Ref T259, Ref T243 improvements of PQ

* null()
* allow chaining for makePositive
* abs()

src/blackmisc/pq/physicalquantity.cpp

@@ -147,15 +147,25 @@ namespace BlackMisc
         }
 
         template <class MU, class PQ>
-        void CPhysicalQuantity<MU, PQ>::makePositive()
+        const PQ &CPhysicalQuantity<MU, PQ>::makePositive()
         {
             if (m_value < 0) { m_value *= -1.0; }
+            return *this->derived();
         }
 
         template <class MU, class PQ>
-        void CPhysicalQuantity<MU, PQ>::makeNegative()
+        const PQ &CPhysicalQuantity<MU, PQ>::makeNegative()
         {
             if (m_value > 0) { m_value *= -1.0; }
+            return *this->derived();
+        }
+
+        template<class MU, class PQ>
+        PQ CPhysicalQuantity<MU, PQ>::abs() const
+        {
+            if (m_value >= 0) { return *this->derived(); }
+            PQ copy(*this->derived());
+            return copy.makePositive();
         }
 
         template <class MU, class PQ>
@@ -373,6 +383,13 @@ namespace BlackMisc
             return pq1 < pq2 ? pq1 : pq2;
         }
 
+        template<class MU, class PQ>
+        const PQ &CPhysicalQuantity<MU, PQ>::null()
+        {
+            static const PQ n(0, MU::nullUnit());
+            return n;
+        }
+
         template <class MU, class PQ>
         uint CPhysicalQuantity<MU, PQ>::getValueHash() const
         {

src/blackmisc/pq/physicalquantity.h

@@ -193,10 +193,13 @@ namespace BlackMisc
             bool isNegativeWithEpsilonConsidered() const;
 
             //! Make value always positive
-            void makePositive();
+            const PQ &makePositive();
 
             //! Make value always negative
-            void makeNegative();
+            const PQ &makeNegative();
+
+            //! Absolute value (always >=0)
+            PQ abs() const;
 
             //! \copydoc BlackMisc::Mixin::DBusByMetaClass::marshallToDbus
             void marshallToDbus(QDBusArgument &argument) const;
@@ -240,6 +243,9 @@ namespace BlackMisc
             //! Minimum of 2 quantities
             static const PQ &minValue(const PQ &pq1, const PQ &pq2);
 
+            //! NULL PQ
+            static const PQ &null();
+
         protected:
             //! Constructor with double
             CPhysicalQuantity(double value, MU unit);