Comparison of Win32 Functions with OLE Functions

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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.