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<!DOCTYPE HTML PUBLIC>
<HTML>
<TITLE>Xbase DBMS Chapter 3</TITLE>
<BODY BGCOLOR=#FFFFFF>
<H1><p align="center">Fields and Strings</p></H1>
<p align="center">Chapter Updated 8/20/03</p><hr>
<br><br>
The title of this chapter almost sounds like a popular North American
magazine called Field and Stream. The difference between this chapter and
that magazine is there are no dead animal carcuses or fishing lures in
this chapter. For those you will need to get a copy of Field and Stream.
<br><br>
The main objective of this chapter is to provide information regarding the
basic concepts of manipulating data via the field methods.<br><br>
Field names can be up to ten bytes in length and can contain characters, numbers
or special characters in the name. The field methods are used to manipulate
the data in a record of a data file. There are several types of fields.<br><br>
<TABLE BORDER>
<CAPTION ALIGN="TOP"><h3>Field Types</H3></CAPTION>
<TR VALIGN="BASELINE">
<TR><TH ALIGN="LEFT">Type<TD>Size<TD>Allowable Values<TD>Schema Value
<TR><TH ALIGN="LEFT">Numeric<TD>0 - 17(include sign and decimal point<TD>+ - . 0 through 9<TD>XB_NUMERIC_FLD
<TR><TH ALIGN="LEFT">Character<TD>0 - 254<TD>Anything<TD>XB_CHAR_FLD
<TR><TH ALIGN="LEFT">Date<TD>8<TD>CCYYMMDD<TD>XB_DATE_FLD
<TR><TH ALIGN="LEFT">Floating Point<TD>0 - 17 (includes sign and decimal point<TD>+ - . 0 through 9<TD>XB_FLOAT_FLD
<TR><TH ALIGN="LEFT">Logical<TD>1<TD>? Y y N n T t F f (? - uninitialized)<TD>XB_LOGICAL_FLD
<TR><TH ALIGN="LEFT">Memo<TD>Fixed length portion - 10<br>Variable length 0 - 32760
<TD>Type III - Text<br>Type IV - Anything<TD>XB_MEMO_FLD
</TABLE>
<br><br>
Field names, types and lengths are defined when a data file is created.
After the file is created, the field characteristics can not be changed. To
change field characteristics, a new database must be defined with the new
field requirements.<br><br>
<h2>Memo Fields</h2>
Memo fields are variable length data fields which are stored in two parts.
This first part is a ten byte field which is stored
in the fixed length record of the .DBF file. The variable data is stored in
a seperate .DBT file in 512 byte blocks. The ten byte field in the fixed
length portion of the record points to a .DBT block number.<br><br>
There are two versions of memo data files type III and type IV. Type IV
is more advanced in that released space can be reused and it also
supports BLOB data. The type III file is older technology, does not
support dynamic space reclamation and only supports string data.
See method xbDbf::SetVersion for controlling which version type you are
using.
<br><br>
To utilize memo fields, the application program must allocate a buffer
which is large enough to handle the memo data.<br><br>
<h2>Fields and Field Numbers</h2>
The Xbase routines can access field data via using field names or field
numbers. Field numbers are numbered 0-n where the first field in a datafile
is field 0 going through the last field n. Accessing fields by number is
more efficient than accessing by name.<br><br>
<h2>Record Schema</h2>
The record schema of a file can be found in the documentation for method
CreateDatabase in the Database Methods chapter, or view example program
sample1.cpp.</A><br><br>
<h2>Strings</h2>
Beginning with Release 1.8.0a, Xbase includes support for a string class
<em>xbString</em>. The original thinking was not to include a string
class in the library, and encourage users to use the string class
native to the user's compiler. But as C++ does not have a native way
to handle strings, and there is not a standard string class across C++
compilers, it was decided to include a lightweight string class with the
library. In some cases this is redundant, but it is portable.
<br><br>
The xbString class interface was derived from the <em>Draft Standard C++
Library by P.J. Plauger</em> and modified. If you are familiar with
other string classes, this one should feel similar.
<br><br>
<hr>
<p><img src="xbase.jpg"><hr>
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