C++ and C Programming

While the examples in the OLE 2 Programmer's Reference were written in C++, the examples in this chapter were written in standard C. The following list describes some of the differences you will encounter.

• Reference data types (such as REFCLSID and REFIID) must be used in standard C with a prepended ampersand (&). In C++, these data types are often used with C++ references, so the ampersand is not required. Because references do not exist in standard C, an explicit address-of operator (the ampersand) is required.

  For example, the CoCreateInstance function uses both a class ID reference (REFCLSID) and an interface ID reference (REFIID) as arguments. In C++, the call to this function can be written as:

  CoCreateInstance(CLSID_ExcelApp, NULL, CLSCTX_LOCAL_SERVER,
      IID_IDispatch, &pdispExcelApp);
  

  In standard C, however, you must use the address-of operator with the first and fourth arguments:

  CoCreateInstance(&CLSID_ExcelApp, NULL, CLSCTX_LOCAL_SERVER,
      &IID_IDispatch, &pdispExcelApp);
  

• In C++, you can call a class member function directly using the member-selection operator (->). In standard C, classes are defined as structures; each structure contains an lpVtbl element that is a pointer to the table of class functions. You must first access the lpVtbl element of the structure and then call the function using a pointer. In addition, the first argument to the function must be the object pointer itself.

  For example, when you are done with an object you have created with the CoCreateInstance function, you should free it with the Release function. In C++, you can call this function directly:

  pdisp->Release();
  

  In standard C, you call the function using the lpVtbl pointer and pass in the dispatch object as the first argument:

  (*(pdisp->lpVtbl->Release))(pdisp);
  

• In C++, you can use the Microsoft Foundation Class library and the ClassWizard in Visual C++ to access OLE methods and properties as member functions of a base class. This simplifies much of the work required to access OLE objects with the IDispatch interface. For more information about using the ClassWizard, see the Visual C++ documentation.