Merge pull request Snapchat#166 from jb-gcx/gcx/use-std-expected

Use std::expected for outcome when available

README.md

@@ -139,7 +139,8 @@ outcome<RESULT, ERROR>
 Where RESULT and ERROR can be any Djinni types (primitives or records).
 
 
-In C++, the `outcome<>` type maps to the template class `djinni::expected<>`.
+In C++, the `outcome<>` type maps to the template `djinni::expected<>`,
+which is an alias of `std::expected<>` if supported or a drop-in replacement otherwise.
 In Java, it maps to the generic class `com.snapchat.djinni.Outcome<>`. In ObjC,
 it maps to the generic class `DJOutcome<>`.
 

support-lib/cpp/expected.hpp

@@ -1,15 +1,43 @@
 #pragma once
 
+#ifdef __has_include
+#if __has_include(<version>)
+#include <version>
+#endif
+#endif
+
+#include <utility>
+#include <type_traits>
+
+#ifdef __cpp_lib_expected
+
+#include <expected>
+
+namespace djinni {
+
+using ::std::expected;
+using ::std::unexpected;
+
+}
+
+#else
+
 #include "tl_expected.hpp"
 
 namespace djinni {
 
 using ::tl::unexpected;
 using ::tl::expected;
 
+}
+
+#endif
+
+namespace djinni {
+
 template <class E>
-unexpected<typename std::decay<E>::type> make_unexpected(E &&e) {
-    return tl::unexpected<typename std::decay<E>::type>(std::forward<E>(e));
+djinni::unexpected<typename std::decay<E>::type> make_unexpected(E &&e) {
+    return djinni::unexpected<typename std::decay<E>::type>(std::forward<E>(e));
 }
 
 }