msvc countof ut

src/libipc/utility/countof.h

@@ -9,6 +9,7 @@
 #include <cstddef>  // std::size_t
 
 #include "libipc/def.h"
+#include "libipc/utility/generic.h"
 
 LIBIPC_NAMESPACE_BEG_
 
@@ -21,14 +22,56 @@ constexpr std::size_t countof(T const (&)[N]) noexcept {
   return N;
 }
 
-template <typename C>
+namespace detail {
-constexpr auto countof(C const &c) -> decltype(c.size()) {
+
-  return c.size();
+template <typename C, typename = void>
-}
+struct countof_trait_has_size {
+  enum : bool { value = false };
+};
 
 template <typename C>
-constexpr auto countof(C const &c) -> decltype(c.Size()) {
+struct countof_trait_has_size<C, void_t<decltype(std::declval<C>().size())>> {
-  return c.Size();
+  enum : bool { value = true };
+};
+
+template <typename C, typename = void>
+struct countof_trait_has_Size {
+  enum : bool { value = false };
+};
+
+template <typename C>
+struct countof_trait_has_Size<C, void_t<decltype(std::declval<C>().Size())>> {
+  enum : bool { value = true };
+};
+
+template <typename C, bool = countof_trait_has_size<C>::value
+                    , bool = countof_trait_has_Size<C>::value>
+struct countof_trait;
+
+template <typename C>
+struct countof_trait<C, true, false> {
+  constexpr static auto countof(C const &c) noexcept(noexcept(c.size())) {
+    return c.size();
+  }
+  using return_t = decltype(countof_trait::countof(std::declval<C>()));
+  enum : bool { noexcept_value = noexcept(countof_trait::countof(std::declval<C>())) };
+};
+
+template <typename C>
+struct countof_trait<C, false, true> {
+  constexpr static auto countof(C const &c) noexcept(noexcept(c.Size())) {
+    return c.Size();
+  }
+  using return_t = decltype(countof_trait::countof(std::declval<C>()));
+  enum : bool { noexcept_value = noexcept(countof_trait::countof(std::declval<C>())) };
+};
+
+} // namespace detail
+
+template <typename C, typename R = detail::countof_trait<C>>
+constexpr auto countof(C const &c) noexcept(R::noexcept_value)
+  -> typename R::return_t {
+  return R::countof(c);
 }
 
 LIBIPC_NAMESPACE_END_

src/libipc/utility/generic.h

@@ -0,0 +1,20 @@
+/**
+ * @file src/generic.h
+ * @author mutouyun (orz@orzz.org)
+ * @brief Tools for generic programming
+ * @date 2022-03-01
+ */
+#pragma once
+
+#include "libipc/def.h"
+
+LIBIPC_NAMESPACE_BEG_
+
+/**
+ * @brief Utility metafunction that maps a sequence of any types to the type void
+ * @see https://en.cppreference.com/w/cpp/types/void_t
+*/
+template <typename...>
+using void_t = void;
+
+LIBIPC_NAMESPACE_END_