Crashing in cstring when assigning

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COleVariant olevar;
rst.GetFieldValue(a, olevar);
CString str = (LPCTSTR)olevar.bstrVal;(carshing)

it is crashing in the str ..
when olevar is having double value. please let me know why it is crashing.

What I have tried:

COleVariant olevar;
rst.GetFieldValue(a, olevar);
CString str = (LPCTSTR)olevar.bstrVal;(carshing)

Posted 11-Aug-16 20:31pm

Gayle123

Updated 11-Aug-16 21:15pm

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Jochen Arndt · Accepted Answer · 2016-08-11T21:15:00

COleVariant is derived from VARIANT (VARIANT and VARIANTARG Data Types [Automation][^]) which is just a union.

So you must check the vt member and access the matching data field only. If your olevar contains a double (vt is VT_R8), the corresponding field is dblVal. Accessing any other fields may result in wrong values or undefined behaviour.

You can do the conversion yourself:

C++

CString str;
if (oleVar.vt == VT_R8)
    str.Format(_T("%E"), oleVar.dblVal);

Another option is converting the variant type using COleVariant::ChangeType[^]:

oleVar.ChangeType(VT_BSTR);
CString str = oleVar.bstrVal;

Note also that no casting to LPCTSTR is used in the above example. BSTR is always Unicode and the above assignment will convert the string to ANSI with non-Unicode builds while casting to LPCTSTR would not work with non-Unicode builds (it would copy only the first character).