Support python-style '#' comments

[rh-m2]

PLEASE DON'T FORGET TO "STAR" THIS REPOSITORY :)

When reporting a bug please include the following:

Version of Boost

1.81

Steps necessary to reproduce the problem

auto parse = [](json::string_view s)
{
    return json::parse(
        s,
        json::storage_ptr(),
        json::parse_options { .allow_comments        = true,
                              .allow_trailing_commas = true });
};

parse(R"_json_(
{
    "exchange": "bitmex",
    "from": "2019-10-01",
    "to": "2019-10-02",
    "symbols": ["XBTUSD", "ETHUSD"],
    "withDisconnectMessages": true,
    "dataTypes": ["trade", "book_change", "book_snapshot_10_100ms"],
    # other available data types examples:
    # 'book_snapshot_10_100ms', 'derivative_ticker', 'quote',
    # 'trade_bar_10ms', 'trade_bar_10s'
}
)_json_")

This JSON will be parsed by python's parser, but not by boost.json

All relevant compiler information

Any

Arguments for supporting this

I am writing a back testing and systematic trading engine. The engine is configured with JSON, picked up either off a config file or via a data bus such as RabbitMQ.

Some components of the system are written in C++ and some in Python. It would be useful to be able to add comments to config files that were parsed by both kinds of components.

Workaround:

parse(R"_json_(
{
    "exchange": "bitmex",
    "from": "2019-10-01",
    "to": "2019-10-02",
    "symbols": ["XBTUSD", "ETHUSD"],
    "withDisconnectMessages": true,
    "dataTypes": ["trade", "book_change", "book_snapshot_10_100ms"],
    "_1": "other available data types examples:",
    "_2": 'book_snapshot_10_100ms', 'derivative_ticker', 'quote','trade_bar_10ms', 'trade_bar_10s'"
}
)_json_")

But you know, yuk!

[vinniefalco]

In theory this can be done without harming performance when allow_comments=false. Or at least it will be no worse than how we currently support the C and C++ style comments.

[rh-m2]

Another workaround:

std::string
depython(std::string const &in)
{
    static const auto reg = std::regex("^[ \t]*#.+(#\n|$)");
    return std::regex_replace(in, reg, "");
}

auto val = json::parse(depython(json_text));