[et] generic arity

examples/et/2_generic_arity.cpp

@@ -0,0 +1,61 @@
+#include <cstdio>
+#include <tuple>
+
+/* unchanged from et/1_first_draft.cpp */
+struct Add {
+	template<typename T>
+	static auto eval(T lhs, T rhs) {
+		return lhs + rhs;
+	}
+};
+
+struct Mul {
+	template<typename T>
+	static auto eval(T lhs, T rhs) {
+		return lhs * rhs;
+	}
+};
+
+template<typename T>
+class Value {
+	T _value;
+
+public:
+	Value(T value): _value(value) {}
+
+	auto eval() const { return _value; }
+};
+/* end of unchanged */
+
+template<typename Op, typename... Operands>
+class Expr {
+	std::tuple<Operands...> _operands;
+
+public:
+	Expr(Operands... operands): _operands{operands...} {}
+
+	auto eval() const {
+		return eval(std::make_index_sequence<sizeof...(Operands)>{});
+	}
+
+private:
+	template<std::size_t... Is>
+	auto eval(std::index_sequence<Is...>) const {
+		return Op::eval(std::get<Is>(_operands).eval()...);
+	}
+};
+
+/* unchanged from et/1_first_draft.cpp */
+int main() {
+	using SubExpr = Expr<Mul, Value<int>, Value<int>>;
+	using MyExpr  = Expr<Add, Value<int>, SubExpr>;
+
+	int volatile v0 = 3, v1 = 5, v2 = 7;
+	int result;
+
+	MyExpr expr{{v0}, {{v1}, {v2}}};
+	result = expr.eval();
+
+	std::printf("%d\n", result);
+}
+/* end of unchanged */