c++: pre c++20 concept example

1 files changed, 53 insertions, 0 deletions

diff --git a/src/development/c++/concepts-11.cc b/src/development/c++/concepts-11.cc
new file mode 100644
index 0000000..888ff4d
--- /dev/null
+++ b/src/development/c++/concepts-11.cc
@@ -0,0 +1,53 @@
+// Copyright (C) 2022 johannst
+#include <type_traits>
+
+template<typename T, template<typename> class Checker, typename = void>
+struct is_valid : std::false_type {};
+
+template<typename T, template<typename> class Checker>
+struct is_valid<T, Checker, std::void_t<Checker<T>>> : std::true_type {};
+
+template<typename T, template<typename> class Checker>
+static constexpr bool is_valid_v = is_valid<T, Checker>::value;
+
+// -----------------------------------------------------------------------------
+
+template<typename T, typename R, template<typename> class Checker, typename = void>
+struct is_valid_with_ret : std::false_type {};
+
+template<typename T, typename R, template<typename> class Checker>
+struct is_valid_with_ret<T, R, Checker, std::void_t<Checker<T>>> : std::is_same<R, Checker<T>> {};
+
+template<typename T, typename R, template<typename> class Checker>
+static constexpr bool is_valid_with_ret_v = is_valid_with_ret<T, R, Checker>::value;
+
+// -----------------------------------------------------------------------------
+
+template<typename T>
+struct is_entry {
+    template<typename TT>
+    using init = decltype(std::declval<TT>().init());
+    template<typename TT>
+    using tag = decltype(std::declval<TT>().tag());
+    template<typename TT>
+    using val = decltype(std::declval<TT>().val());
+
+    static constexpr bool value = is_valid_v<T, init> &&
+                                  is_valid_with_ret_v<T, int, tag> &&
+                                  is_valid_with_ret_v<T, typename T::Type, val>;
+};
+
+template<typename T>
+static constexpr bool is_entry_v = is_entry<T>::value;
+
+template<typename E>
+struct Entry {
+    using Type = E;
+    void init();
+    int tag() const;
+    E val() const;
+};
+
+int main() {
+    static_assert(is_entry_v<Entry<bool>>, "");
+}