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=head1 REGISTRATION INFORMATION |
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|
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Tag 22098 (indirection) |
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Data Item multiple |
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Semantics hint that indicates an additional level of indirection |
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Reference http://cbor.schmorp.de/indirection |
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Contact Marc A. Lehmann <cbor@schmorp.de> |
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|
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=head1 SUMMARY |
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|
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This tag indicates an additional level of indirection when |
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decoding a value. |
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|
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=head1 BACKGROUND |
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|
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Data structures in some languages or data models sometimes employ |
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references, pointers or similar forms of indirection to represent |
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relationships between values. For example, in Perl, CBOR arrays are |
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usually represented as references to perl arrays, rather than arrays |
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themselves, while simple scalar values are represented as these values |
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themselves. However, CBOR leaves explicit references unencodable - while |
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arrays and maps might implicitly result in references, references to |
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simple values, or references to references are not representable. |
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|
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For example, in Perl: |
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|
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{ 1, 2, 3, 4 } # reference to a hash, becomes CBOR map |
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[ 1, 2, 3, 4 } # reference to an array, becomes CBOR array |
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\5 # reference to the scalar value 5, needs extension |
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\[] # reference to a reference to an array, needs extension |
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|
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This tag allows to encode such references - prefixing a value with this |
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tag indicates a hint to the decoder that the original value was actually |
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a reference. Decoders can chose to heed the hint (if their data model has |
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a convenient representation), or simply ignore it and decode the value |
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as-is. |
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|
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For example, CBOR encoders for Perl would not use the indirection tag for |
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hash and array references, as the only way to reference these values in a |
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data structure is already by reference. However, references to strings, |
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numbers, other references or other complex objects would be encoded by |
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tagging the referenced value with the indirection tag. |
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|
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Indirection tags can be nested - each additional level of nesting would |
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indicate an additional indirection. |
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|
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=head1 EXAMPLES |
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|
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The following Perl data structure is an array containing a reference to an |
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array, followed by a reference to a string: |
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|
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[[], \"string"] |
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|
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Since Perl cannot have "naked" arrays or maps inside another data |
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structure, but only a reference to it, the natural encoding of this |
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data structure uses a normal array for the array-reference (i.e. it |
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omits the reference). The string reference is uncommon but valid, and |
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not the natural way to store a string inside another data structure. An |
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encoder might therefore chose to encode this string reference using the |
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indirection tag: |
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|
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256([[], 22098("string")]) |
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|
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d9 0100 # tag(256) |
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82 # array(2) |
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80 # array(0) |
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d9 5652 # tag(22098) |
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66 # text(6) |
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737472696e67 # "string" |
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|
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=head1 IMPLEMENTATIONS |
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|
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This section lists known implementations of this extension (L<drop me a |
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mail|mailto:cbor@schmorp.de?Subject=CBOR-indirection> if you want to be |
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listed here). |
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|
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=over 4 |
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|
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=item * [Perl] L<CBOR::XS|http://software.schmorp.de/pkg/CBOR-XS.html> (reference implementation) |
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|
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=item * [Perl] L<CBOR::Free|https://metacpan.org/pod/CBOR::Free> |
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=back |
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