c++: pre c++20 concept example

2 files changed, 79 insertions, 0 deletions

diff --git a/src/development/c++.md b/src/development/c++.md
index b42a263..1cc4003 100644
--- a/src/development/c++.md
+++ b/src/development/c++.md
@@ -32,9 +32,35 @@ available traits an operation can have:
 
 ```cpp
 {{#include c++/meta2.cc:3:}}
+```
+
+## Example: Concepts pre c++20
+
+Prior to c++20's concepts, `SFINAE` and `std::void_t` can be leveraged to build
+something similar allowing to define an interface (aka trait) for a template
+parameter.
 
+```cpp
+{{#include c++/concepts-11.cc:3:}}
 ```
 
+The main mechanic can be explained with the following reduced example. If one
+of the `decltype(std:declval<T>...` expressions is ill-formed, the template
+specialization for `is_valid` will be removed from the candidate set due to
+[SFINAE][sfinae].
+```cpp
+template<typename T, typename = void>
+struct is_valid : std::false_type {};
+
+template<typename T>
+struct is_valid<T, std::void_t<
+                       decltype(std::declval<T>().some_fun1()),
+                       decltype(std::declval<T>().some_fun2())
+               >> : std::true_type {};
+```
+> `std::declval<T>()` creates an instance of type T in an unevaluated context.
+
+
 [gist-strict-asliasing]: https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
 [parameter-pack]: https://en.cppreference.com/w/cpp/language/parameter_pack
 [enable-if]: https://en.cppreference.com/w/cpp/types/enable_if
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>>, "");
+}