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In a DTD, attributes are declared with an ATTLIST declaration.
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 |
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.
<!ATTLIST element-name attribute-name attribute-type #REQUIRED>
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.
<!ATTLIST element-name attribute-name attribute-type #IMPLIED>
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.
<!ATTLIST element-name attribute-name attribute-type #FIXED "value">
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.
<!ATTLIST element-name attribute-name (en1|en2|..) default-value>
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.
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原文地址：http://www.cnblogs.com/ghgyj/p/3991935.html
