deploy: 7c9208bfb3

2026-07-09 20:15:12 +00:00 · 2026-07-08 18:19:46 +00:00
parent 9a231845c2
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+# nlohmann::basic_json::parse
+
+```
+// (1)
+template<typename InputType>
+static basic_json parse(InputType&& i,
+                        const parser_callback_t cb = nullptr,
+                        const bool allow_exceptions = true,
+                        const bool ignore_comments = false,
+                        const bool ignore_trailing_commas = false);
+
+// (2)
+template<typename IteratorType>
+static basic_json parse(IteratorType first, IteratorType last,
+                        const parser_callback_t cb = nullptr,
+                        const bool allow_exceptions = true,
+                        const bool ignore_comments = false,
+                        const bool ignore_trailing_commas = false);
+```
+
+1. Deserialize from a compatible input.
+
+1. Deserialize from a pair of character iterators
+
+   The `value_type` of the iterator must be an integral type with size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32.
+
+## Template parameters
+
+`InputType` : A compatible input, for instance:
+
+```
+- an `std::istream` object
+- a `FILE` pointer (throws if null)
+- a C-style array of characters
+- a pointer to a null-terminated string of single byte characters (throws if null)
+- a `std::string`
+- an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+```
+
+`IteratorType` : a compatible iterator type, for instance.
+
+```
+- a pair of `std::string::iterator` or `std::vector<std::uint8_t>::iterator`
+- a pair of pointers such as `ptr` and `ptr + len`
+```
+
+## Parameters
+
+`i` (in) : Input to parse from.
+
+`cb` (in) : a parser callback function of type [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md) which is used to control the deserialization by filtering unwanted values (optional)
+
+`allow_exceptions` (in) : whether to throw exceptions in case of a parse error (optional, `true` by default)
+
+`ignore_comments` (in) : whether comments should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
+
+`ignore_trailing_commas` (in) : whether trailing commas in arrays or objects should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
+
+`first` (in) : iterator to the start of a character range
+
+`last` (in) : iterator to the end of a character range
+
+## Return value
+
+Deserialized JSON value; in case of a parse error and `allow_exceptions` set to `false`, the return value will be `value_t::discarded`. The latter can be checked with [`is_discarded`](https://json.nlohmann.me/api/basic_json/is_discarded/index.md).
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
+## Exceptions
+
+- Throws [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) in case of an unexpected token, or empty input like a null `FILE*` or `char*` pointer.
+
+## Complexity
+
+Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser callback function `cb` or reading from (1) the input `i` or (2) the iterator range \[`first`, `last`\] has a super-linear complexity.
+
+## Notes
+
+A UTF-8 byte order mark is silently ignored.
+
+Invalid Unicode escapes and unpaired surrogates in the input are reported as [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) with a detailed message.
+
+## Examples
+
+Parsing from a character array
+
+The example below demonstrates the `parse()` function reading from an array.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text
+    char text[] = R"(
+    {
+        "Image": {
+            "Width":  800,
+            "Height": 600,
+            "Title":  "View from 15th Floor",
+            "Thumbnail": {
+                "Url":    "http://www.example.com/image/481989943",
+                "Height": 125,
+                "Width":  100
+            },
+            "Animated" : false,
+            "IDs": [116, 943, 234, 38793]
+        }
+    }
+    )";
+
+    // parse and serialize JSON
+    json j_complete = json::parse(text);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+}
+```
+
+Output:
+
+```
+{
+    "Image": {
+        "Animated": false,
+        "Height": 600,
+        "IDs": [
+            116,
+            943,
+            234,
+            38793
+        ],
+        "Thumbnail": {
+            "Height": 125,
+            "Url": "http://www.example.com/image/481989943",
+            "Width": 100
+        },
+        "Title": "View from 15th Floor",
+        "Width": 800
+    }
+}
+```
+
+Parsing from a string
+
+The example below demonstrates the `parse()` function with and without callback function.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text
+    auto text = R"(
+    {
+        "Image": {
+            "Width":  800,
+            "Height": 600,
+            "Title":  "View from 15th Floor",
