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| author | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2014-08-02 08:43:31 +0200 |
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| committer | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2014-08-02 08:43:31 +0200 |
| commit | daf17154bf13139d9375f48525d19d6aaba08155 (patch) | |
| tree | e3c08b6c49dc8a8e83f03327591310546675b43d /html/xbc3.htm | |
Imported Upstream version 3.1.2upstream/3.1.2
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diff --git a/html/xbc3.htm b/html/xbc3.htm new file mode 100755 index 0000000..b944a1b --- /dev/null +++ b/html/xbc3.htm @@ -0,0 +1,91 @@ +<!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> +</BODY> +</HTML> |