1 관련서적 #

ADO: ActiveX Data Objects
- Jason T. Roff 저, O'Reilly

2 Referencing ActiveX Data Objects #

Visual C++ 에서 #import 지시어를 사용하면 타입 라이브러리 ( Type Library ) 에 포함된 정보를 사용할 수 있다. 따라서 다음과 같이 #import 지시어를 사용하여 ADO 를 사용할 수 있도록 한다.

#import <msado15.dll>	no_namespace	rename( "EOF", "EOFile" )

msado15.dll 파일의 경로 ( 일반적으로 C:\Program Files\Common Files\System\ado ) 는 Visual C++ 환경에 설정되어 있어야 하며 설정되어 있지 않을 경우 Visual Studio 의 [ Tools ] - [ Options ] 메뉴를 선택하여 [ Directories ] 탭을 선택후 [ Library files ] 에 경로를 추가한다.

#import 지시어는 다음과 같이 두 가지 작업을 수행한다.

.tlh 확장자를 갖는 타입 라이브러리 헤더 ( Type Library Header ) 파일을 생성한다.
msado15.dll 파일의 타입 라이브러리에 포함된 오브젝트 정의와 데이타 타입을 나열하고 있다.
.tli ( Type Library Implementation ) 확장자를 갖는 파일을 생성한다.
msado15.dll 파일의 타입 라이브러리에 정의된 오브젝트 모델의 래퍼 함수 ( Wrapper Function ) 를 포함하고 있다.

#import 지시어의 rename 속성은

rename( "EOF", "EOFile" )

타입 라이브러리에 포함된 EOF 키워드를 EOFile 로 변경한다. 이것은 Visual C++ 에 정의된 EOF 속성 ( Property ) 과 충돌을 피하기 위해서 사용된다.

3 Creating ActiveX Data Objects #

ActiveX Data Object 를 사용하는 것은 OLD DB 를 사용하는 것과 비슷하다. 이것은 OLE DB 기술을 간단히 래핑 ( Wrapping ) 한 것이 ADO 이기 때문이다.

We do this with the following piece of code:

struct StartOLEProcess
{
	StartOLEProcess()
	{
		::CoInitialize( NULL );
	}

	~StartOLEProcess()
	{
		::CoUninitialize();
	}

} _start_StartOLEProcess;

이 구조체는 Placing this structure definition anywhere in our application forces the application to call the _start_StartOLEProcess constructor once it has started.

생성자는 CoInitialize() 함수를 호출하여 OLE 를 초기화한다. 소멸자에서 CoUninitialize() 함수를 호출하여 OLE 를 정지시킨다.

This constructor simply calls CoInitialize to initialize OLE. Once our application is complete, the destructor of _start_StartOLEProcess will be called. This in turn will call CoUninitialize, which will shut down OLE.

The next thing we must do to create an ActiveX Data Object is to declare a pointer to the object we wish to create, as follows:

// define a variable that will be used as a reference to the
// Connection object and set it to NULL
ADODB::_ConnectionPtr con = NULL;

// define a variable that will be used as a reference to the
// Recordset object and set it to NULL
ADODB::_RecordsetPtr rst = NULL;

We then can create an ActiveX Data Object by calling the CreateInstance function of our ADO pointer. This function returns a result of type HRESULT to inform us whether the creation of the object was successful. This is illustrated in the following code fragment:

' create a new instance of an ADO Connection object
hr = con.CreateInstance( __uuidof( ADODB::Connection ) );

' create a new instance of an ADO Recordset object
hr = rst.CreateInstance( __uuidof( ADODB::Recordset ) );

Finally, just as in Visual Basic, it is always a good idea to release objects once they are no longer needed. In Visual C++, we accomplish this with a couple of lines of code that look like the following:

' remove the objects
con = NULL;
rst = NULL;

4 Using ADO with Visual C++: An Example #

Now let's take a look at a fully functional example of a Visual C++ application that utilizes ActiveX Data Objects. To try the following code, create a new Win32 Console Application from within Visual C++, choosing the Simple option from the wizard, and replace the contents of the main .cpp file with the code shown in Example 3-2.

