From f838e7c4aaa04d5084f6922aa516a845b19e78e9 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Fri, 4 Oct 2024 20:10:05 +0200
Subject: fn: add simple fn signature checker example

---
 fn_alias.cc             |  32 ++++++++++++++
 fn_signature_checker.cc | 110 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 142 insertions(+)
 create mode 100644 fn_alias.cc
 create mode 100644 fn_signature_checker.cc

diff --git a/fn_alias.cc b/fn_alias.cc
new file mode 100644
index 0000000..2d9fd1d
--- /dev/null
+++ b/fn_alias.cc
@@ -0,0 +1,32 @@
+#include <cassert>
+#include <cstdio>
+
+// Function alias.
+
+void say() {
+  std::puts("say");
+}
+
+inline constexpr auto say_alias = say;
+
+// Template function alias.
+//
+// One drawback is that template type deducation does not work through the
+// function pointer. One would need to write some macro wrapper.
+
+template <typename T, typename U>
+auto add(T t, U u) -> decltype(t + u) {
+  return t + u;
+}
+
+template <typename U>
+inline constexpr auto add_int = add<int, U>;
+
+// Test me.
+
+int main() {
+  say();
+  say_alias();
+
+  assert(add(2, 3) == add_int<int>(3, 2));
+}
diff --git a/fn_signature_checker.cc b/fn_signature_checker.cc
new file mode 100644
index 0000000..985c719
--- /dev/null
+++ b/fn_signature_checker.cc
@@ -0,0 +1,110 @@
+#if 0
+set -xe
+g++     -std=c++11 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
+clang++ -std=c++11 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
+g++     -std=c++20 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
+clang++ -std=c++20 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
+exit 0
+#endif
+
+#include <type_traits>
+
+// Function signature checker for generic functions.
+//
+// This demonstrates a way to do some simple function signature checking on
+// template arguments w/o implementing the whole concept emulation with
+// std::void_t prior to cpp20. This may not be the best solution, but it gives a
+// small tool at hand which can easily be employed into older cpp code bases.
+//
+// Starting with cpp20, one can fully switch to concepts for this task.
+
+// -- PRE CPP20 ----------------------------------------------------------------
+
+// (1) Primary template to extract the function signature type.
+template <typename Fn>
+struct fn;
+
+// (2) Partial specialization for function types.
+template <typename Ret, typename... Args>
+struct fn<Ret(Args...)> {
+  using type = Ret(Args...);
+};
+
+// (3) Partial specialization for function pointer types.
+//
+// This specialization is disabled on purpose as it allows for writing ambiguous
+// checks for either non-static or static member functions.
+//
+// template <typename Ret, typename... Args>
+// struct fn<Ret (*)(Args...)> {
+//   using type = Ret(Args...);
+// };
+
+// (4) Partial specialization for member function pointer types.
+template <typename T, typename Ret, typename... Args>
+struct fn<Ret (T::*)(Args...)> {
+  using type = Ret(Args...);
+};
+
+template <typename Fn>
+using fn_t = typename fn<Fn>::type;
+
+template <typename T>
+void do_work() {
+  // Check for non-static member function.
+  //
+  // If the partial specialization (3) is enabled, this could also match a
+  // static member function T::get.
+  static_assert(std::is_same<fn_t<int(int)>, fn_t<decltype(&T::get)>>::value,
+                "T has no member function 'int get(int)'");
+  // Check for non-static member function.
+  static_assert(std::is_same<fn_t<void(int)>, fn_t<decltype(&T::set)>>::value,
+                "T has no member function 'void set(int)'");
+  // Check for static member function.
+  static_assert(std::is_same<fn_t<int()>, fn_t<decltype(T::cnt)>>::value,
+                "T has no static function 'static int cnt()'");
+}
+
+struct good {
+  int get(int);
+  void set(int);
+  static int cnt();
+};
+
+struct bad {
+  int get(int);
+  int set();
+  static int cnt();
+};
+
+void check() {
+  do_work<good>();
+
+  // TODO: Enable to see a failing check due to wrong function signature.
+  // do_work<bad>();
+}
+
+// -- SINCE CPP20 --------------------------------------------------------------
+
+#if __cplusplus >= 202002L
+#include <concepts>
+
+template <typename T>
+concept MyType = requires(T t) {
+  { t.get(int{}) } -> std::same_as<int>;
+  { t.set(int{}) } -> std::same_as<void>;
+  { T::cnt() } -> std::same_as<int>;
+};
+
+template <MyType T>
+void do_work2() {
+}
+
+void check2() {
+  do_work2<good>();
+
+  // TODO: Enable to see failing to satisfy the concept MyType.
+  // do_work2<bad>();
+}
+
+#endif
-- 
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