Add support for accessing the underlying pointer type via a dispatch (#85)

Author: mingxwa

Diff for: proxy.h

@@ -152,6 +152,11 @@ template <class D, class T, bool NE, class R, class... Args>
 concept invocable_dispatch = requires { typename D::template invoker<T>; } &&
     is_invoker_well_formed<
         typename D::template invoker<T>, T, NE, R, Args...>();
+template <class D, class P, bool NE, class R, class... Args>
+concept invocable_dispatch_ptr = invocable_dispatch<
+    D, typename ptr_traits<P>::target_type, NE, R, Args...> ||
+    invocable_dispatch<D, const P, NE, R, Args...> ||
+    invocable_dispatch<D, void, NE, R, Args...>;
 
 template <bool NE, class R, class... Args>
 using func_ptr_t = std::conditional_t<
@@ -166,14 +171,20 @@ R invoke_dispatch(Args&&... args) {
   }
 }
 template <class P, class F, class R, class... Args>
-R invocation_dispatcher(const char* self, Args... args)
+R invocation_dispatcher_ref(const char* self, Args... args)
     noexcept(is_invoker_well_formed<
         F, typename ptr_traits<P>::target_type, true, R, Args...>()) {
   return invoke_dispatch<F, R>(ptr_traits<P>::dereference(*std::launder(
       reinterpret_cast<const P*>(self))), std::forward<Args>(args)...);
 }
+template <class P, class F, class R, class... Args>
+R invocation_dispatcher_ptr(const char* self, Args... args)
+    noexcept(is_invoker_well_formed<F, const P, true, R, Args...>()) {
+  return invoke_dispatch<F, R>(*std::launder(reinterpret_cast<const P*>(self)),
+      std::forward<Args>(args)...);
+}
 template <class F, class R, class... Args>
-R invocation_default_dispatcher(const char*, Args... args)
+R invocation_dispatcher_void(const char*, Args... args)
     noexcept(is_invoker_well_formed<F, void, true, R, Args...>())
     { return invoke_dispatch<F, R>(std::forward<Args>(args)...); }
 template <class P>
@@ -208,19 +219,21 @@ struct overload_traits_impl : applicable_traits {
     static constexpr func_ptr_t<NE, R, const char*, Args...> get() {
       if constexpr (invocable_dispatch<
           D, typename ptr_traits<P>::target_type, NE, R, Args...>) {
-        return &invocation_dispatcher<P, typename D::template invoker<
+        return &invocation_dispatcher_ref<P, typename D::template invoker<
             typename ptr_traits<P>::target_type>, R, Args...>;
+      } else if constexpr (invocable_dispatch<D, const P, NE, R, Args...>) {
+        return &invocation_dispatcher_ptr<
+            P, typename D::template invoker<const P>, R, Args...>;
       } else {
-        return &invocation_default_dispatcher<
+        return &invocation_dispatcher_void<
             typename D::template invoker<void>, R, Args...>;
       }
     }
   };
   struct resolver { func_ptr_t<NE, R, Args...> operator()(Args...); };
   template <class D, class P>
-  static constexpr bool applicable_ptr = invocable_dispatch<
-      D, typename ptr_traits<P>::target_type, NE, R, Args...> ||
-      invocable_dispatch<D, void, NE, R, Args...>;
+  static constexpr bool applicable_ptr =
+      invocable_dispatch_ptr<D, P, NE, R, Args...>;
   static constexpr bool is_noexcept = NE;
 };
 template <class O> struct overload_traits : inapplicable_traits {};

Diff for: tests/proxy_invocation_tests.cpp

@@ -11,6 +11,7 @@
 #include <typeinfo>
 #include <vector>
 #include "proxy.h"
+#include "utils.h"
 
 namespace {
 
@@ -50,6 +51,25 @@ PRO_DEF_FREE_DISPATCH(Append, AppendImpl, pro::proxy<Container<T>>(T));
 PRO_DEF_MEMBER_DISPATCH_WITH_DEFAULT(WeakAt, at, NotImplemented<std::string>, std::string(int));
 PRO_DEF_FACADE(ResourceDictionary, WeakAt);
 
+template <class F, class T>
+pro::proxy<F> LockImpl(const std::weak_ptr<T>& p) {
+  auto result = p.lock();
+  if (static_cast<bool>(result)) {
+    return result;
+  }
+  return nullptr;
+}
+template <class F>
+PRO_DEF_FREE_DISPATCH(Lock, LockImpl<F>, pro::proxy<F>());
+template <class F>
+PRO_DEF_FACADE(Weak, Lock<F>, pro::copyable_ptr_constraints);
+template <class F, class T>
+auto GetWeakImpl(const std::shared_ptr<T>& p) { return pro::make_proxy<Weak<F>, std::weak_ptr<T>>(p); }
+template <class F>
+PRO_DEF_FREE_DISPATCH_WITH_DEFAULT(GetWeak, GetWeakImpl<F>, std::nullptr_t, pro::proxy<Weak<F>>());
+
+PRO_DEF_FACADE(SharedStringable, PRO_MAKE_DISPATCH_PACK(utils::spec::ToString, GetWeak<SharedStringable>), pro::copyable_ptr_constraints);
+
 }  // namespace spec
 
 template <class F, class D, bool NE, class... Args>
@@ -237,3 +257,20 @@ TEST(ProxyInvocationTests, TestFreeDispatchDefault) {
     ASSERT_TRUE(exception_thrown);
   }
 }
+
+TEST(ProxyInvocationTests, TestObserverDispatch) {
+  int test_val = 123;
+  pro::proxy<spec::SharedStringable> p{std::make_shared<int>(test_val)};
+  auto weak = p.invoke<spec::GetWeak<spec::SharedStringable>>();
+  ASSERT_TRUE(weak.has_value());
+  {
+    auto locked = weak();
+    ASSERT_TRUE(locked.has_value());
+    ASSERT_EQ(locked.invoke<utils::spec::ToString>(), "123");
+  }
+  p = &test_val;  // The underlying std::shared_ptr will be destroyed
+  ASSERT_TRUE(weak.has_value());
+  ASSERT_FALSE(weak().has_value());
+  ASSERT_FALSE(p.invoke<spec::GetWeak<spec::SharedStringable>>().has_value());
+  ASSERT_EQ(p.invoke<utils::spec::ToString>(), "123");
+}