When you call a method that has optional parameters from Visual C++, you cannot omit the optional parameters. You need to pass a special VARIANT instead.
Some methods have "optional" parameters. In Visual Basic, you can omit them when calling the method. However, when calling with Visual C++, you have to pass a special VARIANT whose .vt field has the value VT_ERROR and .scode field has the value DISP_E_PARAMNOTFOUND. That is:
// VARIANT used in place of optional-parameters.
VARIANT varOpt;
varOpt.vt = VT_ERROR;
varOpt.scode = DISP_E_PARAMNOTFOUND;
This is what Visual Basic is doing behind-the-scenes.
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