Change the way mbo::types::Extend types are constructed to support …

…more complex and deeper nested types. (#18)

* * Change the way `mbo::types::Extend` types are constructed to support more complex and deeper nested types.

* Use static constexpr function rather than member field.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 0.2.22
+
+* Change the way `mbo::types::Extend` types are constructed to support more complex and deeper nested types.
+
 # 0.2.21
 
 * Optimize `AnyScan`, `ConstScan` and `ConvertingScan` by dropping clone layer. We also explicitly support multiple iterations on one object.

mbo/types/extend_test.cc

@@ -276,7 +276,8 @@ TEST_F(ExtendTest, StreamableComplexFields) {
   EXPECT_THAT(
       out.str(),
       Conditional(
-          kStructNameSupport, R"({25, {.name: {.first: "Hugo", .last: "Meyer"}, .age: 42}, *{{"bar", "foo"}}})",
+          kStructNameSupport,
+          R"({.index: 25, .person: {.name: {.first: "Hugo", .last: "Meyer"}, .age: 42}, .data: *{{"bar", "foo"}}})",
           R"({25, {{"Hugo", "Meyer"}, 42}, *{{"bar", "foo"}}})"));
 #ifdef __clang__
   if (HasFailure()) {

mbo/types/extender.h

@@ -106,9 +106,10 @@ namespace mbo::types::extender {
 // ```
 template<tstring ExtenderNameT, template<typename> typename ImplT, typename RequiredExtenderT = void>
 struct MakeExtender {
-  using ExtenderName = decltype(ExtenderNameT);
   using RequiredExtender = RequiredExtenderT;
 
+  static constexpr std::string_view GetExtenderName() { return decltype(ExtenderNameT)::str(); }
+
  private:
   // This friend is necessary to keep symbol visibility in check. But it has to
   // be either 'struct' or 'class' and thus results in `ImplT` being a struct.

mbo/types/internal/extend.h

@@ -30,14 +30,11 @@ namespace mbo::types::extender_internal {
 
 struct NoRequirement {};
 
-// Identify `Extender` classes by the fact that they have a type `ExtenderName`
-// that can be converted into a `std::string_view`.
+// Identify `Extender` classes by the fact that they have a member named
+// `kExtenderName` that can be converted into a `std::string_view`.
 template<typename MaybeExtender>
 concept IsExtender = requires {
-  typename MaybeExtender::ExtenderName;
-  //{
-  //    typename MaybeExtender::ExtenderName()
-  //  } -> std::convertible_to<std::string_view>;
+  { MaybeExtender::GetExtenderName() } -> std::convertible_to<std::string_view>;
 };
 
 template<typename T, typename... Ts>
@@ -136,7 +133,7 @@ struct ExtendImpl
   using RegisteredExtenders = std::tuple<Extender...>;
 
   static constexpr std::array<std::string_view, sizeof...(Extender)> RegisteredExtenderNames() {
-    return {Extender::ExtenderName::str()...};
+    return {Extender::GetExtenderName()...};
   }
 };