I've just submitted a bug report:
http://bugs.freepascal.org/view.php?id=27766
plus the related http://bugs.freepascal.org/view.php?id=27768
27766 is intended to improve handling of multibyte character sets by
TStringField, only I am not quite sure about whether I have fully
understood the meaning of TField.Size and hoped to get some feedback on
this.
The basic problem is that TStringField is only really set up for single
byte character sets and TFieldDef just ignores the whole issue.
TField descendents have three properties to play with: DisplayWidth,
Size and DataSize. The documentation for DisplayWidth is pretty clear -
it's the character width. Ditto DataSize, which is the storage size,
while Size is defined as the "logical size", whatever that means.
In the current TStringFIeld, DisplayWidth = Size and DataSize = Size +
1. For single byte character sets that's fine, with DataSize adding an
extra byte for the null terminator. However, with multi-byte character
sets, is the (logical) Field Size the number of characters or the number
of bytes (less the null terminator) - or something else? My reading is
that it is the number of bytes.
This is important because TFieldDef currently only allows the database
driver (in a TDataSet Descendent) to set the Field Size. There is no
property that allows a multibyte character set to be declared or the max
number of bytes per character. it is possible to work around this, but
only by some rather dirty coding.
My proposal is to enable a better approach by adding an extra property
to TFieldDef - CharSetWidth - which is the max. number of bytes per
character (e.g. 4 for UTF-8) and to pass this on to TStringField when it
is created by TFieldDef.CreateField. But then what should TStringField
do with this information. Should it:
a) Set the default display width to the Field Size div CharSetWidth, and
set DataSize to Field Size + 1, or
b) Set both the Display Width and the Field Size to the TFieldDef Size
div CharSetWidth and Set DataSize to the TFieldDef Size +1, or
c) Something else.
Really, the answer depends on what does the "logical" size mean?
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