Save- und Open-Methode – Beispiel (VC++)

In diesen drei Beispielen wird veranschaulicht, wie die Methoden Save und Open zusammen verwendet werden können.

Angenommen, Sie gehen auf Geschäftsreise und möchten eine Tabelle aus einer Datenbank mitnehmen. Vor Ihrer Abreise greifen Sie auf die Daten als Recordset zu und speichern sie in einem übertragbaren Format. Wenn Sie an Ihrem Bestimmungsort ankommen, greifen Sie auf das Recordset als lokales, getrenntes Recordset-Objekt zu. Sie nehmen Änderungen am Recordset vor, und speichern es dann wieder. Wenn Sie nach Hause zurückkehren, stellen Sie erneut eine Verbindung mit der Datenbank her und aktualisieren sie mit den Änderungen, die Sie unterwegs vorgenommen haben.

// BeginSaveCpp.cpp  
// compile with: /EHsc  
#import "msado15.dll" no_namespace rename("EOF", "EndOfFile")  
  
#include <ole2.h>  
#include <stdio.h>  
#include <conio.h>  
#include <io.h>  
  
// Function declarations  
inline void TESTHR(HRESULT x) { if FAILED(x) _com_issue_error(x); };  
bool FileExists();  
void SaveX1();  
void SaveX2();  
void SaveX3();  
void PrintProviderError(_ConnectionPtr pConnection);  
void PrintComError(_com_error &e);  
  
int main() {  
   if ( FAILED(::CoInitialize(NULL)) )  
      return -1;  
  
   // If File exists in the specified directory, then display error  
   if (!FileExists()) {  
      SaveX1();  
      SaveX2();  
      SaveX3();  
   }  
  
   ::CoUninitialize();  
}  
  
// First, access and save the authors table.  
void SaveX1() {  
   HRESULT hr = S_OK;  
  
   // Define ADO object pointers.  Initialize pointers on define.  
   // These are in the ADODB::  namespace.  
   _RecordsetPtr pRstAuthors = NULL;  
  
   // Definitions of other variables  
   _bstr_t strCnn("Provider='sqloledb'; Data Source='My_Data_Source' ; Initial Catalog='pubs'; Integrated Security='SSPI';");  
  
   try {  
      TESTHR(pRstAuthors.CreateInstance(__uuidof(Recordset)));  
  
      pRstAuthors->Open("SELECT * FROM authors",strCnn, adOpenDynamic,   
         adLockBatchOptimistic, adCmdText);  
  
      // For sake of illustration, save the Recordset to a diskette in XML format.  
      pRstAuthors->Save("c:\\pubs.xml", adPersistXML);  
   }  
   catch(_com_error &e) {  
      // Notify the user of errors, if any.  
      // Pass a connection pointer accessed from the Recordset.  
      _variant_t vtConnect = pRstAuthors->GetActiveConnection();  
  
      // GetActiveConnection returns connect string if connection  
      // is not open, else returns Connection object.  
      switch(vtConnect.vt) {  
      case VT_BSTR:  
         PrintComError(e);  
         break;  
      case VT_DISPATCH:  
         PrintProviderError(vtConnect);  
         break;  
      default:  
         printf("Errors occurred.");  
         break;  
      }  
   }  
  
   if (pRstAuthors)  
      if (pRstAuthors->State == adStateOpen)  
         pRstAuthors->Close();  
}  
  
// At this point, you have arrived at your destination.  
// You will access the authors table as a local, disconnected Recordset.   
// Don't forget you must have the MSPersist provider on the machine you   
// are using in order to access the saved file, c:\pubs.xml.  
void SaveX2() {  
   HRESULT hr = S_OK;  
  
   // Define ADO object pointers.  Initialize pointers on define.  
   // These are in the ADODB::  namespace.  
   _RecordsetPtr pRstAuthors = NULL;  
  
   try {  
      TESTHR(pRstAuthors.CreateInstance(__uuidof(Recordset)));  
  
      // For sake of illustration, we specify all parameters.  
      pRstAuthors->Open("c:\\pubs.xml", "Provider='MSPersist';", adOpenForwardOnly, adLockBatchOptimistic, adCmdFile);  
  
      // Now you have a local, disconnected Recordset.  
      // (In this example, changes will not be saved).  
      pRstAuthors->Find("au_lname = 'Carson'", NULL, adSearchForward);  
      if (pRstAuthors->EndOfFile) {  
         printf("Name not found ...\n");  
         pRstAuthors->Close();  
         return;  
      }  
      pRstAuthors->GetFields()->GetItem("City")->PutValue("Chicago");  
      pRstAuthors->Update();  
  
