Investigate parsing into user structures directly

[pdimov]

E.g.

std::vector<X> v;
std::string_view json = /*...*/;

boost::json::parse_into( v, json );

where X is something that Boost.Json recognizes, such as a struct with value_to defined, or (given #626) a described struct.

As an example of why this is useful, on one popular benchmark

https://github.com/kostya/benchmarks#json
https://github.com/kostya/benchmarks/blob/master/json/test_boost_json.cpp

having the ability to parse directly into either std::vector<coordinate> or a custom container that accumulates in its push_back member function instead of storing the elements could potentially gain a lot of performance.

[pdimov]

I made a prototype, https://gist.github.com/pdimov/bcdaa4fef606fdcd0f21195a79bf8648

Unfortunately it's slower. But still cooler. :-)

1.json: 74181420 bytes
boost::json::parse: 456 ms
  x: -5.00063e-30, y: 5.00037e+30, z: 0.500451: 115 ms
parse_into: 702 ms
  x: -5.00063e-30, y: 5.00037e+30, z: 0.500451: 2 ms

[madmongo1]

The manual loop, parsing via the JSON DOM is not performing error-checking on the types in the DOM. It'll exhibit UB if the JSON is valid but not correctly representing the co-ordinate array, so I think it's getting a better score than it should.

[pdimov]

It's copied from the benchmark code.

This needs to be refactored to contain the subhandlers inline in the composite handlers, rather than allocating them with new each time. ("Structured concurrency" :-) ) (Also, needs a proper error enum. And a handler for tuples. And perform range checking on the numbers. I ran out of night.)

[pdimov]

Also needs a handler for json::value, so that json files that have random unstructured parts can still fit into a structure. I had to remove the opts part of the original json generated by https://github.com/kostya/benchmarks/blob/ef8dbdec90bbcb3b242da8211ff7db5396e23979/json/generate_json.rb because it's too chaotic.

(opts would have to be a map<string, pair<int, bool>>, and I don't have support for pair. I could have declared an extra struct for that I suppose, but that's kind of not the spirit of the benchmark - opts are clearly intended as free form json object.)

[pdimov]

This needs to be refactored to contain the subhandlers inline in the composite handlers, rather than allocating them with new each time. ("Structured concurrency" :-) ) (Also, needs a proper error enum. And a handler for tuples. And perform range checking on the numbers. I ran out of night.)

https://gist.github.com/pdimov/c74627565d35cdff97d7f4d0ae7a7b00

This one is faster.

1.json: 74181420 bytes
boost::json::parse: 431 ms
  x: -5.00063e-30, y: 5.00037e+30, z: 0.500451: 131 ms
parse_into coordinates: 234 ms
  x: -5.00063e-30, y: 5.00037e+30, z: 0.500451: 2 ms
parse_into coordinates2: 182 ms
  x: 0.500451, y: 5.00037e+30, z: 0.500451: 0 ms

[vinniefalco]

It needs its own suite of parsing functions, e.g.

parse_object_into
parse_array_into
parse_string_into
parse_number_into
parse_bool_into
parse_null_into

Using pull-parsing with this signature

template< typename V >
bool
parse_string_into(
  char const*& begin,
  char const* end,
  V& v,
  error_code& ec );

[vinniefalco]

template<class V> using parser_for = basic_parser<detail::handler<V>>;

[vinniefalco]

there should probably be is_bool trait for consistency.