From 40da3f8df72ac0cb075704837cdac79580c248dc Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Thu, 22 Mar 2018 15:06:48 +1100
Subject: [PATCH] singleton: add trivial implementation

---
 CMakeLists.txt     |  2 ++
 singleton.hpp      | 59 ++++++++++++++++++++++++++++++++++++++++++++++
 test/singleton.cpp | 14 +++++++++++
 3 files changed, 75 insertions(+)
 create mode 100644 singleton.hpp
 create mode 100644 test/singleton.cpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 295c0d3f..d62e1c27 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -361,6 +361,7 @@ list (
     signal.cpp
     signal.hpp
     si.hpp
+    singleton.hpp
     stats.cpp
     stats.hpp
     stream.cpp
@@ -525,6 +526,7 @@ if (TESTS)
         roots/bisection
         sarray
         signal
+        singleton
         stream
         string
         stringid
diff --git a/singleton.hpp b/singleton.hpp
new file mode 100644
index 00000000..bc7a6348
--- /dev/null
+++ b/singleton.hpp
@@ -0,0 +1,59 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#ifndef CRUFT_UTIL_SINGLETON_HPP
+#define CRUFT_UTIL_SINGLETON_HPP
+
+#include "debug.hpp"
+
+#include <memory>
+
+
+namespace util {
+    template <typename SelfT>
+    class singleton {
+    private:
+        singleton () = default;
+
+        static SelfT *instance;
+
+    public:
+        template <typename ...Args>
+        static std::unique_ptr<SelfT>
+        instantiate [[nodiscard]] (Args &&...args)
+        {
+            CHECK (!instance);
+
+            auto cookie =std::make_unique<SelfT> (std::forward<Args> (args)...);
+            instance = cookie.get ();
+            return cookie;
+        }
+
+
+        static SelfT&
+        get (void)
+        {
+            return *instance;
+        }
+    };
+};
+
+
+template <typename SelfT>
+SelfT*
+util::singleton<SelfT>::instance;
+
+#endif
diff --git a/test/singleton.cpp b/test/singleton.cpp
new file mode 100644
index 00000000..adbd11fc
--- /dev/null
+++ b/test/singleton.cpp
@@ -0,0 +1,14 @@
+#include "singleton.hpp"
+
+#include "tap.hpp"
+
+
+int main ()
+{
+    util::TAP::logger tap;
+
+    auto obj = util::singleton<int>::instantiate (42);
+    tap.expect_eq (util::singleton<int>::get (), 42, "singleton stores value");
+
+    return tap.status ();
+}
\ No newline at end of file