- Tag (programming)
In
programming , a tag is an argument to asubroutine that determines other arguments passed to it, which is used as a way to pass indefinite number of tagged parameters to the subroutine; notably, tags are used for a number of system calls inAmigaOS v2.0 and onwards.In AmigaOS
In earlier versions of AmigaOS, if a system call required setting a large number of parameters, instead of passing them as function arguments, the function would require a pointer to a structure that holds the arguments (for example,
intuition.library 'sOpenWindow()requiredstruct NewWindowwith 17 different parameters). Tags were introduced in AmigaOS 2.0 because:Quote|Tags make it possible to add new parameters to system functions withoutinterfering with the original parameters. They also make specifyingparameter lists much clearer and easier.|
|cite book|title=Amiga® ROM Kernal Reference Manual: Libraries|edition=3rd edition|ISBN=0-201-56774-1|url=http://cataclysm.cx/random/amiga/reference/Libraries_Manual_guide/node0496.html|pages=867 A number of
third-party software libraries for AmigaOS also use tags extensively.Example
Note how the code without tags is obscure (for example,
0, 1define window colors) while the code with tags is self-documenting. Note also that fewer parameters have to be defined with tags than are in the structure, as OpenWindowTags will fall back to default parameters.Implementation
AmigaOS provides functions for tag handling in its
utility.library.In general
An advantage of tags is that they ease the work with default arguments since the programmer doesn't have to specify them or their substitutes. From this follows another advantage, ease of achieving of both forward and
backward compatibility with external libraries: a program written for an older version of the library will work with a newer one, since the newer library will simply set all the parameters not provided by the program to their default values; and a program written for a newer version of the library will still work with the older version, since the older library will simply pay no attention to the newly-introduced tags.A disadvantage of tags is that their processing is slower than simply reading data from a structure or the stack. Additionally,
compile time type checking is lost.External links
* [http://tbs-software.com/guide/index.php?guide=autodocs.doc%2Futility.doc utility.library autodoc]
* [http://cataclysm.cx/random/amiga/reference/Libraries_Manual_guide/node00B4.html Amiga® ROM Kernal Reference Manual: Libraries - Tag index]
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