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README.md | 4 months ago | |
cppbor.h | 4 months ago | |
cppbor_parse.h | 4 months ago |
README.md
CppBor: A Modern C++ CBOR Parser and Generator
CppBor provides a natural and easy-to-use syntax for constructing and parsing CBOR messages. It does not (yet) support all features of CBOR, nor (yet) support validation against CDDL schemata, though both are planned. CBOR features that aren't supported include:
- Indefinite length values
- Semantic tagging
- Floating point
CppBor requires C++-17.
CBOR representation
CppBor represents CBOR data items as instances of the Item
class or,
more precisely, as instances of subclasses of Item
, since Item
is a
pure interface. The subclasses of Item
correspond almost one-to-one
with CBOR major types, and are named to match the CDDL names to which
they correspond. They are:
Uint
corresponds to major type 0, and can hold unsigned integers up through (2^64 - 1).Nint
corresponds to major type 1. It can only hold values from -1 to -(2^63 - 1), since it's internal representation is an int64_t. This can be fixed, but it seems unlikely that applications will need the omitted range from -(2^63) to (2^64 - 1), since it's inconvenient to represent them in many programming languages.Int
is an abstract base ofUint
andNint
that facilitates working with all signed integers representable with int64_t.Bstr
corresponds to major type 2, a byte string.Tstr
corresponds to major type 3, a text string.Array
corresponds to major type 4, an Array. It holds a variable-length array ofItem
s.Map
corresponds to major type 5, a Map. It holds a variable-length array of pairs ofItem
s.Simple
corresponds to major type 7. It's an abstract class since items require more specific type.Bool
is the only currently-implemented subclass ofSimple
.
Note that major type 6, semantic tag, is not yet implemented.
In practice, users of CppBor will rarely use most of these classes when generating CBOR encodings. This is because CppBor provides straightforward conversions from the obvious normal C++ types. Specifically, the following conversions are provided in appropriate contexts:
- Signed and unsigned integers convert to
Uint
orNint
, as appropriate. std::string
,std::string_view
,const char*
andstd::pair<char iterator, char iterator>
convert toTstr
.std::vector<uint8_t>
,std::pair<uint8_t iterator, uint8_t iterator>
andstd::pair<uint8_t*, size_t>
convert toBstr
.bool
converts toBool
.
CBOR generation
Complete tree generation
The set of encode
methods in Item
provide the interface for
producing encoded CBOR. The basic process for "complete tree"
generation (as opposed to "incremental" generation, which is discussed
below) is to construct an Item
which models the data to be encoded,
and then call one of the encode
methods, whichever is convenient for
the encoding destination. A trivial example:
cppbor::Uint val(0);
std::vector<uint8_t> encoding = val.encode();
It's relatively rare that single values are encoded as above. More often, the
"root" data item will be an `Array` or `Map` which contains a more complex structure.For example
:
using cppbor::Array;
std::vector<uint8_t> vec = // ...
Map val("key1", Array(Map("key_a", 99 "key_b", vec), "foo"), "key2", true);
std::vector<uint8_t> encoding = val.encode();
This creates a map with two entries, with Tstr
keys "Outer1" and
"Outer2", respectively. The "Outer1" entry has as its value an
Array
containing a Map
and a Tstr
. The "Outer2" entry has a
Bool
value.
This example demonstrates how automatic conversion of C++ types to
CppBor Item
subclass instances is done. Where the caller provides a
C++ or C string, a Tstr
entry is added. Where the caller provides
an integer literal or variable, a Uint
or Nint
is added, depending
on whether the value is positive or negative.
As an alternative, a more fluent-style API is provided for building up structures. For example:
using cppbor::Map;
using cppbor::Array;
std::vector<uint8_t> vec = // ...
