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class=f14> <B>利用(IDispatch
*)转换ADO对象的指针类型</B><BR><BR> 1．在一个Variant中显式地封装一个活动的Connection对象，然后用(IDispatch
*)进行类型转换确保正确的构造函数被调用。同时明确地设置第二个参数为缺省的true，使该对象的引用计数在Recordset::Open操作完成后仍得到正确的维护。<BR><BR> 2．表达式(_bstr_t)不是一个类型转换，而是一个_variant_t的操作符，用以从中提取一个_bstr_t字符串。<BR><BR> 表达式(char*)也不是一个类型转换，而是一个_bstr_t的操作符，用以从中提取封装在_bstr_t中的字符串的指针。<BR><BR> 下面这些代码演示了_variant_t和_bstr_t的一些常见操作。<BR><BR>
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<TD>#import "c:\Program Files\Common Files\System\ADO\msado15.dll"
\<BR>no_namespace rename("EOF", "EndOfFile")<BR><BR>#include
<stdio.h><BR><BR>void
main(void)<BR>{<BR>CoInitialize(NULL);<BR>try
<BR>{<BR>_ConnectionPtr pConn("ADODB.Connection");<BR>_RecordsetPtr
pRst("ADODB.Recordset");<BR><BR>pConn->Open("Provider=sqloledb;Data
Source=a-tima10;"<BR>"Initial Catalog=pubs;User Id=sa;Password=;",
<BR>"", "", adConnectUnspecified);<BR>// Note
1<BR>pRst->Open(<BR>"authors",<BR>_variant_t((IDispatch *) pConn,
true),<BR>adOpenStatic,<BR>adLockReadOnly,<BR>adCmdTable);<BR>pRst->MoveLast();<BR>//
Note 2<BR>printf("Last name is '%s %s'\n", <BR>(char*) ((_bstr_t)
pRst->GetFields()->GetItem("au_fname")->GetValue()),<BR>(char*)
((_bstr_t)
pRst->Fields->Item["au_lname"]->Value));<BR><BR>pRst->Close();<BR>pConn->Close();<BR>}<BR>catch
(_com_error &e)<BR>{<BR>printf("Description = '%s'\n", (char*)
e.Description());<BR>}
<BR>::CoUninitialize();<BR>}<BR></TD></TR></TBODY></TABLE><BR> 对于VC++程序员而言，每次都要将ADO返回的数据转换成一般的C++数据类型，接着将数据存入一个类或结构总是一件枯燥的事。更讨厌的是这也带来了效率的低下。<BR><BR> 因此，ADO提供了一个接口以支持将数据直接返回为一个本地化的C/C++数据类型而非VARIANT，并提供了一系列的预处理宏来方便使用这些接口。这样做的结果是一个复杂的工具可以很轻松的被使用并能获得很好的性能。<BR><BR> 一个普通的C/C++客户场景是将一个Recordset中的一条记录绑定到一个包含本地C/C++数据类型的C/C++结构或类之上。如果通过Variant传递数据，这意味着要编写大量的转换代码，以在VARIANT和C/C++本地类型间进行数据转换。VC++对ADO的扩展出现的目的就是要简化这一过程。<BR><BR></SPAN><BR>
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