# Initializers¶

Initialization of core components is done via _Initializers_. An initializer is a collection of properties that each objects uses to initialize itself. These could be either required or optional. Each core class can get the value of these properties from its initializer, e.g. a solver may require number of iterations or a convergence rate.

The initializers may be created manually (C++), from config files (XML) or using wrappers (Python). These wrappers provide a useful abstractions for creating and initializing EXOTica core components directly from scripts or configuration files.

## Creating a core class¶

A core class is a class derived from MotionSolver, PlanningProblem, TaskDefinition or TaskMap. For the class to be valid, it also have to inherit from Instantiable<> templated class, e.g.:

class JointLimit: public TaskMap, public Instantiable<JointLimitInitializer>
{
public:
virtual void Instantiate(JointLimitInitializer& init);
...
}


In the case above, JointLimitInitializer is the template parameter and the type used for initialization. You can then use the parameters of JointLimitInitializer to initialize the JointLimit class:

void JointLimit::Instantiate(const JointLimitInitializer& init)
{
double percent = init.SafePercentage;
...
}


The property init.SafePercentage is of type double and it can be directly used as such.

There are two types of initializers: specialized (e.g. exotica::JointLimitInitializer) and generic (exotica::Initializer). The generic initializer contains a collection of loosely typed parameters exploiting the boost::any class. These initializers are used for parsing XML, python and other data structures and they allow you to create an initializer on run time, as opposed to compiling a specific datatype on compile time (this is useful for reflection and creating wrappers for python and Matlab).

Specialized initializers are typeset and they are used within your class to easily access the parameters a class needs to set itself up. Conversion from generic to specialized initializers is handled by EXOTica.

## Creating specialized initializers¶

Each core class requires some properties to initialize successfully and some properties that are optional. The way to declare properties, their default values and whether are required or optional is to create a specialized initializer type. This can be done by defining it in a initializer file, e.g. init/JointLimit.in contains:

extend <exotica_core/task_map>
Required double SafePercentage;


The file name (excluding the extension .in) defines the name of the type: JointLimit.in>>exotica::JointLimitInitializer. The syntax is as follows:

// Comments start with "//"
extend <exotica_core/planning_problem> // Extend existing initializers (weak inheritance)
Required double Tolerance; // Create a required property (no default value)
Required Eigen::VectorXd W; // Create a required property with included type
Required Initializer EndEffectorFrame; // Stores an initializer type, e.g. LimbInitilizer for defining spatial frames.
Optional int T = 1; // Create an optional property (default value provided)


### Compiling initializers¶

To compile a package defining new initializers, add the following lines to your CMakeLists.txt:

project(MyProjectName)

# ...

catkin_package(
# ... catkin package stuff
)

# Add all names of defined initializers (excluding the *.in extension)

# Generate initializers
GenInitializers()

# ...


You may want to create initializers for core classes, but can also use them for setting up other classes. See exotica::OMPLImpSolver for an example.
EXOTica can now be set up using initializers alone. You can either create these manually or using loaders. See the exotica_examples package for examples of initializing exotica.
See exotica_examples/src/generic.cpp for an example of initializing exotica from c++ without including any of the specialized headers.
See exotica_examples/src/xml.cpp for an example loading exotica from XML. This is similar to the way EXOTica was initialized in the past. The XML format has changed slightly though. See exotica_examples/resources/ik_solver_demo.xml for an example of the new format.