Developing C++ primitive operators

The SPL language supports writing two styles of primitive operators, namely: generic operators and non-generic operators.

Generic operators can handle various stream types, different numbers of ports, and most importantly, can be configured with parameters and output assignments that involve SPL expressions. Alternately, non-generic operators are less configurable and are often used for one-time tasks that do not require high levels of reuse. Both styles are supported under the same development framework, which eases the process of converting non-generic operators to their generic versions when the need arises.

Creating a new primitive operator involves two main tasks:

• Creating an operator model that describes the syntactic and semantic properties common to all instances of the operator. This model is the oper-name.xml file. For more information, see Primitive operator artifacts.
• Creating the header and implementation code generator templates to implement the operator logic. These templates are the oper-name_h.cgt and the oper-name_cpp.cgt files. For more information, see Primitive operator artifacts.

The operator model enables the compiler to check the syntax and semantics for instances of the primitive operator when used in a streams processing application. For non-generic operators, a default operator model can be used for convenience, as non-generic operators are not designed to be reusable and do not require strict error checking. The default operator model is the most permissive model and avoids most of the compile-time checks done by the SPL compiler.

Primitive operators are implemented as C++ classes that extend from a base operator class that is provided by the IBM® Streams instance. Non-generic operators do not require customization that is based on the specific operator instance that appears in the SPL source. As a result, they are implemented as pure C++ code. Conversely, generic operators require specialization. For instance, the filter condition that appears as part of a Filter operator might be different for two different instances of the Filter operator. Thus, generic operators are written as a mixture of C++ (generated code) and Perl (generator code). The Perl code appears within <%...%> sections and has access to a variable named $model that provides access to the operator instance configuration¹ (for example, the filter condition as a C++ expression). In essence, the Perl code is used to generate C++ code that is tailored to the operator instance at hand.

The spl-make-operator command can be used to create skeleton code generator templates and the default operator model as a starting point.

The spl-make-operator script is also used to create code generators (oper-name_h.pm and oper-name_cpp.pm). This step can be deferred to toolkit indexing or skipped. For more information, see the spl-make-operator command reference.