Starting with version 0.22, KLayout offers parametrized cells (PCell's). PCells are a feature found in other tools to simplify layout by providing generators for common layout building blocks. Parametrized cells do not contain static layout but are created dynamically by code using a given set of parameters.
For example, a circle PCell requires two parameters: the layer where the circle should be produced and the radius of the circle to produce. The code is responsible for creating the circle from these parameters.
Using a PCell is easy: choose the library and the PCell from that library when asked for the cell in the instance options dialog. For PCell's, KLayout offers an additional parameters page where it asks for the parameter required by the PCell. The placement of the PCell is done as for simple instances. PCell's offer the same instantiation options that normal cells.
KLayout provides a simple library called "Basic" with some useful basic PCell's. See About The Basic Library for more details about that library.
Unlike other tools, KLayout offers the unique feature of "guiding shapes". A guiding shape is some kind of "ghost shape" that is not produced as real layout but is present as a part of the PCell instance. It is drawn in the style of the cell frame but can be edited as a normal shape. In particular, a guiding shape can be manipulated with the properties dialog and the partial edit mode. A special case is a point-like shape which can act as a handle of the PCell. In move mode, these shapes can be moved to change the parameter related to that handle.
Another use case for guiding shapes is the rounded path. This PCell uses a path as the input shape and applies rounding to the path's spine corners to compute a new path which smoothly bends around the corners. The radius of the bends is a numerical PCell parameter while the input shape controlling the geometry and the width of the path is the guiding shape.
A PCell implementation consists of at least three parts: a description text, a parameter declaration and a production callback. In addition, a PCell can provide a method that "fixes" parameters according to the PCell's consistency rules (coerce parameters). Technically, a PCell is a class implementing a certain interface with these methods.
PCell's are usually packed in libraries. PCell libraries can be provided as shared objects/DLL's (in C++) or as Ruby scripts. Because PCell code is only executed if required, performance usally is not the main objective. A Ruby implementation will therefore be sufficient in most cases and is a much easier to maintain. The Ruby approach also benefits from KLayout's integrated development environment.
For an introduction into PCell programming with Ruby, see Coding PCell's In Ruby.