Getting Access to The Underlying Optimizer

In some advanced situations, you may want to access the underlying optimizer of an optimization model. For instance, this could be the case if you want to get optimizer-specific information which are not exposed by the Model interface.

In this case, you can use the Model::optimizer method. Note that this method will return a reference of type Optimizer&.

To access optimizer-specific routines, you will need to cast the reference to the appropriate optimizer type. This is made easier by the Optimizer::as and Optimizer::is methods, which allow you to cast the optimizer to a specific type or check if it is of a specific type.

For instance, the following code shows how to access the underlying Gurobi optimizer of model.

Env env;
Model model(env);
auto x = model.add_var(0.0, 1.0, Continuous, -1, "x");

model.use(Gurobi());
model.optimize();

if (!model.optimizer().is<Optimizers::Gurobi>()) {
    throw Exception("The optimizer is not Gurobi");
}

auto& gurobi_optimizer = model.optimizer().as<Optimizers::Gurobi>();

// Access the optimizer-specific routines
GRBVar& var = gurobi_optimizer[x];

Warning

A common mistake is to try to cast the optimizer to the wrong type. In particular, to cast optimizer to its optimizer factory type (e.g., Gurobi instead of Optimizers::Gurobi). If so, this will result in a runtime error.