Elements do all of the work for caching in Inventor. If you are creating a node or an action that uses existing elements, you don't need to worry about caching. However, if you are creating a new element, you need to provide the necessary information so that caching continues to work. (You might want to reread Section 1.4, “Caching” before continuing with this section.)

A cache is essentially a faster representation of the data you're trying to generate or compute in Inventor. What the cache contains depends on the action. A GL render cache contains an OpenGL display list. A bounding-box cache contains the bounding box itself. Using the cache speeds up subsequent traversal of the scene graph. In addition, the cache contains a list of the elements outside the cache that the information inside the cache depends on. For efficiency, the cache does not make an exact copy of each element it depends on. Instead, it copies only enough information to determine later on whether the cache is still valid.

The form of this information depends on the element. For some elements, such as those derived from SoFloatElement^{( C++ | Java | .NET )} or SoLongElement, the value of the element is compared. For others, the node ID of the node that set the element's value is compared. For elements such as those derived from SoAccumulatedElement^{( C++ | Java | .NET )}, the list of node IDs for all nodes that affect the instance of the element is compared. The copyMatchInfo() method should copy just enough information so that matches() can work properly.

Inventor uses an element's copyMatchInfo() method to copy the comparison data for the element into the cache. Then, to determine whether a cache is valid, Inventor compares each element in the cache with the corresponding element in the state by calling the element's matches() method.

You can inherit the matches() and copyMatchInfo() methods from the base classes, or for efficiency, you can write routines that are tailored to your new element class. The following sections explain in more detail how those routines work for the base classes.

SbBool
SoReplacedElement::matches(const SoElement *elt) const
{
    return (nodeId == ((const SoReplacedElement *)
            elt)->nodeId);
}

SoElement *
SoReplacedElement::copyMatchInfo() const
{
    SoReplacedElement *result =
        (SoReplacedElement *)getTypeId().createInstance();

    result->nodeId = nodeId;

    return result;
}