ftBytes in TBufDataset
lazarus / 16-Databases

----------------------------------------------------------------------
ojz0r · 2024-06-26 12:59

I'm trying to use ftBytes as fieldtype in a BufDataset but it doesn't
behave the way i expect it to do.
When i set the Size attribute of the field to 1 i expect it to
allocate 1 byte, so max value 255 (or $FF). However it seems that
"Size" corresponds to number of digits, if i try to enter 123 it will
only keep 1, if i set size to 2 it will keep 12, and if i set size to
3 it will keep 123. However that means that it will accept values up
to 999.

Is there something im doing wrong or is this a bug?
I've tried to search for "ftBytes" and "ftByte" but i can't find
anything relevant.

----------------------------------------------------------------------
Zvoni · 2024-06-26 13:53

https://www.freepascal.org/docs-html/fcl/db/tfieldtype.html

The only thing coming close would be ftSmallInt, but that's signed
ftBytes is an Array of Bytes
Quote*snip*
ftTime,  Time value
ftDateTime, Date/Time (timestamp) value
ftBytes,  Array of bytes value, fixed size (unytped)
ftVarBytes, Array of bytes value, variable size (untyped)
ftAutoInc, Auto-increment integer value (4 bytes)
*snip*
That said: Set the "size" to 0

----------------------------------------------------------------------
ojz0r · 2024-06-26 18:22

Okay so no real byte type then.
Smallint seems variable depending on CPU architecture. For 64-bit
system a Smallint is 16-bit signed integer, so +/-32764.

----------------------------------------------------------------------
Richard Marriott · 2024-09-03 02:19

As already mentioned ftBytes represents an array of byte ( defined as
TBytes), I manged to get it to work with this code:

      FieldDefs.Add('XBYTES',ftBytes,1,false);  //1I byte in length

Then to add/retreive data after CreateTable or CreateDataset do this

     Setlength(tb,1);                                              //
tb is type TBytes 
     tb[0] := b;                                                     
 // where b is a number of type byte (0..255)
     FieldsByName('XBYTES').AsBytes := tb;     // Save the value to
the buffer
  
     tb2 := FieldsByName('XBYTES').AsBytes;  // Retrieve the value
into tb2 which is type TBytes    
     b2 := tb2[0];                                                 //
Same as original input
  
If you do this (as TDBGrid would do)
    s := FieldsByName('XBYTES'].AsString;     // s is chr(b2)  ie the
character equivalent

Hope that helps. Else as already suggested use ftSmallInt (2 bytes)
with external code to do range checking. I think that would be
simpler. ftBytes is really best for binary arrays where you need to
know exactly what is contained in each byte and process each byte
separately.

----------------------------------------------------------------------
Richard Marriott · 2024-09-03 02:53

Just had a better idea that works and is simpler. Try this:

      FieldDefs.Add('XBYTES',ftBytes,1,false);  //1 byte in length

Then to add/retrieve data after CreateTable or CreateDataset do this

     b1 := 123;                                                      
// b1 is type byte. assign any valid number 
     FieldsByName('XBYTES').SetData(@b1);     // Save the value to the
buffer
  
    FieldsByName('XBYTES').GetData(@b2);  // Retrieve the value into
b2 of type byte  result is 123