Gurobi variable object. Variables are always associated with a
particular model. You create a variable object by adding a variable
to a model (using
GRBModel::addVar), rather
than by using a GRBVar constructor.
The methods on variable objects are used to get and set variable
attributes. For example, solution information can be queried by
calling get(
GRB_DoubleAttr_X). Note that you can also query attributes for
a set of variables at once. This is done using the attribute query
method on the GRBModel object
(GRBModel::get).