Remember, just as with the Visual Basic example, make sure that a copy of Biblio.mdb is in the C:\Program Files\Microsoft Visual Studio\VB98 directory, or that you change the directory in the following source code to reflect the proper path of the Access database. In addition, if you are having trouble with this code, make sure that you have the MSADO15.DLL file in the C:\Program Files\Common Files\System\ado directory or that you have the proper directory entered in the source code.

Example 3-2. A Simple Visual C++ Example

#include "stdafx.h"
#include <stdio.h>

#import "C:\Program Files\Common Files\System\ado\MSADO15.dll" rename( "EOF", "EOFile" )

struct StartOLEProcess
{
    StartOLEProcess()
    {
        ::CoInitialize(NULL);
    }

    ~StartOLEProcess()
    {
        ::CoUninitialize();
    }

} _start_StartOLEProcess;

void main( void )
{
    // define our variables which will be used as references to the
    // Connection and Recordset objects
    ADODB::_ConnectionPtr con = NULL;
    ADODB::_RecordsetPtr  rec = NULL;

    // define variables to read the Author field from the recordset

    ADODB::FieldPtr pAuthor;
    _variant_t      vAuthor;
    char            sAuthor[ 40 ];

    // create two strings for use with the creation of a Connection
    // and a Recordset object

    bstr_t sConString;
    bstr_t sSQLString;

    // create a variable to hold the result to function calls

    HRESULT hr = S_OK;

    // long variable needed for Execute method of Connection object

    VARIANT *vRecordsAffected = NULL;

    // create a new instance of an ADO Connection object

    hr = con.CreateInstance( __uuidof( ADODB::Connection ) );

    printf( "Connection object created.\n" );

    // open the BiblioDSN data source with the Connection object

    sConString = L"Provider=Microsoft.Jet.OLEDB.4.0; "
                 L"Data Source=C:\\Program Files\\"
                        L"Microsoft Visual Studio\\"
                        L"VB98\\Biblio.mdb";

    con->Open( sConString, L"", L"", -1 );

    printf( "Connection opened.\n" );

    // create a Recordset object from a SQL string

    sSQLString = L"SELECT TOP 10 Author FROM Authors;";

    rec = con->Execute( sSQLString,
                        vRecordsAffected,
                        1 );

    printf( "SQL statement processed.\n" );

    // point to the Author field in the recordset

    pAuthor = rec->Fields->GetItem( "Author" );

    // retrieve all the data within the Recordset object

    printf( "Getting data now...\n\n" );

    while( !rec->EOFile )
    {
        // get the Author field's value and change it
        // to a multibyte type

        vAuthor.Clear();

        vAuthor = pAuthor->Value;

        WideCharToMultiByte( CP_ACP,
                             0,
                             vAuthor.bstrVal,
                             -1,
                             sAuthor,
                             sizeof( sAuthor ),
                             NULL,
                             NULL );

        printf( "%s\n", sAuthor );
        rec->MoveNext();
    }

    printf( "\nEnd of data.\n" );

    // close and remove the Recordset object from memory

    rec->Close();
    rec = NULL;

    printf( "Closed an removed the "
            "Recordset object from memory.\n" );

    // close and remove the Connection object from memory

    con->Close();
    con = NULL;

    printf( "Closed and removed the "
            "Connection object from memory.\n" );

}

Although much of the previous example will be very foreign to you until you have a thorough understanding of how to develop applications with ActiveX Data Objects, it is particularly important to notice how Visual C++ applications must convert datatypes returned by a field's value. In Example 3-2, a function called WideCharToMultiByte is used to convert a Variant datatype to a normal char string datatype (ASCII) so that it can in turn be passed to the printf function.

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