# SoElement Class Reference
[Elements (mostly for internal use)]

`#include <Inventor/elements/SoElement.h>`

Inheritance diagram for SoElement:

List of all members.

## Detailed Description

SoElement is the abstract base class for all elements.

Elements are part of the design for scenegraph traversal in Coin.

It works like this: any traversal action instantiates and keeps a single SoState instance during traversal. The SoState instance uses SoElement objects as "memory units" to keep track of the current state for any feature of the scenegraph nodes.

As an example, consider the SoPointSize node: when the SoPointSize node is traversed by for instance a SoGLRenderAction, it will itself push a SoPointSizeElement onto the SoGLRenderAction's SoState stack. Later, when a SoPointSet node occurs in the scenegraph, it will request the current pointsize value from the SoState by reading off the value of it's SoPointSizeElement.

SoSeparator nodes will push and pop elements on and off the state stack, so anything that changes state below a SoSeparator node will not influence anything above the SoSeparator.

For more information on the theoretical underpinnings of this traversal design, you should consider reading available literature on the so-called "Visitor pattern". We recommend "Design Patterns", by Gamma, Helm, Johnson, Vlissides (aka the "Gang Of Four"). This book actually uses the Inventor API traversal mechanism as the case study for explaining the Visitor pattern.

For extending the Coin library with your own classes, we strongly recommend that you make yourself acquainted with the excellent «The Inventor Toolmaker» book (ISBN 0-201-62493-1), which describes the tasks involved in detail. This book was written by the original SGI Inventor designers and explains many of the underlying design ideas, aswell as having lots of hands-on examples on how to extend the Coin toolkit in ways that are true to the fundamental design ideas. («The Inventor Toolmaker» is also available at SGI's online library, at no cost. See Download The Inventor Toolmaker.) Reading the sourcecode of the built-in classes in Coin should also provide very helpful.

The following is a complete example on how to extend Coin with your own traversal elements. First, the class declaration of the new element (ie the header include file):

      // [texturefilenameelement.h]
      #ifndef TEXTUREFILENAMEELEMENT_H
      #define TEXTUREFILENAMEELEMENT_H

      #include <Inventor/elements/SoReplacedElement.h>
      #include <Inventor/SbString.h>

      class TextureFilenameElement : public SoReplacedElement {
        typedef SoReplacedElement inherited;

        SO_ELEMENT_HEADER(TextureFilenameElement);
      public:
        static void initClass(void);

        virtual void init(SoState * state);
        static void set(SoState * const state, SoNode * const node,
                        const SbString & filename);
        static const SbString & get(SoState * const state);
        static const TextureFilenameElement * getInstance(SoState * state);

      protected:
        virtual ~TextureFilenameElement();
        virtual void setElt(const SbString & filename);

      private:
        SbString filename;
      };

      #endif // !TEXTUREFILENAMEELEMENT_H

The implementation of the element:

      // [texturefilenameelement.cpp]
      //
      // The purpose of the code in this file is to demonstrate how you can
      // make your own elements for scene graph traversals.
      //
      // Code by Peder Blekken <pederb@sim.no>, 1999-12-09. Copyright
      // Systems in Motion.

      #include "texturefilenameelement.h"

      SO_ELEMENT_SOURCE(TextureFilenameElement);

      void
      TextureFilenameElement::initClass(void)
      {
        SO_ELEMENT_INIT_CLASS(TextureFilenameElement, inherited);
      }

      void
      TextureFilenameElement::init(SoState * state)
      {
        this->filename = "<none>";
      }

      TextureFilenameElement::~TextureFilenameElement()
      {
      }

      void
      TextureFilenameElement::set(SoState * const state, SoNode * const node,
                                  const SbString & filename)
      {
        TextureFilenameElement * elem = (TextureFilenameElement *)
          SoReplacedElement::getElement(state, classStackIndex, node);
        elem->setElt(filename);
      }

      const SbString &
      TextureFilenameElement::get(SoState * const state)
      {
        return TextureFilenameElement::getInstance(state)->filename;
      }

      void
      TextureFilenameElement::setElt(const SbString & filename)
      {
        this->filename = filename;
      }

      const TextureFilenameElement *
      TextureFilenameElement::getInstance(SoState * state)
      {
        return (const TextureFilenameElement *)
          SoElement::getConstElement(state, classStackIndex);
      }