+            "Thumbnail": {
+                "Url":    "http://www.example.com/image/481989943",
+                "Height": 125,
+                "Width":  100
+            },
+            "Animated" : false,
+            "IDs": [116, 943, 234, 38793]
+        }
+    }
+    )";
+
+    // parse and serialize JSON
+    json j_complete = json::parse(text);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+
+    // define parser callback
+    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
+    {
+        // skip object elements with key "Thumbnail"
+        if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
+        {
+            return false;
+        }
+        else
+        {
+            return true;
+        }
+    };
+
+    // parse (with callback) and serialize JSON
+    json j_filtered = json::parse(text, cb);
+    std::cout << std::setw(4) << j_filtered << '\n';
+}
+```
+
+Output:
+
+```
+{
+    "Image": {
+        "Animated": false,
+        "Height": 600,
+        "IDs": [
+            116,
+            943,
+            234,
+            38793
+        ],
+        "Thumbnail": {
+            "Height": 125,
+            "Url": "http://www.example.com/image/481989943",
+            "Width": 100
+        },
+        "Title": "View from 15th Floor",
+        "Width": 800
+    }
+}
+
+{
+    "Image": {
+        "Animated": false,
+        "Height": 600,
+        "IDs": [
+            116,
+            943,
+            234,
+            38793
+        ],
+        "Title": "View from 15th Floor",
+        "Width": 800
+    }
+}
+```
+
+Parsing from an input stream
+
+The example below demonstrates the `parse()` function with and without callback function.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <sstream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text
+    auto text = R"(
+    {
+        "Image": {
+            "Width":  800,
+            "Height": 600,
+            "Title":  "View from 15th Floor",
+            "Thumbnail": {
+                "Url":    "http://www.example.com/image/481989943",
+                "Height": 125,
+                "Width":  100
+            },
+            "Animated" : false,
+            "IDs": [116, 943, 234, 38793]
+        }
+    }
+    )";
+
+    // fill a stream with JSON text
+    std::stringstream ss;
+    ss << text;
+
+    // parse and serialize JSON
+    json j_complete = json::parse(ss);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+
+    // define parser callback
+    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
+    {
+        // skip object elements with key "Thumbnail"
+        if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
+        {
+            return false;
+        }
+        else
+        {
+            return true;
+        }
+    };
+
+    // fill a stream with JSON text
+    ss.clear();
+    ss << text;
+
+    // parse (with callback) and serialize JSON
+    json j_filtered = json::parse(ss, cb);
+    std::cout << std::setw(4) << j_filtered << '\n';
+}
+```
+
+Output:
+
+```
+{
+    "Image": {
+        "Animated": false,
+        "Height": 600,
+        "IDs": [
+            116,
+            943,
+            234,
+            38793
+        ],
+        "Thumbnail": {
+            "Height": 125,
+            "Url": "http://www.example.com/image/481989943",
+            "Width": 100
+        },
+        "Title": "View from 15th Floor",
+        "Width": 800
+    }
+}
+
+{
+    "Image": {
+        "Animated": false,
+        "Height": 600,
+        "IDs": [
+            116,
+            943,
+            234,
+            38793
+        ],
+        "Title": "View from 15th Floor",
+        "Width": 800
+    }
+}
+```
+
+Parsing from a contiguous container
+
+The example below demonstrates the `parse()` function reading from a contiguous container.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text given as std::vector
+    std::vector<std::uint8_t> text = {'[', '1', ',', '2', ',', '3', ']', '\0'};
+
+    // parse and serialize JSON
+    json j_complete = json::parse(text);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+}
+```
+
+Output:
+
+```
+[
+    1,
+    2,
+    3
+]
+```
+
+Parsing from a non-null-terminated string
+
+The example below demonstrates the `parse()` function reading from a string that is not null-terminated.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text given as string that is not null-terminated
+    const char* ptr = "[1,2,3]another value";
+
+    // parse and serialize JSON
+    json j_complete = json::parse(ptr, ptr + 7);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+}
+```
+
+Output:
+
+```
+[
+    1,
+    2,
+    3
+]
+```
+
+Parsing from an iterator pair
+
+The example below demonstrates the `parse()` function reading from an iterator pair.
