json/api/basic_json/error_handler_t/index.md

nlohmann::basic_json::error_handler_t

enum class error_handler_t {
    strict,
    replace,
    ignore,
    keep
};

This enumeration is used to choose how to treat ill-formed UTF-8 in a string value or object key:

• dump uses it while serializing a basic_json value to text.
• to_cbor, to_msgpack, to_ubjson, to_bjdata, and to_bson use it while serializing a basic_json value to that binary format. Their default is keep, as no binary writer checked before this parameter was added. CBOR, UBJSON, BJData, and BSON require valid UTF-8, so for these four the default is strict if JSON_STRICT_BINARY_UTF8 is enabled; MessagePack's specification explicitly allows a string to contain ill-formed UTF-8, so to_msgpack stays at keep. to_bon8 does not take this parameter: BON8 always validates, since UTF-8 lead bytes are structural to that format.
• from_cbor, from_msgpack, from_ubjson, from_bjdata, and from_bson use it while parsing that binary format, to decide whether to check a string value or object key for well-formed UTF-8 at all; by default (keep) they do not, as no binary reader did before this parameter was added. from_bon8 does not take this parameter, for the same reason to_bon8 does not.

Four values are differentiated:

strict : throw a type_error/parse_error exception in case of invalid UTF-8

replace : replace invalid UTF-8 sequences with U+FFFD (� REPLACEMENT CHARACTER)

ignore : ignore invalid UTF-8 sequences; all valid bytes are copied to the output unchanged, and invalid bytes are dropped

keep : keep invalid UTF-8 sequences unchanged; only meaningful for the binary formats mentioned above, since [dump] (dump.md) itself must produce text, and keep there writes the ill-formed bytes to the output as is, so the result is then not valid UTF-8 (but still equals the input bytes exactly, including around any well-formed characters, which are still escaped as usual)

Examples

Example

The example below shows how the different values of the error_handler_t influence the behavior of dump when reading serializing an invalid UTF-8 sequence.

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

using json = nlohmann::json;

int main()
{
    // 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)
              << "\nstring with the invalid byte kept as is (" << j_invalid.dump(-1, ' ', false, json::error_handler_t::keep).size()
              << " bytes, not valid UTF-8 itself)\n";
}

Output:

[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"
string with the invalid byte kept as is (7 bytes, not valid UTF-8 itself)

See also

• dump serializes a JSON value, with an error_handler_t parameter to configure invalid UTF-8 handling
• Handling invalid UTF-8 - the article on handling invalid UTF-8

Version history

• Added in version 3.4.0.
• Added keep, and made this enumeration apply to the binary readers and writers in addition to dump, in version 3.13.0.