And a small, stand-alone test application putting the new element to use:

      // [lstextures.cpp]
      //
      // The purpose of this file is to make a small wrapper "tool" around
      // the TextureFilenameElement extension element, just for showing
      // example code on how to make use of a user-defined custom element.
      //
      // The code goes like this:
      //
      // We initialize the element, enable it for the SoCallbackAction, read
      // a scene graph file, set callbacks on SoTexture2 and all shape nodes
      // and applies the SoCallbackAction. The callbacks will then print out
      // the texture filename information from the TextureFilenameElement
      // each time an interesting node is hit.
      //
      //
      // Code by Peder Blekken <pederb@sim.no>. Cleaned up, integrated in
      // Coin distribution and commented by Morten Eriksen <mortene@sim.no>.
      // 1999-12-09. Copyright Systems in Motion.

      #include <Inventor/SoDB.h>
      #include <Inventor/SoInput.h>
      #include <Inventor/actions/SoCallbackAction.h>
      #include <Inventor/nodes/SoSeparator.h>
      #include <Inventor/nodes/SoTexture2.h>
      #include <Inventor/nodes/SoShape.h>
      #include <Inventor/misc/SoState.h>
      #include <stdio.h>

      #include "texturefilenameelement.h"

      SoCallbackAction::Response
      pre_tex2_cb(void * data, SoCallbackAction * action, const SoNode * node)
      {
        const SbString & filename = ((SoTexture2 *)node)->filename.getValue();
        TextureFilenameElement::set(action->getState(), (SoNode *)node, filename);

        (void)fprintf(stdout, "=> New texture: %s\n",
                      filename.getLength() == 0 ?
                      "<inlined>" : filename.getString());

        return SoCallbackAction::CONTINUE;
      }

      SoCallbackAction::Response
      pre_shape_cb(void * data, SoCallbackAction * action, const SoNode * node)
      {
        const SbString & filename =
          TextureFilenameElement::get(action->getState());

        (void)fprintf(stdout, "   Texturemap on %s: %s\n",
                      node->getTypeId().getName().getString(),
                      filename.getLength() == 0 ?
                      "<inlined>" : filename.getString());

        return SoCallbackAction::CONTINUE;
      }

      void
      usage(const char * appname)
      {
        (void)fprintf(stderr, "\n\tUsage: %s <modelfile.iv>\n\n", appname);
        (void)fprintf(stderr,
                      "\tLists all texture filenames in the model file,\n"
                      "\tand on which shape nodes they are used.\n\n"
                      "\tThe purpose of this example utility is simply to\n"
                      "\tshow how to create and use an extension element for\n"
                      "\tscene graph traversal.\n\n");
      }

      int
      main(int argc, char ** argv)
      {
        if (argc != 2) {
          usage(argv[0]);
          exit(1);
        }

        SoDB::init();

        TextureFilenameElement::initClass();
        SO_ENABLE(SoCallbackAction, TextureFilenameElement);

        SoInput input;
        if (!input.openFile(argv[1])) {
          (void)fprintf(stderr, "ERROR: couldn't open file ``%s''.\n", argv[1]);
          exit(1);
        }

        SoSeparator * root = SoDB::readAll(&input);
        if (root) {
          root->ref();
          SoCallbackAction cbaction;
          cbaction.addPreCallback(SoTexture2::getClassTypeId(), pre_tex2_cb, NULL);
          cbaction.addPreCallback(SoShape::getClassTypeId(), pre_shape_cb, NULL);
          cbaction.apply(root);
          root->unref();
          return 0;
        }
        return 1;
      }

## Public Member Functions

const SoType | getTypeId (void) const
int | getStackIndex (void) const
virtual void | init (SoState *state)
virtual void | push (SoState *state)
virtual void | pop (SoState *state, const SoElement *prevTopElement)
virtual SbBool | matches (const SoElement *element) const=0
virtual SoElement * | copyMatchInfo (void) const=0
void | setDepth (const int depth)
int | getDepth (void) const
virtual void | print (FILE *file=stdout) const
virtual | ~SoElement ()

