Bring documentation up-to-date

doc/associations.md

@@ -1,5 +1,5 @@
 # Associationg unrelated objects with each other
-Sometimes it is necessary to build relations between objects whose datatypes are
+Sometimes it is necessary to build associations between objects whose datatypes are
 not related via a `OneToOneRelation` or a `OneToManyRelation`. These *external
 relations* are called *Associations* in podio, and they are implemented as a
 templated version of the code that would be generated by the following yaml
@@ -188,28 +188,19 @@ template <typename FromT, typename ToT>
 using MutableAssociation = AssociationT<FromT, ToT, true>;
 ```
 
-All podio generated datatypes have the following three public typedefs
-- `DefT` yields the default type
-- `MutT` yields the mutable type
-- `CollT` yields the collection type
-
-There are corresponding template helpers to retrieve these types in
-`AssocationFwd.h`. Note that these are not
-[*SFINAE*](https://en.cppreference.com/w/cpp/language/sfinae) friendly. However,
-since they are only used in contexts internally in non-SFINAE contexts, this
-doesn't really matter.
-
 Throught the implementation it is assumed that `FromT` and `ToT` always are the
-default types. This is ensured through `static_assert`s, resp. through usage of
-the aforementioned template helpers at the possible entry points. With this in
-mind, effectively all mutating operations on `Association`s are defined using
-SFINAE using the following template structure (taking here `setFrom` as an
-example)
+default handle types. This is ensured through `static_assert`s in the
+`AssociationCollection` to make sure it can only be instantiated with those. The
+`GetDefaultHandleType` helper templates are used to retrieve the correct type
+from any `FromT` regardless of whether it is a mutable or a default handle type
+With this in mind, effectively all mutating operations on `Association`s are
+defined using [*SFINAE*](https://en.cppreference.com/w/cpp/language/sfinae)
+using the following template structure (taking here `setFrom` as an example)
 
 ```cpp
 template <typename FromU,
           typename = std::enable_if_t<Mutable && 
-                                      std::is_same_v<detail::GetDefT<FromU>, FromT>>>
+                                      std::is_same_v<detail::GetDefaultHandleType<FromU>, FromT>>>
 void setFrom(FromU value);
 ```