refactor(furvm): improve thing value access

furvm/include/furvm/thing.hpp

@@ -96,11 +96,17 @@ public:
     struct dynamic_array {
         std::size_t size;
         std::byte*  data;
+
+        static bool matches(const thing_type& type) {
+            return type.type == thing_type::Array && type.value.array.size == 0;
+        }
     };
 
     struct slice {
         std::size_t length;
         std::byte*  data;
+
+        static bool matches(const thing_type& type) { return type.type == thing_type::Slice; }
     };
 
     struct header {
@@ -407,7 +413,7 @@ public:
      */
     template <typename T>
     T& get() {
-        if (compute_size_na(*m_type) != sizeof(T)) throw bad_thing_access();
+        if (!detail::thing_traits<T>{}(*m_type)) throw bad_thing_access();
         return *std::launder(reinterpret_cast<T*>(m_data));
     }
 
@@ -418,7 +424,7 @@ public:
      */
     template <typename T>
     const T& get() const {
-        if (compute_size_na(*m_type) != sizeof(T)) throw bad_thing_access();
+        if (!detail::thing_traits<T>{}(*m_type)) throw bad_thing_access();
         return *std::launder(reinterpret_cast<const T*>(m_data));
     }
 public:

furvm/include/furvm/types.hpp

@@ -6,6 +6,8 @@
 #include <cstdint>
 #include <limits>
 #include <stdexcept>
+#include <type_traits>
+#include <utility>
 
 namespace furvm {
 
@@ -127,6 +129,76 @@ struct thing_type {
     }
 };
 
+namespace detail {
+
+template <typename T, typename = void>
+struct overrides_thing_type_matching : std::false_type {};
+
+template <typename T>
+struct overrides_thing_type_matching<T, std::void_t<decltype(T::matches(std::declval<const thing_type&>()))>>
+  : std::is_same<decltype(T::matches(std::declval<const thing_type&>())), bool> {};
+
+template <typename T>
+struct thing_traits {
+    bool operator()(const thing_type& type) const {
+        if constexpr (overrides_thing_type_matching<T>::value) {
+            return T::matches(type);
+        } else {
+            return false;
+        }
+    }
+};
+
+template <>
+struct thing_traits<s8> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::S8; }
+};
+
+template <>
+struct thing_traits<u8> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::U8; }
+};
+
+template <>
+struct thing_traits<s16> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::S16; }
+};
+
+template <>
+struct thing_traits<u16> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::U16; }
+};
+
+template <>
+struct thing_traits<s32> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::S32; }
+};
+
+template <>
+struct thing_traits<u32> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::U32; }
+};
+
+template <>
+struct thing_traits<s64> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::S64; }
+};
+
+template <>
+struct thing_traits<u64> {
+    bool operator()(const thing_type& type) const { return type.type == thing_type::U64; }
+};
+
+template <typename Inner>
+struct thing_traits<Inner*> {
+    bool operator()(const thing_type& type) const {
+        return (type.type == thing_type::Ptr || type.type == thing_type::Ref) &&
+               thing_traits<Inner>{}(*type.value.typeRef);
+    }
+};
+
+} // namespace detail
+
 } // namespace furvm
 
 #endif // FURVM_TYPES_HPP

furvm/test/test.cpp

@@ -1,3 +1,4 @@
+#include "furvm/exceptions.hpp"
 #include "furvm/furvm.hpp"
 #include "furvm/thing.hpp"
 
@@ -77,4 +78,12 @@ TEST(ThingOps, Iterators) {
     }
 }
 
+TEST(ThingOps, Access) {
+    furvm::thing thing{ furvm::thing_type{ furvm::thing_type::U8 } };
+    EXPECT_NO_THROW(thing.get<furvm::u8>());
+    EXPECT_THROW(thing.get<furvm::s8>(), furvm::bad_thing_access);
+    EXPECT_THROW(thing.get<furvm::s32>(), furvm::bad_thing_access);
+    EXPECT_THROW(thing.get<void*>(), furvm::bad_thing_access);
+}
+
 } // namespace