mp/examples/et/4_separate.cpp

#include <cstdio>
#include <tuple>

/* **** */
/* tags */

namespace tag {

struct Add;
struct Mul;
struct Minus;

}

/* ********** */
/* expression */

template<typename Op, typename... Operands>
class Expr {
	std::tuple<Operands...> _operands;

public:
	Expr(Operands... operands): _operands{operands...} {}

	template<template<typename> class Evaluator>
	auto eval() const {
		return eval<Evaluator>(std::make_index_sequence<sizeof...(Operands)>{});
	}

private:
	template<template<typename> class Evaluator, std::size_t... Is>
	auto eval(std::index_sequence<Is...>) const {
		return Evaluator<Op>::eval(std::get<Is>(_operands).template eval<Evaluator>()...);
	}
};

template<typename T>
class Value {
	T _value;

public:
	Value(T value): _value(value) {}

	template<template<typename> class>
	auto eval() const { return _value; }
};

/* *********** */
/* type traits */

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<tag::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<tag::Mul, LHS, RHS>{lhs, rhs};
}

template<
	typename RHS,
	std::enable_if_t<IsExpr<RHS>>* = nullptr
>
auto operator-(RHS rhs) {
	return Expr<tag::Minus, RHS>{rhs};
}

/* ********** */
/* evaluators */

template<typename> struct EvaluatorElem;

template<>
struct EvaluatorElem<tag::Add> {
	template<typename T>
	static auto eval(T lhs, T rhs) { return lhs + rhs; }
};

template<>
struct EvaluatorElem<tag::Mul> {
	template<typename T>
	static auto eval(T lhs, T rhs) { return lhs * rhs; }
};

template<>
struct EvaluatorElem<tag::Minus> {
	template<typename T>
	static auto eval(T rhs) { return -rhs; }
};

/* *** */

template<typename> struct EvaluatorBool;

template<>
struct EvaluatorBool<tag::Add> {
	static auto eval(bool lhs, bool rhs) { return lhs or rhs; }
};

template<>
struct EvaluatorBool<tag::Mul> {
	static auto eval(bool lhs, bool rhs) { return lhs and rhs; }
};

template<>
struct EvaluatorBool<tag::Minus> {
	static auto eval(bool rhs) { return not rhs; }
};

/* *** */

template<template<typename> class Evaluator, typename Expr>
auto eval(Expr expr) {
	return expr.template eval<Evaluator>();
}

int main() {
	int volatile v[]{1, 1, 1};
	int result;

	Value<int> v0{v[0]}, v1{v[1]}, v2{v[2]};

	auto expr = v0 * -(v1 + v2);

	result = eval<EvaluatorElem>(expr);
	std::printf("evaluator elem: %d\n", result);

	result = eval<EvaluatorBool>(expr);
	std::printf("evaluator bool: %d\n", result);
}