Add simgear::optional

This is a poor-man’s proxy for std::optional until we have it.

simgear/misc/CMakeLists.txt

@@ -20,6 +20,7 @@ set(HEADERS
     texcoord.hxx
     test_macros.hxx
     lru_cache.hxx
+    simgear_optional.hxx
     )
 
 set(SOURCES

simgear/misc/simgear_optional.hxx

@@ -0,0 +1,95 @@
+// -*- coding: utf-8 -*-
+//
+// simgear_optional.hxx --- Mimic std::optional until we can use C++14
+// Copyright (C) 2020  James Turner <james@flightgear.org>
+//
+// This library is free software; you can redistribute it and/or
+// modify it under the terms of the GNU Library General Public
+// License as published by the Free Software Foundation; either
+// version 2 of the License, or (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Library General Public License for more details.
+//
+// You should have received a copy of the GNU Library General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+// MA  02110-1301  USA.
+
+
+#pragma once
+
+#include <simgear/structure/exception.hxx>
+
+namespace simgear
+{
+
+/**
+ * Inefficient version of std::optional. It requires a default-constructable,
+ * copyable T type, unlike the real version.
+ */
+
+template <class T>
+class optional
+{
+public:
+    using value_type  = T;
+
+    optional() = default;
+
+    optional(const T& v) :
+        _value(v),
+        _haveValue(true)
+    {}
+
+    optional(const optional<T>& other) :
+        _value(other._value),
+        _haveValue(other._haveValue)
+    {}
+
+    optional<T>& operator=(const optional<T>& other)
+    {
+        _haveValue = other._haveValue;
+        _value = other._value;
+        return *this;
+    }
+
+    explicit operator bool() const
+    {
+        return _haveValue;
+    }
+
+    bool has_value() const
+    {
+        return _haveValue;
+    }
+
+    const T& value() const
+    {
+        if (!_haveValue) {
+            throw sg_exception("No value in optional");
+        }
+        return _value;
+    }
+
+    T& value()
+    {
+        if (!_haveValue) {
+            throw sg_exception("No value in optional");
+        }
+        return _value;
+    }
+
+    void reset()
+    {
+        _haveValue = false;
+        _value = {};
+    }
+private:
+    T _value = {};
+    bool _haveValue = false;
+};
+
+} // of namespace simgear