refs #601 Fixed thread safety of get() by adding getCopy().

src/blackmisc/valuecache.cpp

@@ -422,7 +422,20 @@ namespace BlackMisc
         return element;
     }
 
+    bool CValuePage::isValid(const Element &element, int typeId) const
+    {
+        auto reader = element.m_value.read();
+        return reader->isValid() && reader->userType() == typeId;
+    }
+
     const CVariant &CValuePage::getValue(const Element &element) const
+    {
+        Q_ASSERT(QThread::currentThread() == thread());
+
+        return element.m_value.read();
+    }
+
+    CVariant CValuePage::getValueCopy(const Element &element) const
     {
         return element.m_value.read();
     }

src/blackmisc/valuecache.h

@@ -292,9 +292,12 @@ namespace BlackMisc
         {}
 
         //! Read the current value.
-        //! \threadsafe
         const T &get() const { static const T empty {}; return *(isValid() ? static_cast<const T *>(getVariant().data()) : &empty); }
 
+        //! Get a copy of the current value.
+        //! \threadsafe
+        T getCopy() const { return isValid() ? getVariantCopy().template value<T>() : T{}; }
+
         //! Write a new value. Must be called from the thread in which the owner lives.
         CStatusMessage set(const T &value, qint64 timestamp = 0) { return m_page.setValue(m_element, CVariant::from(value), timestamp); }
 
@@ -328,7 +331,8 @@ namespace BlackMisc
         static Private::CValuePage::NotifySlot slot_cast(F slot) { return static_cast<Private::CValuePage::NotifySlot>(slot); }
 
         const QVariant &getVariant() const { return m_page.getValue(m_element).getQVariant(); }
-        bool isValid() const { return getVariant().isValid() && getVariant().userType() == qMetaTypeId<T>(); }
+        QVariant getVariantCopy() const { return m_page.getValueCopy(m_element).getQVariant(); }
+        bool isValid() const { return m_page.isValid(m_element, qMetaTypeId<T>()); }
 
         Private::CValuePage &m_page;
         Private::CValuePage::Element &m_element;

src/blackmisc/valuecacheprivate.h

@@ -57,9 +57,16 @@ namespace BlackMisc
             //! \param slot Optional member function of parent, will be called when value changes.
             Element &createElement(const QString &key, int metaType, Validator validator, const CVariant &defaultValue, NotifySlot slot);
 
+            //! True if the currently paged value is a valid instance of the given type.
+            //! \threadsafe
+            bool isValid(const Element &element, int typeId) const;
+
+            //! Read the currently paged value corresponding to the element's key.
+            const CVariant &getValue(const Element &element) const;
+
             //! Read the currently paged value corresponding to the element's key.
             //! \threadsafe
-            const CVariant &getValue(const Element &element) const;
+            CVariant getValueCopy(const Element &element) const;
 
             //! Write the value corresponding to the element's key and begin synchronizing it to any other pages.
             CStatusMessage setValue(Element &element, const CVariant &value, qint64 timestamp, bool save = false);