DTD

Friday, 6 July 2012

A Default Attribute Value

DTD:
<!ELEMENT square EMPTY>
<!ATTLIST square width CDATA "0">

Valid XML:
<square width="100" />

In the example above, the "square" element is defined to be an empty element with a "width" attribute of type CDATA. If no width is specified, it has a default value of 0.

#REQUIRED

Syntax

<!ATTLIST element-name attribute-name attribute-type #REQUIRED>

Example

DTD:
<!ATTLIST person number CDATA #REQUIRED>

Valid XML:
<person number="5677" />

Invalid XML:
<person />

Use the #REQUIRED keyword if you don't have an option for a default value, but still want to force the attribute to be present.

#IMPLIED

Syntax

<!ATTLIST element-name attribute-name attribute-type #IMPLIED>

Example

DTD:
<!ATTLIST contact fax CDATA #IMPLIED>

Valid XML:
<contact fax="555-667788" />

Valid XML:
<contact />

Use the #IMPLIED keyword if you don't want to force the author to include an attribute, and you don't have an option for a default value.

#FIXED

Syntax

<!ATTLIST element-name attribute-name attribute-type #FIXED "value">

Example

DTD:
<!ATTLIST sender company CDATA #FIXED "Microsoft">

Valid XML:
<sender company="Microsoft" />

Invalid XML:
<sender company="ABC" />

Use the #FIXED keyword when you want an attribute to have a fixed value without allowing the author to change it. If an author includes another value, the XML parser will return an error.

Enumerated Attribute Values

Syntax

<!ATTLIST element-name attribute-name (en1|en2|..) default-value>

Example

DTD:
<!ATTLIST payment type (check|cash) "cash">

XML example:
<payment type="check" />
or
<payment type="cash" />

Use enumerated attribute values when you want the attribute value to be one of a fixed set of legal values.

Empty Elements

Empty elements are declared with the category keyword EMPTY:

<!ELEMENT element-name EMPTY>

Example:

<!ELEMENT br EMPTY>

XML example:

<br />

Declaring Elements

In a DTD, XML elements are declared with an element declaration with the following syntax:

<!ELEMENT element-name category>
or
<!ELEMENT element-name (element-content)>