Xbase DBMS Chapter 3

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Fields and Strings

Chapter Updated 8/20/03

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.
The main objective of this chapter is to provide information regarding the basic concepts of manipulating data via the field methods.
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.

### Field Types

Type | Size | Allowable Values | Schema Value
Numeric | 0 - 17(include sign and decimal point | + - . 0 through 9 | XB_NUMERIC_FLD
Character | 0 - 254 | Anything | XB_CHAR_FLD
Date | 8 | CCYYMMDD | XB_DATE_FLD
Floating Point | 0 - 17 (includes sign and decimal point | + - . 0 through 9 | XB_FLOAT_FLD
Logical | 1 | ? Y y N n T t F f (? - uninitialized) | XB_LOGICAL_FLD
Memo | Fixed length portion - 10 Variable length 0 - 32760 | Type III - Text Type IV - Anything | XB_MEMO_FLD

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.

## Memo Fields

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.
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.
To utilize memo fields, the application program must allocate a buffer which is large enough to handle the memo data.

## Fields and Field Numbers

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.

## Record Schema

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.

## Strings

Beginning with Release 1.8.0a, Xbase includes support for a string class xbString. 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.
The xbString class interface was derived from the Draft Standard C++ Library by P.J. Plauger and modified. If you are familiar with other string classes, this one should feel similar.