📝 add documentation for JSON Lines (#3247)

doc/examples/json_lines.cpp

+#include <sstream>
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // JSON Lines (see https://jsonlines.org)
+    std::stringstream input;
+    input << R"({"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
+{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
+{"name": "May", "wins": []}
+{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
+)";
+
+    std::string line;
+    while (std::getline(input, line))
+    {
+        std::cout << json::parse(line) << std::endl;
+    }
+}

doc/examples/json_lines.output

+{"name":"Gilbert","wins":[["straight","7♣"],["one pair","10♥"]]}
+{"name":"Alexa","wins":[["two pair","4♠"],["two pair","9♠"]]}
+{"name":"May","wins":[]}
+{"name":"Deloise","wins":[["three of a kind","5♣"]]}

doc/mkdocs/docs/features/parsing/json_lines.md

+# JSON Lines
+
+The [JSON Lines](https://jsonlines.org) format is a text format of newline-delimited JSON. In particular:
+
+1. The input must be UTF-8 encoded.
+2. Every line must be a valid JSON value.
+3. The line separator must be `\n`. As `\r` is silently ignored, `\r\n` is also supported.
+4. The final character may be `\n`, but is not required to be one.
+
+!!! example "JSON Text example"
+
+    ```json
+    {"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
+    {"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
+    {"name": "May", "wins": []}
+    {"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
+    ```
+
+JSON Lines input with more than one value is treated as invalid JSON by the [`parse`](../../api/basic_json/parse.md) or
+[`accept`](../../api/basic_json/accept.md) functions. The process it line by line, functions like
+[`std::getline`](https://en.cppreference.com/w/cpp/string/basic_string/getline) can be used:
+
+!!! example "Example: Parse JSON Text input line by line"
+
+    The example below demonstrates how JSON Lines can be processed.
+
+    ```cpp
+    --8<-- "examples/json_lines.cpp"
+    ```
+    
+    Output:
+
+    ```json
+    --8<-- "examples/json_lines.output"
+    ```
+
+!!! warning "Note"
+
+    Using [`operator>>`](../../api/basic_json/operator_gtgt.md) like
+    
+    ```cpp
+    json j;
+    while (input >> j)
+    {
+        std::cout << j << std::endl;
+    }
+    ```
+    
+    with a JSON Lines input does not work, because the parser will try to parse one value after the last one.

doc/mkdocs/mkdocs.yml

@@ -60,6 +60,7 @@ nav:
     - features/object_order.md
     - Parsing:
         - features/parsing/index.md
+        - features/parsing/json_lines.md
         - features/parsing/parse_exceptions.md
         - features/parsing/parser_callbacks.md
         - features/parsing/sax_interface.md

test/src/unit-deserialization.cpp

@@ -1065,6 +1065,48 @@ TEST_CASE("deserialization")
             "start_array()"
         }));
     }
+
+    SECTION("JSON Lines")
+    {
+        SECTION("Example file")
+        {
+            std::stringstream ss;
+            ss << R"({"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
+                    {"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
+                    {"name": "May", "wins": []}
+                    {"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
+)";
+
+            std::string line;
+            int object_count = 0;
+            while (std::getline(ss, line))
+            {
+                ++object_count;
+                CHECK(json::accept(line));
+            }
+
+            CHECK(object_count == 4);
+        }
+
+        SECTION("Example file without trailing newline")
+        {
+            std::stringstream ss;
+            ss << R"({"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
+                    {"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
+                    {"name": "May", "wins": []}
+                    {"name": "Deloise", "wins": [["three of a kind", "5♣"]]})";
+
+            std::string line;
+            int object_count = 0;
+            while (std::getline(ss, line))
+            {
+                ++object_count;
+                CHECK(json::accept(line));
+            }
+
+            CHECK(object_count == 4);
+        }
+    }
 }
 
 TEST_CASE_TEMPLATE("deserialization of different character types (ASCII)", T,