## Static Public Member Functions

void | initClass (void)
SoType | getClassTypeId (void)
int | getClassStackIndex (void)
void | initElements (void)
int | getNumStackIndices (void)
SoType | getIdFromStackIndex (const int stackIndex)

## Protected Member Functions

| SoElement (void)
void | capture (SoState *const state) const
virtual void | captureThis (SoState *state) const
void | setTypeId (const SoType typeId)
void | setStackIndex (const int index)
SoElement * | getNextInStack (void) const
SoElement * | getNextFree (void) const

## Static Protected Member Functions

SoElement * | getElement (SoState *const state, const int stackIndex)
const SoElement * | getConstElement (SoState *const state, const int stackIndex)
int | createStackIndex (const SoType id)

## Protected Attributes

SoType | typeId
int | stackIndex
int | depth

## Static Protected Attributes

int | classStackIndex
SoTypeList * | stackToType

## Friends

class | SoState

## Constructor & Destructor Documentation

SoElement::~SoElement | ( |  | ) | `[virtual]`

| The destructor.

SoElement::SoElement | ( | void |  | ) | `[protected]`

| The constructor. To create element instances, use SoType::createInstance() for the elements type identifier..

## Member Function Documentation

void SoElement::initClass | ( | void |  | ) | `[static]`

| Initialize relevant common data for all instances, like the type system. Reimplemented in SoFloatElement, SoGLColorIndexElement, SoGLLineWidthElement, SoGLPointSizeElement, SoLineWidthElement, SoPointSizeElement, SoReplacedElement, SoListenerPositionElement, SoListenerOrientationElement, SoListenerGainElement, and SoListenerDopplerElement.

SoType SoElement::getClassTypeId | ( | void |  | ) | `[static]`

| This static method returns the class type.

int SoElement::getClassStackIndex | ( | void |  | ) | `[static]`

| This static method returns the state stack index for the class.

const SoType SoElement::getTypeId | ( | void |  | ) | const

| Returns the type identification of an object derived from a class inheriting SoElement. This is used for run-time type checking and "downward" casting. For a more thorough explanation of the run-time type identification functionality, see the documentation of SoBase::getTypeId().

int SoElement::getStackIndex | ( | void |  | ) | const

| Returns the stack index for an element instance.

void SoElement::init | ( | SoState * | state | ) | `[virtual]`

This function initializes the element type in the given SoState. It is called for the first element of each enabled element type in SoState objects.

Reimplemented in SoFloatElement, SoGLColorIndexElement, SoGLLineWidthElement, SoGLPointSizeElement, SoLineWidthElement, SoPointSizeElement, SoReplacedElement, SoListenerPositionElement, SoListenerOrientationElement, SoListenerGainElement, and SoListenerDopplerElement.

void SoElement::push | ( | SoState * | state | ) | `[virtual]`

This method is called every time a new element is required in one of the stacks. This happens when a writable element is requested, using SoState::getElement() or indirectly SoElement::getElement(), and the depth of the current element is less than the state depth.

Override this method if your element needs to copy data from the previous top of stack. The push() method is called on the new element, and the previous element can be found using SoElement::getNextInStack().

Reimplemented in SoGLLineWidthElement, and SoGLPointSizeElement.

void SoElement::pop | ( | SoState * | state,
|  | const SoElement * | prevTopElement
| ) | `[virtual]`

| This method is callled when the state is popped, and the depth of the element is bigger than the current state depth. pop() is called on the new top of stack, and a pointer to the previous top of stack is passed in prevTopElement. Override this method if you need to copy some state information from the previous top of stack. Reimplemented in SoGLLineWidthElement, and SoGLPointSizeElement.

SbBool SoElement::matches | ( | const SoElement * | element | ) | const`[pure virtual]`

| This function returns `TRUE` is the element matches another element (of the same class), with respect to cache validity. If the application programmer's extension element has a matches() function, it should also have a copyMatchInfo() function. Implemented in SoFloatElement, and SoReplacedElement.

