# Descriptors¶

A descriptor is an object that represents an algorithm including all its meta-parameters, dependencies on other algorithms, floating-point types, computational methods and tasks. A descriptor serves as:

• A dispatching mechanism for operations. Based on a descriptor type, an operation executes a particular algorithm implementation.

• An aggregator of meta-parameters. It provides an interface for setting up meta-parameters at either compile-time or run-time.

• An object that stores the state of the algorithm. In the general case, a descriptor is a stateful object whose state changes after an operation is applied.

Each oneDAL algorithm has its own dedicated namespace, where the corresponding descriptor is defined (for more details, see Namespaces). Descriptor, in its turn, defines the following:

• Template parameters. A descriptor is allowed to have any number of template parameters, but shall support at least three:

• Float is a floating-point type that the algorithm uses for computations. This parameter is defined first and has the oneapi::dal::default_float_t default value.

• Method is a tag-type that specifies the computational method. This parameter is defined second and has the method::by_default default value.

• Task is a tag-type that specifies the computational task. This parameter is defined third and has the task::by_default default value.

• Properties. A property is a run-time parameter that can be accessed by means of the corresponding getter and setter methods.

The following code sample shows the common structure of a descriptor’s definition for an abstract algorithm. To define a particular algorithm, the following strings shall be substituted:

• %ALGORITHM% is the name of an algorithm and its namespace. All classes and structures related to that algorithm are defined within the namespace.

• %PROPERTY_NAME% and %PROPERTY_TYPE% are the name and the type of one of the algorithm’s properties.

namespace oneapi::dal::%ALGORITHM% {

template <typename Float  = default_float_t,
typename Method = method::by_default,
/* more template parameters */>
class descriptor {
public:
/* Constructor */
descriptor(const %PROPERTY_TYPE%& %PROPERTY_NAME%,
/* more properties */)

/* Getter & Setter for the property called %PROPERTY_NAME% */
descriptor& set_%PROPERTY_NAME%(%PROPERTY_TYPE% value);
%PROPERTY_TYPE% get_%PROPERTY_NAME%() const;

/* more properties */
};

} // namespace oneapi::dal::%ALGORITHM%


Each meta-parameter of an algorithm is mapped to a property that shall satisfy the following requirements:

• Properties are defined with getter and setter methods. The underlying class member variable that stores the property’s value is never exposed in the descriptor interface.

• The getter returns the value of the underlying class member variable.

• The setter accepts only one parameter of the property’s type and assigns it to the underlying class member variable.

• Most of the properties are preset with default values, others are initialized by passing the required parameters to the constructor.

• The setter returns a reference to the descriptor object to allow chaining calls as shown in the example below.

auto desc = descriptor{}
.set_property_name_1(value_1)
.set_property_name_2(value_2)
.set_property_name_3(value_3);


## Floating-point Types¶

It is required for each algorithm to support at least one implementation-defined floating-point type. Other floating-point types are optional, for example float, double, float16, and bfloat16. It is up to a specific oneDAL implementation whether or not to support these types.

The floating-point type used as a default in descriptors is implementation-defined and shall be declared within the top-level namespace.

namespace oneapi::dal {
using default_float_t = /* implementation defined */;
} // namespace oneapi::dal


## Computational Methods¶

The supported computational methods are declared within the %ALGORITHM%::method namespace using tag-types. Algorithm shall support at least one method and declare the by_default type alias that refers to one of the methods as shown in the example below.

namespace oneapi::dal::%ALGORITHM% {
namespace method {
struct x {};
struct y {};
using by_default = x;
} // namespace method
} // namespace oneapi::dal::%ALGORITHM%


The supported computational tasks are declared within the %ALGORITHM%::task namespace using tag-types. Algorithm shall support at least one task and declare the by_default type alias that refers to one of the tasks as shown in the example below.
If an algorithm assumes both classification and regression tasks, the default task shall be classification. In some cases where an algorithm does not have the well-defined training and inference stages an algorithm may define only one task.
namespace oneapi::dal::%ALGORITHM% {