Extended Data

Extended data (xdata) is created by applications written with ObjectARX ^® or AutoLISP ^® and can be added to any object. Xdata consists of a linked list of resbufs used by the application. (AutoCAD maintains the information but doesn't use it.) The data is associated with a DXF group code in the range of 1000 to 1071.

This mechanism is space-efficient and can be useful for adding lightweight data to an object. However, xdata is limited to 16K and to the existing set of DXF group codes and types.

For a more detailed description of xdata, see the AutoCAD DXF Reference.

Use the AcDbObject::xData() function to obtain the resbuf chain containing a copy of the xdata for an object:

virtual resbuf* 
AcDbObject::xData(const char* regappName = NULL) const;

Use the AcDbObject::setXData() function to specify the xdata for an object:

virtual Acad::ErrorStatus 
AcDbObject::setXData(const resbuf* xdata);

The following example uses the xData() function to obtain the xdata for a selected object and then prints the xdata to the screen. It then adds a string (testrun) to the xdata and calls the setXdata() function to modify the object's xdata. This example also illustrates the use of the upgradeOpen() and downgradeOpen() functions.

// This function calls the selectObject() function to allow 
// the user to pick an object; then it accesses the xdata of 
// the object and sends the list to the printList() function 
// that lists the restype and resval values.
// 
void
printXdata()
{
    // Select and open an object.
    //
    AcDbObject *pObj;
    if ((pObj = selectObject(AcDb::kForRead)) == NULL) {
        return;
    }
    // Get the application name for the xdata.
    //
    char appname[133];
    if (acedGetString(NULL,
        "\nEnter the desired Xdata application name: ",
        appname) != RTNORM)
    {
        return;
    }
    // Get the xdata for the application name.
    //
    struct resbuf *pRb;
    pRb = pObj->xData(appname);
    if (pRb != NULL) {
        // Print the existing xdata if any is present.
        // Notice that there is no -3 group, as there is in
        // LISP. This is ONLY the xdata, so
        // the -3 xdata-start marker isn't needed.
        // 
        printList(pRb);
        acutRelRb(pRb);
    } else {
        acutPrintf("\nNo xdata for this appname");
    }
    pObj->close();
}
void 
addXdata() 
{
    AcDbObject* pObj = selectObject(AcDb::kForRead);
    if (!pObj) {
        acutPrintf("Error selecting object\n");
        return;
    }
    // Get the application name and string to be added to
    // xdata.
    //
    char appName[132], resString[200];
    appName[0] = resString[0] = '\0';
    acedGetString(NULL, "Enter application name: ",
        appName);
    acedGetString(NULL, "Enter string to be added: ",
        resString);
    struct  resbuf  *pRb, *pTemp;
    pRb = pObj->xData(appName);
    if (pRb != NULL) {
        // If xdata is present, then walk to the
        // end of the list.
        //
        for (pTemp = pRb; pTemp->rbnext != NULL;
                pTemp = pTemp->rbnext)
                { ; }
    } else {
        // If xdata is not present, register the application
        // and add appName to the first resbuf in the list.
        // Notice that there is no -3 group as there is in
        // AutoLISP. This is ONLY the xdata so
        // the -3 xdata-start marker isn't needed.
        // 
        acdbRegApp(appName);
        pRb = acutNewRb(AcDb::kDxfRegAppName);
        pTemp = pRb;
        pTemp->resval.rstring
            = (char*) malloc(strlen(appName) + 1);
        strcpy(pTemp->resval.rstring, appName);
    }
    // Add user-specified string to the xdata.
    //
    pTemp->rbnext = acutNewRb(AcDb::kDxfXdAsciiString);
    pTemp = pTemp->rbnext;
    pTemp->resval.rstring
        = (char*) malloc(strlen(resString) + 1);
    strcpy(pTemp->resval.rstring, resString);
    // The following code shows the use of upgradeOpen()
    // to change the entity from read to write.
    //
    pObj->upgradeOpen();
    pObj->setXData(pRb);
    pObj->close();
    acutRelRb(pRb);
}