Added functions for writing root

Author: liuzicheng1987

include/rfl/bson/Writer.hpp

@@ -21,7 +21,9 @@ namespace bson {
 
 /// Please refer to https://mongoc.org/libbson/current/api.html
 class Writer {
-  using ParentType = std::variant<bson_t*, bson_array_builder_t*>;
+  struct IsRoot {};
+
+  using ParentType = std::variant<bson_t*, bson_array_builder_t*, IsRoot>;
 
  public:
   struct BSONOutputArray {
@@ -44,22 +46,32 @@ class Writer {
   using OutputObjectType = BSONOutputObject;
   using OutputVarType = BSONOutputVar;
 
-  Writer() : { bson_init(&doc_); }
+  Writer() : {}
 
   ~Writer() = default;
 
-  // TODO
-  OutputArrayType array_as_root(const size_t _size) const noexcept {}
+  OutputArrayType array_as_root(const size_t _size) const noexcept {
+    bson_array_builder_t* val = bson_array_builder_new();
+    return OutputObjectType(val, IsRoot{});
+  }
 
-  // TODO
-  OutputObjectType object_as_root(const size_t _size) const noexcept {}
+  OutputObjectType object_as_root(const size_t _size) const noexcept {
+    bson_t val;
+    bson_init(&val);
+    return OutputObjectType(val, IsRoot{});
+  }
 
-  // TODO
-  OutputVarType null_as_root() const noexcept {}
+  OutputVarType null_as_root() const noexcept {
+    // Appears to be unsupported by the BSON C API.
+    return OutputVarType{};
+  }
 
-  // TODO
   template <class T>
-  OutputVarType value_as_root(const T& _var) const noexcept {}
+  OutputVarType value_as_root(const T& _var) const noexcept {
+    static_assert(rfl::always_false_v<T>,
+                  "BSON only allows arrays or objects as its root.");
+    return OutputVarType{};
+  }
 
   OutputArrayType add_array_to_array(const size_t _size,
                                      OutputArrayType* _parent) const noexcept {
@@ -154,8 +166,12 @@ class Writer {
       using Type = std::remove_cvref_t<decltype(_parent)>;
       if constexpr (std::is_same<Type, bson_array_builder_t*>()) {
         bson_array_builder_append_array_builder_end(_parent, _arr->val_);
-      } else {
+      } else if constexpr (std::is_same<Tupe, bson_t>()) {
         bson_append_array_end(_parent, _arr->val_);
+      } else if constexpr (std::is_same<Type, IsRoot>()) {
+        bson_array_builder_build(_arr->val_, doc_);
+      } else {
+        static_assert(rfl::always_false_v<Type>, "Unsupported type.");
       }
     };
     std::visit(handle, _arr->parent_);
@@ -166,14 +182,18 @@ class Writer {
       using Type = std::remove_cvref_t<decltype(_parent)>;
       if constexpr (std::is_same<Type, bson_array_builder_t*>()) {
         bson_array_builder_append_document_end(_parent, _arr->val_);
-      } else {
+      } else if constexpr (std::is_same<Tupe, bson_t>()) {
         bson_append_document_end(_parent, _arr->val_);
+      } else if constexpr (std::is_same<Type, IsRoot>()) {
+        doc_ = _obj->val_;
+      } else {
+        static_assert(rfl::always_false_v<Type>, "Unsupported type.");
       }
     };
     std::visit(handle, _arr->parent_);
   }
 
- public:
+ private:
   bson_t doc_;
 };