      // Save changes in ADTG format this time, purely for sake of  illustration.   
      // Note that the previous version is still on the diskette as c:\pubs.xml.  
      pRstAuthors->Save("c:\\pubs.adtg", adPersistADTG);  
   }  
   catch(_com_error &e){  
      // Notify the user of errors if any.  
      // Pass a connection pointer accessed from the Recordset.  
      _variant_t vtConnect = pRstAuthors->GetActiveConnection();  
  
      // GetActiveConnection returns connect string if connection  
      // is not open, else returns Connection object.  
      switch(vtConnect.vt) {  
      case VT_BSTR:  
         PrintComError(e);  
         break;  
      case VT_DISPATCH:  
         PrintProviderError(vtConnect);  
         break;  
      default:  
         printf("Errors occurred.");  
         break;  
      }  
   }  
  
   if (pRstAuthors)  
      if (pRstAuthors->State == adStateOpen)  
         pRstAuthors->Close();  
}  
  
// Now update the database with changes.  
void SaveX3() {  
   HRESULT hr = S_OK;  
  
   // Define ADO object pointers.  Initialize pointers on define.  
   // These are in the ADODB::  namespace.  
   _RecordsetPtr pRstAuthors = NULL;  
   _ConnectionPtr pCnn = NULL;  
  
   // Definitions of other variables  
   _bstr_t strCnn("Provider='sqloledb'; Data Source='My_Data_Source'; Initial Catalog='pubs'; Integrated Security='SSPI';");  
  
   try {  
      TESTHR(pCnn.CreateInstance(__uuidof(Connection)));  
  
      TESTHR(pRstAuthors.CreateInstance(__uuidof(Recordset)));  
  
      // If there is no ActiveConnection, you can open with defaults.  
      pRstAuthors->Open("c:\\pubs.adtg", "Provider=MSPersist;",  
         adOpenForwardOnly, adLockBatchOptimistic, adCmdFile);  
  
      // Connect to the database, associate the Recordset with the connection,   
      // then update the database table with the changed Recordset.  
      pCnn->Open(strCnn, "", "", NULL);  
  
      pRstAuthors->PutActiveConnection(_variant_t((IDispatch *) pCnn));  
      pRstAuthors->UpdateBatch(adAffectAll);  
   }  
   catch(_com_error &e) {  
      // Notify the user of errors if any.  
      // Pass a connection pointer accessed from the Recordset.  
      _variant_t vtConnect = pRstAuthors->GetActiveConnection();  
  
      // GetActiveConnection returns connect string if connection  
      // is not open, else returns Connection object.  
      switch(vtConnect.vt) {  
      case VT_BSTR:  
         PrintComError(e);  
         break;  
      case VT_DISPATCH:  
         PrintProviderError(vtConnect);  
         break;  
      default:  
         printf("Errors occurred.");  
         break;  
      }  
   }  
  
   if (pRstAuthors)  
      if (pRstAuthors->State == adStateOpen)  
         pRstAuthors->Close();  
   if (pCnn)  
      if (pCnn->State == adStateOpen)  
         pCnn->Close();  
}  
  
void PrintProviderError(_ConnectionPtr pConnection) {  
   // Print Provider Errors from Connection object.  
   // pErr is a record object in the Connection's Error collection.  
   ErrorPtr pErr = NULL;  
  
   if ( (pConnection->Errors->Count) > 0) {  
      long nCount = pConnection->Errors->Count;  
  
      // Collection ranges from 0 to nCount -1.  
      for ( long i = 0 ; i < nCount ; i++) {  
         pErr = pConnection->Errors->GetItem(i);  
         printf("Error number: %x\t%s\n", pErr->Number, (LPCSTR) pErr->Description);  
      }  
   }  
}  
  
void PrintComError(_com_error &e) {  
   _bstr_t bstrSource(e.Source());  
   _bstr_t bstrDescription(e.Description());  
  
   // Print COM errors.   
   printf("Error\n");  
   printf("\tCode = %08lx\n", e.Error());  
   printf("\tCode meaning = %s\n", e.ErrorMessage());  
   printf("\tSource = %s\n", (LPCSTR) bstrSource);  
   printf("\tDescription = %s\n", (LPCSTR) bstrDescription);  
}  
  
bool FileExists() {  
   struct _finddata_t xml_file;  
   long hFile;  
  
   if( (hFile = _findfirst("c:\\pubs.xml", &xml_file )) != -1L)  
   {  
      printf( "File already exists!\n" );  
      return(true);  
   }  
   else  
      return (false);  
}  

Weitere Informationen

Open-Methode (ADO-Recordset)
Recordset-Objekt (ADO)
Save-Methode