Json: Feature request: Comments

[hunyadix]

Many JSON parsers support the option to comment out fields. Is it possible to request such a feature in the nlohmann parser too?

[nlohmann]

For this library, we do not plan to support any input which is not compliant to RFC 7159.

See #376, #363, #311, #294, #192.

[kinchungwong]

@nlohmann

1. Would it make sense to just document the fact that comments are not supported on the project front page, or on an FAQ page?
2. Would it make sense to mention that users who need to process json inputs containing comments should use a separate comment stripper (that does not come with this library), and some recommendations?

Thanks.

[nlohmann]

@kinchungwong

1. As comments are not part of JSON, I do not think that an explicit FAQ entry that comments are not supported by this library would be helpful.
2. Can you propose such a comment stripper?

[fawdlstty]

It's been a long time, and I think we can consider replanning the introduction of json comments

[fawdlstty]

Not support for JSON comments is a very sad thing, as is the ability to debug data, indicate what values are available, and so on.If you need to enforce the JSON standard, consider adding a separate interface for parsing JSON strings with JSON comments

[fawdlstty]

Such as:

string s = "{/**///\n}";
s = nlohmann::json::remove_comments (s);
auto o = nlohmann::json::parse (s);

[nlohmann]

But removing comments is not possible by just a regular expression. It would mean integration into the lexer.

[fawdlstty]

So can reopen the question?

[nlohmann]

No, because this library will not support comments. If anyone wants to provide a pull request with a compact comment stripper, I will have a look.

[fawdlstty]

I think can add this code to the project:

#ifndef __JSON_PREPROCESS_HPP__
#define __JSON_PREPROCESS_HPP__

#include <string>

class JsonPreprocess {
    enum class CurrentState {
        Normal, Quote, QuoteIgnore, BeginComment, CommentLine, CommentBlock, CommentBlockEnd,
    };
public:
    static std::string remove_comments (std::string _data) {
        std::string _ret = "";
        auto _state = CurrentState::Normal;
        for (char ch : _data) {
            if (_state == CurrentState::Normal) {
                if (ch == '"') {
                    _ret += ch;
                    _state = CurrentState::Quote;
                } else if (ch == '/') {
                    _state = CurrentState::BeginComment;
                } else {
                    _ret += ch;
                }
            } else if (_state == CurrentState::Quote) {
                if (ch == '"') {
                    _ret += ch;
                    _state = CurrentState::Normal;
                } else if (ch == '\\') {
                    _ret += ch;
                    _state = CurrentState::QuoteIgnore;
                } else {
                    _ret += ch;
                }
            } else if (_state == CurrentState::QuoteIgnore) {
                _ret += ch;
                _state = CurrentState::Quote;
            } else if (_state == CurrentState::BeginComment) {
                _state = (ch == '/' ? CurrentState::CommentLine : CurrentState::CommentBlock);
            } else if (_state == CurrentState::CommentLine) {
                if (ch == '\r' || ch == '\n') {
                    _ret += ch;
                    _state = CurrentState::Normal;
                } else {
                    _state = CurrentState::CommentLine;
                }
            } else if (_state == CurrentState::CommentBlock) {
                _state = (ch == '*' ? CurrentState::CommentBlockEnd : CurrentState::CommentBlock);
            } else if (_state == CurrentState::CommentBlockEnd) {
                _state = (ch == '*' ? CurrentState::CommentBlockEnd : (ch == '/' ? CurrentState::Normal : CurrentState::CommentBlock));
            }
        }
        return _ret;
    }
};

#endif //__JSON_PREPROCESS_HPP__

[nlohmann]

I'm currently working on extending the parser and will open a PR later this week.