());
+      throw;
     }
   };
 };
+#undef PRE_CPP17_SHIM
 
 template <>
 struct as_if<std::string, void> {

include/yaml-cpp/node/node.h

@@ -143,6 +143,7 @@ YAML_CPP_API bool operator==(const Node& lhs, const Node& rhs);
 
 YAML_CPP_API Node Clone(const Node& node);
 
+//forward declare this customization point for all types
 template <typename T>
 struct convert;
 }

test/node/node_test.cpp

@@ -42,6 +42,30 @@ template <class T> using CustomList = std::list<T,CustomAllocator<T>>;
 template <class K, class V, class C=std::less<K>> using CustomMap = std::map<K,V,C,CustomAllocator<std::pair<const K,V>>>;
 template <class K, class V, class H=std::hash<K>, class P=std::equal_to<K>> using CustomUnorderedMap = std::unordered_map<K,V,H,P,CustomAllocator<std::pair<const K,V>>>;
 
+struct Vec3 {
+  double x, y, z;
+  bool operator==(const Vec3& rhs) const {
+    return x == rhs.x && y == rhs.y && z == rhs.z;
+  }
+};
+
+class NonDefCtorVec3 : public Vec3 {
+  using Vec3::x;
+  using Vec3::y;
+  using Vec3::z;
+ public:
+  NonDefCtorVec3(double x, double y, double z)
+      : Vec3() {
+     this->x=x;
+     this->y=y;
+     this->z=z;
+  };
+  bool operator==(const NonDefCtorVec3& rhs) const {
+    return x == rhs.x && y == rhs.y && z == rhs.z;
+  }
+};
+
+
 }  // anonymous namespace
 
 using ::testing::AnyOf;
@@ -56,6 +80,45 @@ using ::testing::Eq;
   }
 
 namespace YAML {
+
+//define custom convert structs
+template<>
+struct convert<Vec3> {
+  static Node encode(const Vec3& rhs) {
+    Node node;
+    node.push_back(rhs.x);
+    node.push_back(rhs.y);
+    node.push_back(rhs.z);
+    return node;
+  }
+
+  static bool decode(const Node& node, Vec3& rhs) {
+    if(!node.IsSequence() || node.size() != 3) {
+      return false;
+    }
+
+    rhs.x = node[0].as<double>();
+    rhs.y = node[1].as<double>();
+    rhs.z = node[2].as<double>();
+    return true;
+  }
+};
+
+template <>
+struct convert<NonDefCtorVec3> {
+  static Node encode(const NonDefCtorVec3& rhs) {
+    return convert<Vec3>::encode(rhs);
+  }
+
+  static NonDefCtorVec3 decode(const Node& node) {
+    if (!node.IsSequence() || node.size() != 3) {
+      throw YAML::conversion::DecodeException();
+    }
+    return {node[0].as<double>(), node[1].as<double>(), node[2].as<double>()};
+  }
+};
+
+
 namespace {
 TEST(NodeTest, SimpleScalar) {
   Node node = Node("Hello, World!");
@@ -674,6 +737,24 @@ TEST(NodeTest, AccessNonexistentKeyOnConstNode) {
   ASSERT_FALSE(other["5"]);
 }
 
+TEST(NodeTest, CustomClassDecoding) {
+  YAML::Node node;
+  node.push_back(1.0);
+  node.push_back(2.0);
+  node.push_back(3.0);
+  ASSERT_TRUE(node.IsSequence());
+  EXPECT_EQ(node.as<Vec3>(), (Vec3{1.0, 2.0, 3.0}));
+}
+
+TEST(NodeTest, CustomNonDefaultConstructibleClassDecoding) {
+  YAML::Node node;
+  node.push_back(1.0);
+  node.push_back(2.0);
+  node.push_back(3.0);
+  ASSERT_TRUE(node.IsSequence());
+  EXPECT_EQ(node.as<NonDefCtorVec3>(), (NonDefCtorVec3{1.0, 2.0, 3.0}));
+}
+
 class NodeEmitterTest : public ::testing::Test {
  protected:
   void ExpectOutput(const std::string& output, const Node& node) {