Comparison of Win32 Functions with OLE Functions

In theory, the functions exported out of OLE32.DLL and OLEAUT32.DLL are no different from any other DLL function. In practice, the OLE functions follow a consistent calling style of their own. OLE functions differ from Win32 functions in the following ways:

• OLE functions only come in Unicode. There are no ANSI OLE functions.
  
  
• Virtually all OLE functions return a 32-bit status code known as an HRESULT through the normal function return value. The high-bit indicates whether the function succeeded (hi-bit off) or failed (hi-bit on). A few functions return multiple success values (typically S_OK and S_FALSE) but most return only one success value (S_OK).
  
  
• If an OLE function returns some value other than the status code, it does so by having the caller supply a pointer to a variable that will receive the value on exit. This return value pointer is, by convention, the last parameter.
  
  
• The Win32 style of returning strings is to fill in a caller-allocated buffer. OLE functions typically return strings by allocating them using CoTaskMemAlloc and expecting the caller to free them using CoTaskMemFree.