SoElement * SoElement::copyMatchInfo | ( | void |  | ) | const`[pure virtual]`

| This function creates a copy of the element that contains enough information to enable the matches() function to work. Used to help with scenegraph traversal caching operations. Implemented in SoFloatElement, and SoReplacedElement.

void SoElement::initElements | ( | void |  | ) | `[static]`

| This function initializes all the built-in Coin element classes.

int SoElement::getNumStackIndices | ( | void |  | ) | `[static]`

| Returns the number of allocated element stack index slots.

SoType SoElement::getIdFromStackIndex | ( | const int | stackIndex | ) | `[static]`

| Returns the SoType identifier for the element class with element state stack index stackIndex.

void SoElement::setDepth | ( | const int | depth | ) |

| Sets the depth value of the element instance in the state stack.

int SoElement::getDepth | ( | void |  | ) | const

| Returns the state stack depth value of the element instance.

void SoElement::print | ( | FILE * | file = stdout | ) | const`[virtual]`

| This function is for printing element information, and is used mostly for debugging purposes. Reimplemented in SoFloatElement, SoReplacedElement, SoListenerPositionElement, SoListenerOrientationElement, and SoListenerDopplerElement.

SoElement * SoElement::getElement | ( | SoState *const | state,
|  | const int | stackIndex
| ) | `[inline, static, protected]`

This method returns the top instance (in the state stack) of the element class with stack index stackIndex.

The retuned instance is writable. To make this instance, some lazy evaluation may have to be perfomed, so use getConstElement() instead if the instance shouldn't be modified.

If no instance is available and can not be made, `NULL` is returned.

See also:
  const SoElement * SoElement::getConstElement(SoState * const state, const int stackIndex)

void const SoElement * SoElement::getConstElement | ( | SoState *const | state,
|  | const int | stackIndex
| ) | `[inline, static, protected]`

This method returns a reference to the top element of the class with stack index stackIndex. The returned element is non-mutable.

(Don't try to be clever and cast away the constness -- if the returned instance is modified, strange, hard to find and generally wonderful bugs will most likely start to happen.)

If no instance can be returned, `NULL` is returned.

See also:
  SoElement * SoElement::getElement(SoState * const state, const int stackIndex)

void SoElement::capture | ( | SoState *const | state | ) | const`[inline, protected]`

| This function does whatever is necessary in the state for caching purposes. If should be called by subclasses of SoElement whenever any value in the element is accessed.

void SoElement::captureThis | ( | SoState * | state | ) | const`[protected, virtual]`

| Adds the element to the cache.

void SoElement::setTypeId | ( | const SoType | typeId | ) | `[protected]`

| Sets the type identifier of an instance. Note that this is fundamentally different from the SoNode run-time type system.

void SoElement::setStackIndex | ( | const int | stackIndex | ) | `[protected]`

| Sets the stack index in an instance. Used in constructors of derived elements.

int SoElement::createStackIndex | ( | const SoType | typeId | ) | `[static, protected]`

| Returns the value of a new available stack index.

SoElement * SoElement::getNextInStack | ( | void |  | ) | const`[protected]`

| Returns the next element down in the stack. Should be used in push() to get the previous element. This method has a slightly misleading name, but we didn't change it to stay compatible with the original SGI Inventor API.

SoElement * SoElement::getNextFree | ( | void |  | ) | const`[protected]`

| Returns the next free element, ie the next element up in the stack.

## Member Data Documentation

int SoElement::classStackIndex`[static, protected]`

| This is the static state stack index for the class.

SoType SoElement::typeId`[protected]`

| The element's unique SoType type identification.

int SoElement::stackIndex`[protected]`

| The index in the state stack for this particular element instance.

SoTypeList * SoElement::stackToType`[static, protected]`

| Provides mapping from state stack indices to element types.

int SoElement::depth`[protected]`

| The depth of the element instance in the state stack.

The documentation for this class was generated from the following files:

- SoElement.h
- SoElement.cpp