utils: Add a Cache class

src/utils/Cache.h

+/*****************************************************************************
+ * Media Library
+ *****************************************************************************
+ * Copyright (C) 2015 Hugo Beauzée-Luyssen, Videolabs
+ *
+ * Authors: Hugo Beauzée-Luyssen<hugo@beauzee.fr>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
+ *****************************************************************************/
+
+#pragma once
+
+#include <mutex>
+#include <cassert>
+
+template <typename T>
+class Cache
+{
+public:
+    Cache() : m_cached( false ) {}
+
+    bool isCached() const { return m_cached; }
+
+    operator T() const
+    {
+        assert( m_cached );
+        return m_value;
+    }
+
+    operator const T&() const
+    {
+        return m_value;
+    }
+
+    const T& get() const
+    {
+        return m_value;
+    }
+
+    T& get()
+    {
+        return m_value;
+    }
+
+    template <typename U>
+    T& operator=( U&& value )
+    {
+        static_assert( std::is_same<typename std::decay<T>::type, typename std::decay<U>::type>::value, "Mismatching types" );
+        m_value = std::forward<U>( value );
+        m_cached = true;
+        return m_value;
+    }
+
+    std::unique_lock<std::mutex> lock()
+    {
+        return std::unique_lock<std::mutex>( m_lock );
+    }
+
+private:
+    T m_value;
+    std::mutex m_lock;
+    bool m_cached;
+};