BeginTrans-, CommitTrans- und RollbackTrans-Methode – Beispiel (VC++)
In diesem Beispiel wird der Buchtyp aller Psychologiebücher in der Tabelle Titles (Titel) der Datenbank geändert. Nachdem die BeginTrans-Methode eine Transaktion gestartet hat, die alle an der Titles-Tabelle (Titel) vorgenommenen Änderungen isoliert, speichert die CommitTrans-Methode die Änderungen. Sie können die Rollback-Methode verwenden, um Änderungen rückgängig zu machen, die Sie mithilfe der Update-Methode gespeichert haben.
Beispiel
// BeginBeginTransCpp.cpp
// compile with: /EHsc /c
#import "msado15.dll" no_namespace rename("EOF", "EndOfFile")
#include <stdio.h>
#include <ole2.h>
#include <conio.h>
#include <assert.h>
#include <malloc.h>
#include "icrsint.h"
// extracts only title and type
class CTitlesRs : public CADORecordBinding {
BEGIN_ADO_BINDING(CTitlesRs)
//Column title is the 2nd field in the recordset
ADO_VARIABLE_LENGTH_ENTRY2(2, adVarChar, m_szT_title, sizeof(m_szT_title), lT_titleStatus, FALSE)
// Column type is the 3rd field in the recordset
ADO_VARIABLE_LENGTH_ENTRY2(3, adVarChar, m_szT_type, sizeof(m_szT_type), lT_typeStatus, TRUE)
END_ADO_BINDING()
public:
CHAR m_szT_title[150];
ULONG lT_titleStatus;
CHAR m_szT_type[40];
ULONG lT_typeStatus;
};
inline void TESTHR(HRESULT x) { if FAILED(x) _com_issue_error(x); };
void BeginTransX();
void PrintProviderError(_ConnectionPtr pConnection);
int main() {
if (FAILED(::CoInitialize(NULL)))
return -1;
BeginTransX();
::CoUninitialize();
}
void BeginTransX() {
// Define ADO object pointers, initialize pointers on define. These are in the ADODB:: namespace.
_RecordsetPtr rstTitles = NULL;
_ConnectionPtr pConnection = NULL;
char * token1, * token2;
// Define Other Variables
HRESULT hr = S_OK;
IADORecordBinding *picRs = NULL; // Interface Pointer declared
CTitlesRs titlrs;
_bstr_t strTitle;
_bstr_t strMessage;
LPSTR p_TempStr = NULL;
char chKey;
int i = 0;
try {
// open connection.
_bstr_t strCnn("Provider='sqloledb';Data Source='My_Data_Source'; Initial Catalog='pubs';Integrated Security='SSPI';");
TESTHR(pConnection.CreateInstance(__uuidof(Connection)));
TESTHR(pConnection->Open(strCnn, "", "", adConnectUnspecified));
rstTitles.CreateInstance(__uuidof(Recordset));
rstTitles->CursorType = adOpenDynamic;
rstTitles->LockType = adLockPessimistic;
// open Titles table
TESTHR(rstTitles->Open("titles", _variant_t((IDispatch*)pConnection,true),
adOpenDynamic, adLockPessimistic,adCmdTable));
rstTitles->MoveFirst();
pConnection->BeginTrans();
// Open an IADORecordBinding interface pointer which
// we'll use for Binding Recordset to a class
TESTHR(rstTitles->QueryInterface(__uuidof(IADORecordBinding), (LPVOID*)&picRs));
// Bind the Recordset to a C++ Class here
TESTHR(picRs->BindToRecordset(&titlrs));
// Loop through recordset, prompt user to change the type for a specified title.
// Allocate memory to p_TempStr string pointer.
p_TempStr = (LPSTR) malloc(sizeof(titlrs.m_szT_type));
// Check for null string.
assert(p_TempStr != NULL);
while (VARIANT_FALSE == rstTitles->EndOfFile) {
// Set the final character of the destination string to NULL.
p_TempStr[sizeof(titlrs.m_szT_type)-1] = '\0';
// The source string will get truncated if its length is
// longer than the length of the destination string minus one.
