PRB: Problem Using -1 with Double or Float Field Variable

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++ for Windows, version 1.5

SYMPTOMS

If an edit control is mapped to a double or float field variable using DDX and RFX functions, a value of -1 for the variable is changed to NULL when a CRecordView moves past the record. To see how this works, perform the follow steps:

1. Use the Enroll sample that ships with Visual C++, version 1.5.

2. Add a field called "Fee" to the SECTION Table using Access, version 1.1.

3. Set the field to the Access data type of "Number".

4. Enter the data "1", "0", and "-1" into some records in the Fee field of the SECTION table.

5. Map a CRecordset double field variable, "m_Fee", to the column.

6. Add the label "Fee:" to ENROLL.RC and an edit-control box for "Fee:".

7. Map the m_Fee variable to the edit control.

8. Build the project and run it.

9. Move through the records. When the record with a -1 value for the fee appears, move to the next record and then back again. You will see that the -1 has been replaced with a NULL value. The edit control is empty.

CAUSE

The Microsoft Foundation Classes need some way of determining whether a field value is NULL (no value). Because a double variable always has some value, MFC has defined what it calls "pseudo-null" values. If a variable contains a pseudo-null value, it is basically considered NULL. In the file AFXDB.H, you can see what the pseudo-null values are:

   #define AFX_RFX_INT_PSEUDO_NULL (0x7EE4)
   #define AFX_RFX_LONG_PSEUDO_NULL (0x4a4d4120L)
   #define AFX_RFX_BYTE_PSEUDO_NULL 255
   #define AFX_RFX_SINGLE_PSEUDO_NULL (-1.0)
   #define AFX_RFX_DOUBLE_PSEUDO_NULL (-1.0)
   #define AFX_RFX_BOOL_PSEUDO_NULL 2
   #define AFX_RFX_DATE_PSEUDO_NULL CTime(0)

The fundamental problem is in the code for "RFX_Double()" and "RFX_Float()" functions where the MarkForUpdate case is handled. Here is the code for RFX_Double():

   case CFieldExchange::MarkForUpdate:
        if (value == AFX_RFX_DOUBLE_PSEUDO_NULL)
            pFX->m_prs->SetFieldFlags(nField,
                AFX_SQL_FIELD_FLAG_NULL, pFX->m_nFieldType);
        else
            pFX->m_prs->ClearFieldFlags(nField,
                AFX_SQL_FIELD_FLAG_NULL, pFX->m_nFieldType);
        goto LDefault;

   if (pSet->CanUpdate())
    {
        pSet->Edit();
        if (!UpdateData())
            return FALSE;

        pSet->Update();
    }

RESOLUTION

There are two possible workarounds. The value of the pseudo-null value can be changed to something that a user of the application is not expected to type into the edit control. So, in the case of RFX_Double(), "AFX_RFX_DOUBLE_PSEUDO_NULL" can be changed in AFXDB.H to something like the following:

   #define AFX_RFX_DOUBLE_PSEUDO_NULL (-9.123e19)

Another way to work around this problem is to copy the code for RFX_Double into the .CPP file where your CRecordset class implementation code is. Change the function name to "RFX_NewDouble()" or whatever you want to call it. Then, delete the "MarkForUpdate" case statement shown above. You can also remove the "LDefault:" label, which is used by the goto statement at the end of the MarkForUpdate case.

Once you have created this new RFX_NewDouble() function, you need to replace the call to RFX_Double() in your CRecordset's DoFieldExchange() function with RFX_NewDouble().

MORE INFORMATION

Beginning with Visual C++ for Windows version 1.51 and Visual C++ 32-bit Edition version 2.0, the definitions of AFX_RFX_SINGLE_PSEUDO_NULL and AFX_RFX_DOUBLE_PSEUDO_NULL use a value of -9.123e19 instead of -1.0.