- Use `value_type` when the returned type is not a reference
- Enforce `reference = value_type` if the returned type is not
a reference (to avoid `typename PM::reference r = get(m, k)`,
which will take a ref to a temporary if the `reference`
typedef is an actualy reference)
- Do not use `put_get_helper` if the property map is not a `lvalue`
**mutable** property map: the `put()` is `map[k] = v`, which
is broken if `operator[]` does not return a reference
- The concept `boost::lvalue_property_map_tag` requires `operator[](key)`,
not a reference in `get(map, key)`. You can have a readable property map
returning a reference through its `get(map, key)`, but if there is
no `operator[]`, it's just a `boost::readable_property_map_tag`
- Some const correctness to avoid copying maps with state
or heavy keys in `get(map, key)` / `put(map, key, value)`
- Base the category of a wrapping pmap on what it offers instead
of just forwarding the base property map's category
- Tried to do something like mutable lvalue pmap:
* `value_type& operator[](key&)`
* `ref get(map, const key&)`
* `put(map, const key&, const value_type&)`
and non-mutable lvalue pmap:
* `const value_type& operator[](const key&)`
* `ref get(map, const key&)`
but not everything fits properly...
Sébastien Loriot
Mael Rouxel-Labbé
Andreas Fabri
Maxime Gimeno