Util: Add get<T>() functionality with customization points to Value

src/util/json/fromjson.h

@@ -0,0 +1,118 @@
+/*
+ * Copyright (C) 2022 Riyyi
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef JSON_FROM_JSON_H
+#define JSON_FROM_JSON_H
+
+#include <algorithm> // transform
+#include <cassert>   // assert
+#include <cstddef>   // nullptr_t
+#include <map>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "util/json/array.h"
+#include "util/json/object.h"
+
+namespace Json {
+
+namespace Detail {
+
+// Required for containers with Json::Value type
+template<typename Json>
+void fromJson(const Json& json, Json& value)
+{
+	value = json;
+}
+
+template<typename Json>
+void fromJson(const Json& json, std::nullptr_t& null)
+{
+	assert(json.type() == Json::Type::Null);
+	null = nullptr;
+}
+
+template<typename Json>
+void fromJson(const Json& json, bool& boolean)
+{
+	assert(json.type() == Json::Type::Bool);
+	boolean = json.asBool();
+}
+
+template<typename Json>
+void fromJson(const Json& json, int& number)
+{
+	assert(json.type() == Json::Type::Number);
+	number = (int)json.asDouble();
+}
+
+template<typename Json>
+void fromJson(const Json& json, double& number)
+{
+	assert(json.type() == Json::Type::Number);
+	number = json.asDouble();
+}
+
+template<typename Json>
+void fromJson(const Json& json, std::string& string)
+{
+	assert(json.type() == Json::Type::String);
+	string = json.asString();
+}
+
+template<typename Json, typename T>
+void fromJson(const Json& json, std::vector<T>& array)
+{
+	assert(json.type() == Json::Type::Array);
+	array.resize(json.size());
+	std::transform(
+		json.asArray().values().begin(),
+		json.asArray().values().end(),
+		array.begin(),
+		[](const Json& json) {
+			return json.template get<T>(); // (missing-dependent-template-keyword)
+		});
+}
+
+struct fromJsonFunction {
+	template<typename Json, typename T>
+	auto operator()(const Json& json, T&& value) const
+	{
+		return fromJson(json, std::forward<T>(value));
+	}
+};
+
+// Avoid ODR (One Definition Rule) violations
+template<typename T>
+constexpr T staticConst {};
+
+} // namespace Detail
+
+// Anonymous namespace prevents multiple definition of the reference
+namespace {
+// Function object
+constexpr const auto& fromJson = Detail::staticConst<Detail::fromJsonFunction>; // NOLINT (misc-definitions-in-headers)
+} // namespace
+
+} // namespace Json
+
+#endif // JSON_FROM_JSON_H
+
+// Customization Points
+// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html
+
+// Json::fromJson is a function object, the type of which is
+// Json::Detail::fromJsonFunction. In the Json::Detail namespace are the
+// fromJson free functions. The function call operator of fromJsonFunction makes
+// an unqualified call to fromJson which, since it shares the Detail namespace
+// with the fromJson free functions, will consider those in addition to any
+// overloads that are found by argument-dependent lookup.
+
+// Variable templates are linked externally, therefor every translation unit
+// will see the same address for Detail::staticConst<Detail::fromJsonFunction>.
+// Since Json::fromJson is a reference to the variable template, it too will
+// have the same address in all translation units.

src/util/json/value.cpp

@@ -7,8 +7,7 @@
 #include <algorithm> // all_of
 #include <cassert>   // assert
 #include <cstdint>   // uint32_t
-#include <iostream>
-#include <memory> // make_shared, shared_ptr
+#include <iostream>  // istream, ostream
 #include <string>
 #include <utility> // move
 

src/util/json/value.h

@@ -13,6 +13,8 @@
 #include <iostream> // istream, ostream
 #include <string>
 
+#include "util/json/fromjson.h"
+
 namespace Json {
 
 class Array;
@@ -73,6 +75,22 @@ public:
 
 	// --------------------------------------
 
+	template<typename T>
+	T get() const
+	{
+		T type;
+		fromJson(*this, type);
+		return type;
+	}
+
+	template<typename T>
+	void getTo(T& type) const
+	{
+		fromJson(*this, type);
+	}
+
+	// --------------------------------------
+
 	Type type() const { return m_type; }
 	size_t size() const;