strncpy_s(p_TempStr, sizeof(titlrs.m_szT_type), strtok_s(titlrs.m_szT_type," ", &token1), sizeof(titlrs.m_szT_type) - 1);
// Compare type with psychology
if (!strcmp(p_TempStr,"psychology")) {
strTitle = titlrs.m_szT_title;
strMessage = "Title: " + strTitle + "\n Change type to Self help?(y/n)";
// Change the title for specified employee
printf("%s\n", (LPCSTR)strMessage);
do {
chKey = _getch();
} while (chKey != 'y' && chKey !='n');
if (chKey == 'y') {
// Set last character of destination string to NULL.
titlrs.m_szT_type[sizeof(titlrs.m_szT_type)-1] = '\0';
// Copy "self_help" title field. The source string will get truncated
// if its length is longer than length of destination string, minus one.
strncpy_s(titlrs.m_szT_type, _countof(titlrs.m_szT_type), "self_help", sizeof(titlrs.m_szT_type) - 1);
picRs->Update(&titlrs);
}
}
rstTitles->MoveNext();
}
// Ask if the User wants to commit to all the changes made above
printf("\n\n Save all changes(y/n)?");
do {
chKey = _getch();
} while (chKey != 'y' && chKey !='n');
if (chKey == 'y')
// Save the changes to the title table
pConnection->CommitTrans();
else
// Unsave the changes to the title table
pConnection->RollbackTrans();
// Print current data in recordset.
rstTitles->Requery(0);
// Move to the first record of the title table
rstTitles->MoveFirst();
printf("\n\nPress any key to continue...");
_getch();
// Clear the screen for the next display
// system("cls");
// Open IADORecordBinding interface pointer again for binding Recordset to a class.
TESTHR(rstTitles->QueryInterface( __uuidof(IADORecordBinding), (LPVOID*)&picRs));
// Rebind the Recordset to a C++ Class.
TESTHR(picRs->BindToRecordset(&titlrs));
while (!rstTitles->EndOfFile) {
i = i + 1;
if (i % 23 == 0) {
printf("\nPress any key to continue...");
_getch();
// Clear the screen for the next display
// system("cls");
}
printf("%s - %s\n", titlrs.m_szT_title, titlrs.m_szT_type);
rstTitles->MoveNext();
}
// Restore original data because this is a demonstration.
rstTitles->MoveFirst();
while (VARIANT_FALSE == rstTitles->EndOfFile) {
// Set the final character of the destination string to NULL.
p_TempStr[sizeof(titlrs.m_szT_type)-1] = '\0';
// The source string will get truncated if its length is
// longer than the length of the destination string minus one.
strncpy_s(p_TempStr, sizeof(titlrs.m_szT_type), titlrs.m_szT_type, sizeof(titlrs.m_szT_type) - 1);
p_TempStr = strtok_s(p_TempStr," ", &token2);
if (!strcmp(p_TempStr,"self_help")) {
// Set final character of destination string to NULL.
titlrs.m_szT_type[sizeof(titlrs.m_szT_type)-1] = '\0';
// The source string will get truncated if its length is longer than the length
// of the destination string minus one.
strncpy_s(titlrs.m_szT_type, _countof(titlrs.m_szT_type), "psychology", sizeof(titlrs.m_szT_type) - 1);
picRs->Update(&titlrs);
}
rstTitles->MoveNext();
}
}
catch(_com_error &e) {
// Notify the user of errors if any.
_bstr_t bstrSource(e.Source());
_bstr_t bstrDescription(e.Description());
PrintProviderError(pConnection);
printf("Source : %s\n", (LPCSTR)bstrSource);
printf("Description : %s\n", (LPCSTR)bstrDescription);
}
// Deallocate the memory
if (p_TempStr)
free(p_TempStr);
// Clean up objects before exit.
// Release the IADORecordset Interface here
if (picRs)
picRs->Release();
if (rstTitles)
if (rstTitles->State == adStateOpen)
rstTitles->Close();
if (pConnection)
if (pConnection->State == adStateOpen)
pConnection->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;
long nCount = 0;
long i = 0;
if ( (pConnection->Errors->Count) > 0) {
nCount = pConnection->Errors->Count;
// Collection ranges from 0 to nCount -1.
for (i = 0 ; i < nCount ; i++) {
pErr = pConnection->Errors->GetItem(i);
printf("Error number: %x\t%s\n", pErr->Number, (LPCSTR) pErr->Description);
}
}
}
Weitere Informationen
BeginTrans-, CommitTrans- und RollbackTrans-Methode (ADO)
Update-Methode