1 files changed, 85 insertions, 0 deletions
diff --git a/src/development/c++/meta2.cc b/src/development/c++/meta2.cc
new file mode 100644
index 0000000..4c1194f
--- /dev/null
+++ b/src/development/c++/meta2.cc
@@ -0,0 +1,85 @@
+// Copyright (C) 2022 johannst
+
+#include <iostream>
+#include <type_traits>
+
+// Helper meta fns.
+
+template<typename T>
+using enable_if_bool = std::enable_if_t<T::value, bool>;
+
+template<typename T>
+using disable_if_bool = std::enable_if_t<!T::value, bool>;
+
+template<typename T>
+using has_dst = std::integral_constant<bool, !std::is_same<typename T::Return, void>::value>;
+
+// Template meta programming invoke machinery.
+
+namespace impl {
+    // Invoke an OPERATION which *USES* a context.
+    template<typename Ctx, template<typename> class Op, typename... P,
+             enable_if_bool<typename Op<Ctx>::HasCtx> = true>
+    typename Op<Ctx>::Return Invoke(const Ctx& C, P... params) {
+        return Op<Ctx>()(C, params...);
+    }
+
+    // Invoke an OPERATION which uses *NO* context.
+    template<typename Ctx, template<typename> class Op, typename... P,
+             disable_if_bool<typename Op<Ctx>::HasCtx> = true>
+    typename Op<Ctx>::Return Invoke(const Ctx&, P... params) {
+        return Op<Ctx>()(params...);
+    }
+}  // namespace impl
+
+// Invoke an OPERATION which *HAS* a DESTINATION with arbitrary number of arguments.
+template<typename Ctx, template<typename> class Op, typename... P,
+         enable_if_bool<has_dst<Op<Ctx>>> = true>
+void Invoke(const Ctx& C, P... params) {
+    std::cout << "Invoke " << Op<Ctx>::Name << '\n';
+    typename Op<Ctx>::Return R = impl::Invoke<Ctx, Op>(C, params...);
+    std::cout << "returned -> " << R << '\n';
+}
+
+// Invoke an OPERATION which has *NOT* a DESTINATION with arbitrary number of arguments.
+template<typename Ctx, template<typename> class Op, typename... P,
+         disable_if_bool<has_dst<Op<Ctx>>> = true>
+void Invoke(const Ctx& C, P... params) {
+    std::cout << "Invoke " << Op<Ctx>::Name << " without destination." << '\n';
+    impl::Invoke<Ctx, Op>(C, params...);
+}
+
+// Custom context.
+
+struct Ctx {
+    void out(const char* s, unsigned v) const { printf("%s%x\n", s, v); }
+};
+
+// Operations to invoke.
+
+template<typename Ctx>
+struct OpA {
+    using HasCtx = std::false_type;
+    using Return = int;
+    static constexpr const char* const Name = "OpA";
+
+    constexpr Return operator()(int a, int b) const { return a + b; }
+};
+
+template<typename Ctx>
+struct OpB {
+    using HasCtx = std::true_type;
+    using Return = void;
+    static constexpr const char* const Name = "OpB";
+
+    Return operator()(const Ctx& C, unsigned a) const { C.out("a = ", a); }
+};
+
+int main() {
+    Ctx C;
+
+    Invoke<Ctx, OpA>(C, 1, 2);
+    Invoke<Ctx, OpB>(C, 0xf00du);
+
+    return 0;
+}