json/api/basic_json/dump.md

nlohmann::basic_json::dump

string_t dump(const int indent = -1,
              const char indent_char = ' ',
              const bool ensure_ascii = false,
              const error_handler_t error_handler = error_handler_t::strict) const;

Serialization function for JSON values. The function tries to mimic Python's json.dumps() function, and currently supports its indent and ensure_ascii parameters.

Parameters

indent (in)

If indent is nonnegative, then array elements and object members will be pretty-printed with that indent level. An indent level of 0 will only insert newlines. -1 (the default) selects the most compact representation.

indent_char (in)

The character to use for indentation if indent is greater than 0. The default is (space).

ensure_ascii (in)

If ensure_ascii is true, all non-ASCII characters in the output are escaped with \uXXXX sequences, and the result consists of ASCII characters only.

error_handler (in)

how to react on decoding errors; there are four possible values (see error_handler_t: strict (throws an exception in case a decoding error occurs; default), replace (replace invalid UTF-8 sequences with U+FFFD), ignore (ignore invalid UTF-8 sequences during serialization; all valid bytes are copied to the output unchanged, and invalid bytes are dropped), and keep (write the ill-formed bytes to the output as is, without escaping them, even if ensure_ascii is #!cpp true; the result is then not valid UTF-8, but equals the input bytes exactly, and well-formed characters around the ill-formed bytes are still escaped as usual)).

Return value

string containing the serialization of the JSON value

Exception safety

Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

Exceptions

Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded and error_handler is set to strict

!!! warning "Serializing untrusted input"

When serializing values that may contain invalid or untrusted UTF-8 (e.g., bytes taken directly from network
input), `dump()` throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) in the default
`strict` mode. To serialize such data without throwing, pass
[`error_handler_t::replace`](error_handler_t.md) (substitutes U+FFFD) or
[`error_handler_t::ignore`](error_handler_t.md). Callers that serialize untrusted input on a crash-sensitive path
should either choose a non-strict error handler or wrap `dump()` in a `#!cpp try`/`#!cpp catch`.

See the [FAQ](../../home/faq.md#serializing-untrusted-or-invalid-utf-8) for details.

Complexity

Linear.

Notes

Binary values are serialized as an object containing two keys:

• "bytes": an array of bytes as integers
• "subtype": the subtype as integer or #!json null if the binary has no subtype

Examples

??? example

The following example shows the effect of different `indent`, `indent_char`, and `ensure_ascii` parameters to the
result of the serialization.

```cpp
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json j_object = {{"one", 1}, {"two", 2}};
    json j_array = {1, 2, 4, 8, 16};
    json j_string = "Hellö 😀!";

    // call dump()
    std::cout << "objects:" << '\n'
              << j_object.dump() << "\n\n"
              << j_object.dump(-1) << "\n\n"
              << j_object.dump(0) << "\n\n"
              << j_object.dump(4) << "\n\n"
              << j_object.dump(1, '\t') << "\n\n";

    std::cout << "arrays:" << '\n'
              << j_array.dump() << "\n\n"
              << j_array.dump(-1) << "\n\n"
              << j_array.dump(0) << "\n\n"
              << j_array.dump(4) << "\n\n"
              << j_array.dump(1, '\t') << "\n\n";

    std::cout << "strings:" << '\n'
              << j_string.dump() << '\n'
              << j_string.dump(-1, ' ', true) << '\n';

    // create JSON value with invalid UTF-8 byte sequence
    json j_invalid = "ä\xA9ü";
    try
    {
        std::cout << j_invalid.dump() << std::endl;
    }
    catch (const json::type_error& e)
    {
        std::cout << e.what() << std::endl;
    }

    std::cout << "string with replaced invalid characters: "
              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)
              << "\nstring with ignored invalid characters: "
              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)
              << '\n';
}

```

Output:

```json
objects:
{"one":1,"two":2}

{"one":1,"two":2}

{
"one": 1,
"two": 2
}

{
    "one": 1,
    "two": 2
}

{
	"one": 1,
	"two": 2
}

arrays:
[1,2,4,8,16]

[1,2,4,8,16]

[
1,
2,
4,
8,
16
]

[
    1,
    2,
    4,
    8,
    16
]

[
	1,
	2,
	4,
	8,
	16
]

strings:
"Hellö 😀!"
"Hell\u00f6 \ud83d\ude00!"
[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9
string with replaced invalid characters: "ä�ü"
string with ignored invalid characters: "äü"

```

See also

• to_string returns a string representation of a JSON value
• operator<< serialize to stream
• Serialization - the serialization article

Version history

• Added in version 1.0.0.
• Indentation character indent_char, option ensure_ascii and exceptions added in version 3.0.0.
• Error handlers added in version 3.4.0.
• Serialization of binary values added in version 3.8.0.
• Error handler keep added in version 3.13.0.