Answer to the question
You want a new dynamic layer based on another given layer.
I would advise the following steps :
- put your country layer in a database (PostGIS for exemple)
- create a function (sql/plpgsql) in the database to derive states from a country. This function's complexity depends on the needed method.
- generate a state table by computing states (with the latter function) for every country. Keep a link between the two layers : the state table will have a country id.
- put a trigger on the country table which updates the state table, deleting and recreating the states corresponding to a given country
whenever this country is modified (deleted/updated/inserted)
You cannot do that directly in QGIS without building a plugin or a specific behaviour with human action.
Further thought - Atlas plugin
If the final intent is to use Atlas plugin to have multiple coverages per country, then this is a behaviour which is planned in a future version of Atlas plugin : coverage generation.
This would include giving a layer of feature and generating coverages from it, with parameters such as scale, number of coverage optimization, regular/irregular grids, coverage rotations…
But this new feature will be developed whenever funding will be given for that.
You can add your own comments to the feature request on Atlas plugin bugtracker :
From the edit and the comment, what the poster seems to do, is editing geometries with a topology.
Basic GIS modelisation relies on «spaghetti model» which are layers of features, which are independant, with their own geometry.
Another way of doing things is to modelize the geometries using a specific topology : the geometries of the features share a common set of canonical entities, which are shared by features. Usually this is done with a node-edge-face model.
QGis does not currently features real topological geometry edition. But here are some hints if you want to go this way :
PostGIS 2.0 has a new topology feature set. A QGis plugin is
currently developped to support this feature, enabling you to fulfill
your needs. Alpha state for the moment, but check
semi-topologic editing. There are options in QGIS enabling pseudo-topology when editing geometries : new points of the geometry sit on another feature's shape for example. See «Preferences->snapping options» and the corresponding part in QGIS Manual. This is not full topology editing since modifications to the base geometry are not automatically propagated to the other.