[et] dsl

examples/et/3_dsl.cpp

@@ -0,0 +1,87 @@
+#include <cstdio>
+#include <tuple>
+
+/* unchanged from et/2_generic_arity.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; }
+};
+
+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()...);
+	}
+};
+/* end of unchanged */
+
+namespace impl {
+template<typename T> struct IsExpr: std::false_type {};
+
+template<typename Op, typename... Operands>
+struct IsExpr<Expr<Op, Operands...>>: std::true_type {};
+
+template<typename T>
+struct IsExpr<Value<T>>: std::true_type {};
+}
+
+template<typename T>
+constexpr bool IsExpr = impl::IsExpr<T>::value;
+
+template<
+	typename LHS, typename RHS,
+	std::enable_if_t<IsExpr<LHS> && IsExpr<RHS>>* = nullptr
+>
+auto operator+(LHS lhs, RHS rhs) {
+	return Expr<Add, LHS, RHS>{lhs, rhs};
+}
+
+template<
+	typename LHS, typename RHS,
+	std::enable_if_t<IsExpr<LHS> && IsExpr<RHS>>* = nullptr
+>
+auto operator*(LHS lhs, RHS rhs) {
+	return Expr<Mul, LHS, RHS>{lhs, rhs};
+}
+
+int main() {
+	int volatile v[]{3, 5, 7};
+	int result;
+
+	Value<int> v0{v[0]}, v1{v[1]}, v2{v[2]};
+
+	auto expr = v0 + v1 * v2;
+	result = expr.eval();
+
+	std::printf("%d\n", result);
+}