string copy operation in parse function is useless

[ayrb13]

jsoncpp/src/lib_json/json_reader.cpp

Line 93 in 5be07bd

document_.assign(document.begin(), document.end());

there is string copy operation while we use reader to parse json from std::string. I think it is useless, we can directly use document param, there is no need to copy it.
could someone explain it for me, thanks

[ayrb13]

if we remove this copy operation, the parsing time cost will be reduced by 20%

[dota17]

Reader class is deprecated, we highly recommand that you use OurReader class, which is better designed than Reader

[fedormsv]

Still Json::parseFromStream is potentially doing extra copy: if I have an std::string to parse JSON from, I'm still forced to wrap it into stringstream, pass this stream to operator>> or parseFromStream function. There the provided stream is directed to istringstream and finaly str() return a string, actually a copy of original one. Having parseFromString or even from string_view would save time and memory.

[cdunn2001]

@fedormsv , please submit a PR. Could be a good idea. This sounds like it's not a bug, so we probably won't spend time on it ourselves.

[fedormsv]

PR is submitted. I'm not sure why it gets new number.
#1221

It doesn't seem to be really welcomed.