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Tentative definitions

The ANSI C standard supports the concept of the tentative definition. Any external data declaration that has no storage class specifier and no initializer is considered a tentative definition. If the identifier declared appears in a later definition, then the tentative definition is treated as if the extern storage class specifier were present. In other words, the tentative definition becomes a simple referencing declaration.

If the end of the translation unit is reached and no definition has appeared with an initializer for the identifier, then the tentative definition becomes a full definition, and the object defined has uninitialized (zero-filled) space reserved for it. For example,

int x;

int x; /*legal, one copy of x is reserved */
int y;
int y = 4; /* legal, y is initialized to 4 */
int z = 5;

int z = 6; /* not legal, both are initialized definitions */

Unlike ANSI C, C++ doesn't have the concept of a tentative declaration; an external data declaration without a storage class specifier is always a definition.

TClientDataSet

TClientDataSet implements a database-independent dataset.

Unit

dbclient

Description

TClientDataSet encapsulates a database-independent, distributed dataset. A client dataset can be used as a

Fully-functional, standalone, flat-file dataset for single-tiered database applications. When used in this manner, an application uses the client dataset to read from and write to a user’s hard disk directly, without accessing a database engine.
Client dataset in the client application portion of a multi-tiered database application.

Note: The two functions described above are not mutually exclusive. A multi-tiered application can be designed to support the option of working with data off-line, using the “briefcase” model. On site, the application uses TClientDataSet to communicate with the application server’s data provider. When a user works off site, the client dataset writes a snapshot of the data to the hard disk. The application works with this snapshot off site, with the client dataset acting as a flat-file dataset in a single-tiered application.

The client dataset can communicate with a data provider directly by calling the provider component’
s methods. When used in a client application as part of a multi-tiered application, the client dataset passes all calls to the provider through the remote data module’s IAppServer interface.

IAppServer::AS_Execute

Executes the query or stored procedure bound to a specified provider. virtual HRESULT __safecall AS_Execute(const WideString: ProviderName, const WideString: CommandText, OleVariant &Params, OleVariant &OwnerData) = 0 ; Description Use AS_Execute to pass an execute command to a remote provider, using a specified set of parameters. AS_Execute calls the Execute method of the specified provider after assigning any parameters. Output parameters that result from executing a query or stored procedure are returned in the Params parameter. Note: Different types of provider interpret the Execute command in different ways. Some providers merely generate BeforeExecute and AfterExecute events, while others execute a query or stored procedure. The ProviderName parameter specifies the name of the provider component associated with the query or stored procedure. CommandText specifies an optional SQL statement that replaces the SQL of the provider’s query, or the name of a stored procedure that replaces the provider’s stored procedure. This parameter is ignored if the Options property does not include poAllowCommandText. The Params parameter encodes any parameters expected by the query or stored procedure and returns any output parameters. To create a properly encoded value for Params from a TParams object, use the PackageParams routine. To decode Params into a TParams object, use the UnpackParams routine. The OwnerData parameter contains custom information that appears as an argument to the provider’s BeforeExecute and AfterExecute event handlers. This information originates in a client dataset’s BeforeExecute event handler. The value returned in OwnerData is passed to the client dataset’s AfterExecute event handler. Client applications must explicitly call AS_Execute through the client dataset’s Execute method. C++Builder does not call AS_Execute automatically.

Field Link designer

The Field Link Designer provides a visual way to link (join) master and detail tables.
At design time, drop a TDataSource object on the form and define a datasource. Select the TTable component and double-click the MasterFields property in the Object Inspector to invoke the Field Link designer.

Available Indexes combo box

The Available Indexes combo box shows the currently selected index used to join the tables. Unless you specify a different index name in the table’s IndexName property, the default index used for the link is the primary index for the table. Other available indexes defined on the table can be selected from the drop-down list.

To link master and detail tables:

1 Select the field to use to link the detail table in the Detail Fields list
2 Select the field to link the master table in the Master Fields list.
3 Choose Add.

The selected fields are be displayed in the Joined Fields list box. For example,

OrderNo -> OrderNo

Note: For tables on a database server, the Available Indexes combo box will not appear, and you must manually select the detail and master fields to join in the Detail Fields and Master Fields list boxes.

BeforeGetRecords, Active, CloneCursor, FetchOnDemand Example

This example shows how to use the BeforeGetRecords event handler to send the application server information it needs for incremental data fetching. Before fetching the next data packet, the client dataset packages up the key value of the last record so that the application server knows where to begin the next data packet. It also sends some application-specific information, which is stored in Memo1.

void __fastcall TForm1::ClientDataSet1BeforeGetRecords(TObject *Sender, OleVariant &OwnerData)
{
OleVariant[2] Values;
Values[0] = NULL;
if (ClientDataSet1->Active)
{
ClientDataSet *CDSClone = new ClientDataSet(Form1);
try
{
CDSClone->CloneCursor(ClientDataSet1, true);
// turn off FetchOnDemand so that the clone only
// fetches the last LOCAL record
CDSClone->FetchOnDemand = false;
CDSClone->Last();
Values[0] = CDSClone->Fields->Fields[0]->Value;
CDSClone->Close();
}
__finally
{
delete CDSClone;
}
}
Values[1] = Memo1->Lines->Text;
OwnerData = VarArrayOf(Values,1);
}

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TClientDataSet::FetchOnDemand

Indicates whether to fetch data packets from the provider on an as needed basis.

__property bool FetchOnDemand = {read=FFetchOnDemand, write
=FFetchOnDemand, default=1};

Description

By default FetchOnDemand is true, meaning that the client dataset fetches additional packets of data as needed (for example, as a user scrolls through data, or conducts a search). If the server does not automatically include BLOB fields or nested detail sets in data packets, setting FetchOnDemand to true causes the client dataset to fetch these values when needed as well.

Set FetchOnDemand to false only if your client application contains code that explicitly fetches data packets as needed using the GetNextPacket method. Similarly, if FetchOnDemand is false, and the provider component does not include BLOB data by default (Options includes poFetchBlobsOnDemand), the client application must fetch BLOB fields explicitly using the FetchBlobs method. If the provider does not include nested detail sets (Options includes poFetchDetailsOnDemand) and FetchOnDemand is false, the client application must fetch nested details explicitly using the FetchDetails method.

FetchOnDemand Example

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