In a DTD, attributes are declared with an ATTLIST declaration.
Declaring Attributes
An attribute declaration has the following syntax:
<!ATTLIST element-name attribute-name attribute-type attribute-value>
DTD example:
<!ATTLIST payment type CDATA "check">
XML example:
<payment type="check" />
The attribute-type can be one of the following:
| Type | Description |
|---|---|
| CDATA | The value is character data |
| (en1|en2|..) | The value must be one from an enumerated list |
| ID | The value is a unique id |
| IDREF | The value is the id of another element |
| IDREFS | The value is a list of other ids |
| NMTOKEN | The value is a valid XML name |
| NMTOKENS | The value is a list of valid XML names |
| ENTITY | The value is an entity |
| ENTITIES | The value is a list of entities |
| NOTATION | The value is a name of a notation |
| xml: | The value is a predefined xml value |
The attribute-value can be one of the following:
| Value | Explanation |
|---|---|
| value | The default value of the attribute |
| #REQUIRED | The attribute is required |
| #IMPLIED | The attribute is optional |
| #FIXED value | The attribute value is fixed |
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="w3ii" />
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.