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83 lines
2.8 KiB
Markdown
83 lines
2.8 KiB
Markdown
# JSON Lines
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The [JSON Lines](https://jsonlines.org) format is a text format of newline-delimited JSON. In particular:
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1. The input must be UTF-8 encoded.
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1. Every line must be a valid JSON value.
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1. The line separator must be `\n`. As `\r` is silently ignored, `\r\n` is also supported.
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1. The final character may be `\n`, but is not required to be one.
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JSON Text example
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```
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{"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
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{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
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{"name": "May", "wins": []}
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{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
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```
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JSON Lines input with more than one value is treated as invalid JSON by the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) or [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) functions. To process it line by line, functions like [`std::getline`](https://en.cppreference.com/w/cpp/string/basic_string/getline) can be used:
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Example: Parse JSON Text input line by line
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The example below demonstrates how JSON Lines can be processed.
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```
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#include <sstream>
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// JSON Lines (see https://jsonlines.org)
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std::stringstream input;
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input << R"({"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
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{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
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{"name": "May", "wins": []}
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{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
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)";
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std::string line;
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while (std::getline(input, line))
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{
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std::cout << json::parse(line) << std::endl;
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}
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}
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```
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Output:
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```
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{"name":"Gilbert","wins":[["straight","7♣"],["one pair","10♥"]]}
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{"name":"Alexa","wins":[["two pair","4♠"],["two pair","9♠"]]}
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{"name":"May","wins":[]}
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{"name":"Deloise","wins":[["three of a kind","5♣"]]}
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```
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Note
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Using [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) like
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```
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json j;
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while (input >> j)
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{
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std::cout << j << std::endl;
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}
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```
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with a JSON Lines input does not work, because the parser will try to parse one value after the last one and throw a [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) exception. The same happens for a stream of *concatenated* (non-newline-delimited) JSON values: `operator>>` reads them one at a time, but the loop above throws after the last value. To read either format with `operator>>`, check for the end of the stream before each read:
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```
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json j;
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while (input >> std::ws && input.peek() != std::char_traits<char>::eof())
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{
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input >> j;
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std::cout << j << std::endl;
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}
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```
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A value that is a number must be followed by whitespace -- see the [notes](https://json.nlohmann.me/api/operator_gtgt/#notes) of `operator>>` for details.
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