Map val();
val.add("key1", Array().add(Map().add("key_a", 99).add("key_b", vec)).add("foo")).add("key2", true);
std::vector<uint8_t> encoding = val.encode();
An advantage of this interface over the constructor -
based creation approach above is that it need not be done all at once
.The `add` methods return a reference to the object added to to allow calls to be chained,
but chaining is not necessary; calls can be made
sequentially, as the data to add is available.
encode
methods
There are several variations of Item::encode
, all of which
accomplish the same task but output the encoded data in different
ways, and with somewhat different performance characteristics. The
provided options are:
bool encode(uint8\_t** pos, const uint8\_t* end)
encodes into the buffer referenced by the range [*pos
, end).*pos
is moved. If the encoding runs out of buffer space before finishing, the method returns false. This is the most efficient way to encode, into an already-allocated buffer.void encode(EncodeCallback encodeCallback)
callsencodeCallback
for each encoded byte. It's the responsibility of the implementor of the callback to behave safely in the event that the output buffer (if applicable) is exhausted. This is less efficient than the prior method because it imposes an additional function call for each byte.template </*...*/> void encode(OutputIterator i)
encodes into the provided iterator. SFINAE ensures that the template doesn't match for non-iterators. The implementation actually uses the callback-based method, plus has whatever overhead the iterator adds.std::vector<uint8_t> encode()
creates a new std::vector, reserves sufficient capacity to hold the encoding, and inserts the encoded bytes with a std::pushback_iterator and the previous method.std::string toString()
does the same as the previous method, but returns a string instead of a vector.
Incremental generation
Incremental generation requires deeper understanding of CBOR, because
the library can't do as much to ensure that the output is valid. The
basic tool for intcremental generation is the encodeHeader
function. There are two variations, one which writes into a buffer,
and one which uses a callback. Both simply write out the bytes of a
header. To construct the same map as in the above examples,
incrementally, one might write:
using namespace cppbor; // For example brevity
std::vector encoding;
auto iter = std::back_inserter(result);
encodeHeader(MAP, 2 /* # of map entries */, iter);
std::string s = "key1";
encodeHeader(TSTR, s.size(), iter);
std::copy(s.begin(), s.end(), iter);
encodeHeader(ARRAY, 2 /* # of array entries */, iter);
Map().add("key_a", 99).add("key_b", vec).encode(iter)
s = "foo";
encodeHeader(TSTR, foo.size(), iter);
std::copy(s.begin(), s.end(), iter);
s = "key2";
encodeHeader(TSTR, foo.size(), iter);
std::copy(s.begin(), s.end(), iter);
encodeHeader(SIMPLE, TRUE, iter);
As the above example demonstrates, the styles can be mixed -- Note the creation and encoding of the inner Map using the fluent style.
Parsing
CppBor also supports parsing of encoded CBOR data, with the same feature set as encoding. There are two basic approaches to parsing, "full" and "stream"
Full parsing
Full parsing means completely parsing a (possibly-compound) data
item from a byte buffer. The parse
functions that do not take a
ParseClient
pointer do this. They return a ParseResult
which is a
tuple of three values:
- std::unique_ptr that points to the parsed item, or is nullptr if there was a parse error.
- const uint8_t* that points to the byte after the end of the decoded item, or to the first unparseable byte in the event of an error.
- std::string that is empty on success or contains an error message if a parse error occurred.
Assuming a successful parse, you can then use Item::type()
to
discover the type of the parsed item (e.g. MAP), and then use the
appropriate Item::as*()
method (e.g. Item::asMap()
) to get a
pointer to an interface which allows you to retrieve specific values.
Stream parsing
Stream parsing is more complex, but more flexible. To use
StreamParsing, you must create your own subclass of ParseClient
and
call one of the parse
functions that accepts it. See the
ParseClient
methods docstrings for details.
One unusual feature of stream parsing is that the ParseClient
callback methods not only provide the parsed Item, but also pointers
to the portion of the buffer that encode that Item. This is useful
if, for example, you want to find an element inside of a structure,
and then copy the encoding of that sub-structure, without bothering to
parse the rest.
The full parser is implemented with the stream parser.