Prepare arb::util::optional to be used for automatic conversion from/to pybind11::object (#738)

* Add value_type and emplace method to the optional class to support automatic conversion of arb::util::optional in python.
* Add unit test for arb::util::optional::emplace.

arbor/include/arbor/util/optional.hpp

@@ -107,6 +107,7 @@ namespace detail {
         using const_rvalue_reference = typename data_type::const_rvalue_reference;
 
     public:
+        using value_type = X;
         using reference = typename data_type::reference;
         using const_reference = typename data_type::const_reference;
         using pointer = typename data_type::pointer;
@@ -165,6 +166,15 @@ namespace detail {
             }
             set = false;
         }
+
+        template <typename Y>
+        void emplace(Y&& y) {
+            if (set) {
+                data.destruct();
+            }
+            data.construct(std::forward<Y>(y));
+            set = true;
+        }
     };
 
     // type utilities

test/unit/test_optional.cpp

@@ -287,3 +287,18 @@ TEST(optional, just) {
     EXPECT_EQ(3, o2.value());
     EXPECT_EQ(4, o3.value());
 }
+
+TEST(optional, emplace) {
+    optional<int> o1(7);
+    optional<std::array<double, 3>> o2{{22., 22., 22.}};
+    int x = 42;
+    std::array<double, 3> arr{{4.5, 7.1, 1.2}};
+
+    o1.emplace(x);
+    o2.emplace(arr);
+
+    EXPECT_EQ(42, o1.value());
+    EXPECT_EQ(4.5, o2.value()[0]);
+    EXPECT_EQ(7.1, o2.value()[1]);
+    EXPECT_EQ(1.2, o2.value()[2]);
+}