## 7.3 Data Structures

### 7.3.1 Parts list

#### Main parts list

The primary data storage is in a list of ``cards''. A card is anything that can appear in a net list. Cards live here, primarily, but there are some other auxilary lists that also contain pointers to cards.
The list stores pointers, rather than actual objects, because there are many types of cards. All are derived from the ``card'', through several levels of inheritance.
Usually, they are stored in the order they are read from the file, except for subcircuits, which are stored in separate lists to preserve the hierarchy.
As of release 0.24, the main list is in static storage, so there can be only one. This will change. New cards can be inserted anywhere in the list, but usually they are inserted at the end. The mechanism for marking the location is a hybrid of STL and a 15 year old pointer scheme, which will also change someday.

#### The ``Common'' and ``Eval'' classes

The ``common'' serves two distinct purposes. The first is to share storage for similar devices. The second is to attach ``evaluators'' to otherwise simple components for special behavior.
Most circuits have many identical elements. The ``common'' enables them to share storage. One ``common'' can be attached to many devices. When a new device is created, even if it is parses separately, an attempt is made to find an appropriate device to share with.
Simple elements like resistors and capacitors can have ``evaluators'' attached as commons. These evaluators calculate a function and its derivative, and return it in a standard form. Some evaluators are used internally, such as in the diode and mosfet models. Some are used explicitly, such as in behavioral modeling.