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Tham khảo tài liệu 'excel add-in development in c/c++ applications in finance phần 5', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 140 Excel Add-in Development in C C return false b ret_val.val._bool 0 return true Using the cpp_xloper class the conversion would look like this cpp_xloper Oper Some code that sets Oper s value. bool result bool Oper The code for the overloaded conversion operator bool is cpp_xloper operator bool void bool b if coerce_to_bool m_Op b return b return false What the memory considerations are None unless the 10 bytes for the xloper itself are dynamically allocated as the integer _bool is contained entirely within the xloper. How you can avoid using it Declare functions as taking int arguments and or returning ints Excel will do the necessary conversions. 6.8.5 Worksheet error value xltypeErr When you will encounter it This xloper type is used by Excel for all error values passed from worksheets to a DLL. When you want your DLL code to be called even if one of the inputs evaluates to an error such as range with invalid references - REF you should declare arguments as xlopers. Otherwise Excel will intercept the error and fail the function call before the DLL code is even reached. This xloper type is returned by most of the C API functions when they fail to complete successfully. DLL functions accessed via VB that accept Variant arguments or Passing Data between Excel and the DLL 141 arrays of Variants may need to convert between the Variant representation of Excel errors and the C API error codes. This is discussed in section 3.6.11 Variant types that Excel can pass to VB functions on page 59. When you need to create it Excel s error codes provide a very well understood way of communicating problems to the worksheet and are therefore very useful. They have the added benefit of propagating through to dependent cells. It s a good idea to declare fallible worksheet functions as returning xlopers so that errors can be returned in addition to the desired output type. You might even want to pass an error code into a C API function although this is unlikely. How you create an
