这是模板链接：{{If empty}}模板。
- 2014年12月23日更新：此模板过去最多只可使用9个参数。此限制已移除。
此模板用于其他模板内。此模板将读取任何参数并将第一个有参数的定义输出。如果并无发现，不作输出。典型用法如下：
{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}- ("Return {{{logo|}}} if it has a value; else {{{image|}}} if it has a value; else {{{picture|}}} if it has a value; else return "default.svg".")
This returns the first of the parameters logo, image and picture that is defined and non-empty, otherwise "default.svg".
Rationale
The MediaWiki parameter default function doesn't return the default value for empty parameters. That is, {{{logo|default.svg}}} does not return "default.svg" if the template was called like this: {{template|logo=}}.
The usual workaround for a single parameter is:
{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}- ("If {{{logo}}} has a value, return it, else return "default.svg".")
But this becomes complex when several parameters are to be checked:
{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}- ("If {{{logo}}} has a value, return it; else if {{{image}}} has a value, return that; else if {{{picture}}} has a value, return that; else return "default.svg".")
In these cases, 模板链接：{{if empty}} produces the simpler syntax (as above):
{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}
Piping
Parameters used with 模板链接：{{if empty}} must be piped – i.e. include the vertical bar (pipe) symbol (" | ") as a trailing character – so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, {{{logo|}}}, {{{image|}}} and {{{picture|}}} rather than {{{logo}}}, {{{image}}} and {{{picture}}} in the above examples.
Examples
| Code | Result | |
模板链接：{{if empty}} | Returns an empty string. | |
模板链接：{{if empty|one}} | one | Returns the first parameter that is defined and not empty. |
模板链接：{{if empty|one|two}} | one | |
模板链接：{{if empty|one|two|three|four}} | one | |
模板链接：{{if empty||two}} | two | The first parameter is empty/undefined, so is passed over. |
模板链接：{{if empty||two|three|four}} | two | |
模板链接：{{if empty||two||four}} | two | |
{{if empty||||||||||ten}} | ten | Unlike in previous versions, this template is no longer limited by 9 parameters. |
模板链接：{{if empty|}} | The only parameter is empty or undefined, so returns an empty string. | |
模板链接：{{if empty||||}} | Returns an empty string. | |
{{if empty|{{{1|}}}|{{{2|}}}|three}} | three | |
{{if empty|{{{1}}}|{{{2}}}|three}} | {{{1}}} | No pipe-characters following the names of the first two parameters ("1" and "2"), so the first of these returned as text ("{{{1}}}"). |
{{if empty|{{{logo|}}}|two}} | two | |
{{if empty|{{{logo}}}|two}} | {{{logo}}} | No pipe-character following the parameter name "logo", so the text "{{{logo}}}" returned. |
{{if empty|p=q}} | The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc. |
See also
- 模板链接：{{if all}}
- 模板链接：{{if either}}
- Help:Parameter default