[libvmake] add meta type vmake::is_sequence_t to check if a type is a valid sequence

fixed a bug with vmake::range(start, end, step)
added an example about how to define costum sequences

libvmake/examples/custom_sequence.cpp

@@ -0,0 +1,26 @@
+#include "../vmake.hpp"
+using namespace std;
+
+// equivlant to vmake::range(2, 11, 2)
+struct my_sequence {
+	using result = int;
+
+	int p;
+	my_sequence() : p(2) {}
+
+	bool is_terminated() const noexcept {
+		return p > 10;
+	}
+
+	int operator()() {
+		p += 2;
+		return p - 2;
+	}
+};
+
+int main() {
+	static_assert(vmake::is_sequence_t<my_sequence>::value, "my_sequence is not a valid sequence!");
+	vmake::outputln(cout, " ", my_sequence{});
+	vmake::outputln(cout, " ", vmake::range(2, 11, 2));
+	return 0;
+}

libvmake/vmake.hpp

@@ -19,8 +19,26 @@ struct sequence_terminated_error : std::exception {
 	}
 };
 
-namespace polyfill {
+namespace details {
 
+template<typename T>
+auto is_sequence_helper(char)
+	-> decltype(
+	std::declval<T>()()
+	, std::declval<T>().is_terminated()
+	, typename std::enable_if<std::is_same<decltype(std::declval<T>()())
+		, typename T::result>::value, int>::type()
+	, std::true_type{});
+
+template<typename T>
+auto is_sequence_helper(int) -> std::false_type;
+
+}
+
+template<typename T>
+using is_sequence_t = decltype(details::is_sequence_helper<T>(' '));
+
+namespace polyfill {
 
 #if __cplusplus >= 201703L
 
@@ -46,7 +64,7 @@ struct iota_sequence {
 	using result = T;
 
 	T start;
-	iota_sequence(const T& start) : start(start) {}
+	iota_sequence(T&& start) : start(std::forward<T>(start)) {}
 
 	bool is_terminated() const noexcept {
 		return false;
@@ -62,10 +80,11 @@ struct ranged_sequence {
 	using result = T;
 
 	T start, end;
-	ranged_sequence(const T& start, const T& end) : start(start), end(end) {}
+	ranged_sequence(T&& start, T&& end)
+		: start(std::forward<T>(start)), end(std::forward<T>(end)) {}
 
 	bool is_terminated() const noexcept {
-		return start == end;
+		return !(start < end);
 	}
 
 	auto operator()() {
@@ -79,10 +98,12 @@ struct ranged_step_sequence {
 	using result = T;
 
 	T start, end, step;
-	ranged_step_sequence(const T& start, const T& end, const T& step) : start(start), end(end), step(step) {}
+	ranged_step_sequence(T&& start, T&& end, T&& step)
+		: start(std::forward<T>(start)), end(std::forward<T>(end))
+		, step(std::forward<T>(step)) {}
 
 	bool is_terminated() const noexcept {
-		return start == end;
+		return !(start < end);
 	}
 
 	auto operator()() {
@@ -657,6 +678,19 @@ inline void outputln(std::basic_ostream<CharT, Traits>& out, const Endl &endl =
 	out << endl;
 }
 
+namespace tests {
+
+using empty_sequence_int = decltype(nothing<int>());
+
+static_assert(is_sequence_t<empty_sequence_int>::value, "");
+static_assert(is_sequence_t<decltype(take(empty_sequence_int{}, 1))>::value, "");
+static_assert(is_sequence_t<decltype(group<20>(empty_sequence_int{}))>::value, "");
+
+static_assert(!is_sequence_t<int>::value, "");
+static_assert(!is_sequence_t<std::less<int>>::value, "");
+
+}
+
 }
 
 #endif