mp/examples/et/3_dsl.cpp

#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);
}