+
+```
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text given an input with other values
+    std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};
+
+    // parse and serialize JSON
+    json j_complete = json::parse(input.begin(), input.begin() + 7);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+}
+```
+
+Output:
+
+```
+[
+    1,
+    2,
+    3
+]
+```
+
+Effect of `allow_exceptions` parameter
+
+The example below demonstrates the effect of the `allow_exceptions` parameter in the `parse()` function.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // an invalid JSON text
+    std::string text = R"(
+    {
+        "key": "value without closing quotes
+    }
+    )";
+
+    // parse with exceptions
+    try
+    {
+        json j = json::parse(text);
+    }
+    catch (const json::parse_error& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    // parse without exceptions
+    json j = json::parse(text, nullptr, false);
+
+    if (j.is_discarded())
+    {
+        std::cout << "the input is invalid JSON" << std::endl;
+    }
+    else
+    {
+        std::cout << "the input is valid JSON: " << j << std::endl;
+    }
+}
+```
+
+Output:
+
+```
+[json.exception.parse_error.101] parse error at line 4, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \u000A or \n; last read: '"value without closing quotes<U+000A>'
+the input is invalid JSON
+```
+
+Effect of `ignore_comments` parameter
+
+The example below demonstrates the effect of the `ignore_comments` parameter in the `parse()` function.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::string s = R"(
+    {
+        // update in 2006: removed Pluto
+        "planets": ["Mercury", "Venus", "Earth", "Mars",
+                    "Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
+    }
+    )";
+
+    try
+    {
+        json j = json::parse(s);
+    }
+    catch (json::exception& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    json j = json::parse(s,
+                         /* callback */ nullptr,
+                         /* allow exceptions */ true,
+                         /* ignore_comments */ true);
+    std::cout << j.dump(2) << '\n';
+}
+```
+
+Output:
+
+```
+[json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A>    {<U+000A>        /'; expected string literal
+{
+  "planets": [
+    "Mercury",
+    "Venus",
+    "Earth",
+    "Mars",
+    "Jupiter",
+    "Uranus",
+    "Neptune"
+  ]
+}
+```
+
+Effect of `ignore_trailing_commas` parameter
+
+The example below demonstrates the effect of the `ignore_trailing_commas` parameter in the `parse()` function.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::string s = R"(
+    {
+        "planets": [
+            "Mercury",
+            "Venus",
+            "Earth",
+            "Mars",
+            "Jupiter",
+            "Uranus",
+            "Neptune",
+        ]
+    }
+    )";
+
+    try
+    {
+        json j = json::parse(s);
+    }
+    catch (json::exception& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    json j = json::parse(s,
+                         /* callback */ nullptr,
+                         /* allow exceptions */ true,
+                         /* ignore_comments */ false,
+                         /* ignore_trailing_commas */ true);
+    std::cout << j.dump(2) << '\n';
+}
+```
+
+Output:
+
+```
+[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
+{
+  "planets": [
+    "Mercury",
+    "Venus",
+    "Earth",
+    "Mars",
+    "Jupiter",
+    "Uranus",
+    "Neptune"
+  ]
+}
+```
+
+## See also
+
+- [accept](https://json.nlohmann.me/api/basic_json/accept/index.md) - check if the input is valid JSON
+- [sax_parse](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - parse input using the SAX interface
+- [operator>>](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
+
+## Version history
+
+- Added in version 1.0.0.
+- Overload for contiguous containers (1) added in version 2.0.3.
+- Ignoring comments via `ignore_comments` added in version 3.9.0.
+- Changed [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in case of `FILE*` null pointers to exception in version 3.12.0.
+- Added `ignore_trailing_commas` in version 3.12.x.
+
+Deprecation
+
+Overload (2) replaces calls to `parse` with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like `parse({ptr, ptr+len}, ...);` with `parse(ptr, ptr+len, ...);`.
+
+You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated function.