DTD

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" />

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE square [
<!ELEMENT square EMPTY>
<!ATTLIST square width CDATA "0">
]>
<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 />

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE person [
<!ELEMENT person EMPTY>
<!ATTLIST person number CDATA #REQUIRED>
]>
<person number="5677"/>

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 />

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE contact [
<!ELEMENT contact EMPTY>
<!ATTLIST contact fax CDATA #IMPLIED>
]>
<contact fax="555-667788"/>

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="W3Schools" />

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sender [
<!ELEMENT sender EMPTY>
<!ATTLIST sender company CDATA #FIXED "Microsoft">
]>
<sender company="Microsoft"/>

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" />

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE payment [
<!ELEMENT payment EMPTY>
<!ATTLIST payment type (check|cash) "cash">
]>
<payment/>

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