fix: add quantity_point::origin, like std::chrono::time_point::clock

docs/framework/basic_concepts.rst

@@ -23,7 +23,7 @@ The most important concepts in the library are `Unit`, `Dimension`,
     ]
 
     [<abstract>QuantityPoint|
-    [quantity_point<Dimension, Unit, Rep>]
+    [quantity_point<PointOrigin, Unit, Rep>]
     ]
 
     [<abstract>QuantityKind|
@@ -39,6 +39,10 @@ The most important concepts in the library are `Unit`, `Dimension`,
     [Unit]<-[Quantity]
     [Quantity]<-[QuantityPoint]
 
+    [<abstract>PointOrigin]<-[QuantityPoint]
+    [Dimension]<-[PointOrigin]
+    [PointOrigin]<-[PointKind]
+
     [<abstract>Kind]<-[QuantityKind]
     [Dimension]<-[Kind]
     [Quantity]<-[QuantityKind]
@@ -60,7 +64,7 @@ derived dimensions. Examples: ``si::dim_time``, ``si::dim_length``, ``si::dim_sp
 specific representation. Examples: ``quantity<si::dim_time, si::second, int>``,
 ``si::length<si::metre, int>``, ``si::speed<si::kilometre_per_hour>``.
 
-`QuantityPoint` is an absolute `Quantity` with respect to some origin.
+`QuantityPoint` is an absolute `Quantity` with respect to an origin.
 Examples: timestamp (as opposed to duration), absolute temperature
 (as opposed to temperature difference).
 
@@ -68,6 +72,6 @@ Examples: timestamp (as opposed to duration), absolute temperature
 distance (``horizonal_kind``) and height (``vertical_kind``) are different kinds
 of a length quantity.
 
-`QuantityPointKind` is an absolute `QuantityKind` with respect to some origin.
+`QuantityPointKind` is an absolute `QuantityKind` with respect to an origin.
 Examples: altitude is a quantity point of ``vertical_kind`` (as opposed to
 height).

docs/framework/dimensions.rst

@@ -106,8 +106,9 @@ Quantity Points
 +++++++++++++++
 
 Quantity points have a more restricted set of operations.
-Quantity can be added to or subtracted from a quantity point.
-The result will always be a quantity point of the same dimension:
+Quantity can be added to or subtracted
+from a quantity point of the same origin.
+The result will always be a quantity point of the same origin:
 
 .. code-block::
     :emphasize-lines: 3-5
@@ -132,9 +133,10 @@ The result is a relative quantity of the same dimension:
 
     It is not allowed to:
 
-    - add quantity points
-    - subtract a quantity point from a quantity:
-    - multiply nor divide quantity points with anything else.
+    - add quantity points,
+    - subtract a quantity point from a quantity,
+    - multiply nor divide quantity points with anything else, and
+    - mix quantity points with different origins:
 
     .. code-block::
         :emphasize-lines: 3-5
@@ -144,6 +146,8 @@ The result is a relative quantity of the same dimension:
         auto res1 = quantity_point{dist1} + quantity_point{dist2};  // ERROR
         auto res2 = dist1 - quantity_point{dist2};                  // ERROR
         auto res3 = quantity_point{dist1} / (2 * s);                // ERROR
+        auto res4 = quantity_point{std::chrono::utc_second{1s}} +
+                    quantity_point{std::chrono::sys_second{1s}};    // ERROR
 
 Quantity Point Kinds
 ++++++++++++++++++++

docs/framework/quantity_points.rst

@@ -3,9 +3,20 @@
 Quantity Points
 ===============
 
-A quantity point is an absolute quantity with respect to zero
-(which represents some origin) and is represented in the library with a
-`quantity_point` class template.
+A quantity point is an absolute quantity with respect to an origin
+and is represented in the library with a `quantity_point` class template.
+
+Point Origins
+-------------
+
+We need a `point_origin` to represent the origin of a quantity point::
+
+    struct mean_sea_level : point_origin<si::dim_length> {};
+
+Quantities points with this origin represent a point from the mean sea level.
+
+The library offers a `dynamic_origin<Dimension>`
+for quantity points whose origin is not specified in the type system.
 
 
 Construction
@@ -14,7 +25,7 @@ Construction
 To create the quantity point object from a `quantity` we just have to pass
 the value to the `quantity_point` class template explicit constructor::
 
-    quantity_point<si::dim_length, si::kilometre, double> d(123 * km);
+    quantity_point<dynamic_origin<si::dim_length>, si::kilometre, double> d(123 * km);
 
 .. note::
 
@@ -25,7 +36,7 @@ the value to the `quantity_point` class template explicit constructor::
     `copy initialization <https://en.cppreference.com/w/cpp/language/copy_initialization>`_
     **does not compile**::
 
-        quantity_point<si::dim_length, si::kilometre, double> d = 123 * km;  // ERROR
+        quantity_point<dynamic_origin<si::dim_length>, si::kilometre, double> d = 123 * km;  // ERROR
 
 
 Differences To Quantity