Adding a cast operator to allow frame<T> -> series<T>

Requires a check to make sure that for frame<Ts...> sizeof...(Ts) is 1

mainframe/frame.hpp

@@ -384,6 +384,20 @@ class frame
     bool
     operator==(const frame<Ts...>& other) const;
 
+    /// Cast to series<> if the frame has only one column
+    ///
+    /// This is a convenience operator for the common case of a single-column
+    /// frame. It allows you to write
+    ///
+    ///    void takes_a_series(const series<double>& s);
+    ///
+    ///    frame_row<year_month, int, double> fr1;
+    ///    fr1.push_back(2022_y/11, 1, 3.14);
+    ///    takes_a_series(fr1[_2]);
+    ///
+    template<size_t NumColumns = sizeof...(Ts)>
+    operator std::enable_if_t<NumColumns == 1, series<typename pack_element<0, Ts...>::type>>() const;
+
     /// Dereference the first row of a single-column frame
     ///
     /// This is a convenience operator for the common case of a single-column

mainframe/impl/frame.hpp

@@ -503,6 +503,13 @@ frame<Ts...>::operator==(const frame<Ts...>& other) const
     return eq_impl<0>(other);
 }
 
+template<typename... Ts>
+template<size_t NumColumns>
+frame<Ts...>::operator std::enable_if_t<NumColumns == 1, series<typename pack_element<0, Ts...>::type>>() const
+{
+    return std::get<0>(m_columns);
+}
+
 template<typename... Ts>
 template<size_t Num>
 typename std::enable_if<Num == 1, typename detail::pack_element<0, Ts...>::type>::type

tests/mainframe_test_main.cpp

@@ -3525,6 +3525,31 @@ TEST_CASE("operator*", "[frame]")
     }
 }
 
+TEST_CASE("operator()", "[frame]")
+{
+    SECTION("legitimate")
+    {
+        auto func = [](const series<year_month_day>& s) { 
+            REQUIRE(s.size() == 5);
+            REQUIRE(s.name() == "date");
+            REQUIRE(s[0] == 2022_y / January / 1);
+            REQUIRE(s[1] == 2022_y / January / 2);
+            REQUIRE(s[2] == 2022_y / January / 3);
+            REQUIRE(s[3] == 2022_y / January / 4);
+            REQUIRE(s[4] == 2022_y / January / 5);
+        };
+
+        frame<double, year_month_day, double> f1;
+        f1.set_column_names("real", "date", "imag");
+        f1.push_back( 8.9, 2022_y / January / 1, 10.2);
+        f1.push_back(10.2, 2022_y / January / 2, 10.2);
+        f1.push_back(11.1, 2022_y / January / 3,  7.7);
+        f1.push_back(12.2, 2022_y / January / 4, 10.2);
+        f1.push_back(13.3, 2022_y / January / 5, 10.2);
+
+        func(f1[_1]);
+    }
+}
 
 //template<typename Func, typename Arg>
 //struct fnobj;