typeidx: ensure fundamental types have a constexpr index

typeidx.hpp

@@ -13,6 +13,7 @@
 #include <atomic>
 #include <variant>
 #include <vector>
+#include <string_view>
 
 #include <cassert>
 
@@ -22,19 +23,107 @@ namespace cruft {
         template <typename TagT>
         int typeidx_next (void)
         {
-            static std::atomic<int> counter;
+            if constexpr (std::is_same_v<TagT, void>) {
+                // This must match the highest offset of the default set of names.
+                static std::atomic<int> counter = 17;
                 return ++counter;
+            } else {
+                static std::atomic<int> counter = 0;
+                return ++counter;
+            }
         }
 
         template <typename TagT>
-        std::vector<std::string>&
+        std::vector<std::string_view>&
         name (void)
         {
-            static std::vector<std::string> s_names (1, "");
+            if constexpr (std::is_same_v<TagT, void>) {
+                // This list is a propulated form of the specialisations for
+                // index queries that appears a little below. The order of this
+                // list _must_ be _exactly_ the same in both locations so that
+                // the indexes match.
+                static std::vector<std::string_view> s_names ({
+                    "",
+                    "void",
+                    "nullptr_t",
+                    "bool",
+                    "char",
+                    "signed char",
+                    "unsigned char",
+                    "signed short",
+                    "unsigned short",
+                    "signed int",
+                    "unsigned int",
+                    "signed long",
+                    "unsigned long",
+                    "signed long long",
+                    "unsigned long long",
+                    "float",
+                    "double",
+                    "long double",
+                });
+                return s_names;
+            } else {
+                static std::vector<std::string_view> s_names (1, "");
                 return s_names;
             }
         }
 
+
+        template <typename T, typename TagT = void>
+        struct queryidx {
+            static int get (void)
+            {
+                static auto id = detail::typeidx_next<TagT> ();
+
+                static bool s_done = false;
+                if (!s_done) {
+                    auto &name = detail::name<TagT> ();
+                    assert (name.size () == std::size_t (id));
+                    name.resize (name.size () + 1);
+                    name[id] = cruft::introspection::name::full<T> ();
+                    s_done = true;
+                }
+
+                return id;
+            }
+        };
+
+        #define SPECIALISE(KLASS, IDX)          \
+        template <>                             \
+        struct queryidx<KLASS, void> {          \
+            static consteval int get (void) {   \
+                return IDX;                     \
+            }                                   \
+        };
+
+        SPECIALISE(void, 1)
+        SPECIALISE(std::nullptr_t, 2)
+
+        SPECIALISE(bool, 3)
+
+        SPECIALISE(char, 4)
+        SPECIALISE(signed char, 5)
+        SPECIALISE(unsigned char, 6)
+
+        SPECIALISE(signed short, 7)
+        SPECIALISE(unsigned short, 8)
+        SPECIALISE(signed int, 9)
+        SPECIALISE(unsigned int, 10)
+        SPECIALISE(signed long, 11)
+        SPECIALISE(unsigned long, 12)
+        SPECIALISE(signed long long, 13)
+        SPECIALISE(unsigned long long, 14)
+
+        SPECIALISE(float, 15)
+        SPECIALISE(double, 16)
+        SPECIALISE(long double, 17)
+
+
+        #undef SPECIALISE
+    }
+
+
     /// Return a globally unique runtime ID for a given type (namespaced by a
     /// tag type).
     ///
@@ -57,26 +146,15 @@ namespace cruft {
     ///               sets of IDs. The default Tag is void.
     /// \return       The unique ID of the type
     template <typename T, typename TagT = void>
-    int
+    constexpr int
     typeidx (void)
     {
-        static auto id = detail::typeidx_next<TagT> ();
+        return detail::queryidx<T,TagT>::get ();
-
-        static bool s_done = false;
-        if (!s_done) {
-            auto &name = detail::name<TagT> ();
-            assert (name.size () == std::size_t (id));
-            name.resize (name.size () + 1);
-            name[id] = cruft::introspection::name::full<T> ();
-            s_done = true;
-        }
-
-        return id;
     }
 
 
     template <typename TagT = void>
-    std::string const&
+    std::string_view
     typeidx_name (int idx)
     {
         return detail::name<TagT> ()[idx];