json/api/basic_json/from_msgpack/index.md

nlohmann::basic_json::from_msgpack

// (1)
template<typename InputType>
static basic_json from_msgpack(InputType&& i,
                               const bool strict = true,
                               const bool allow_exceptions = true);
// (2)
template<typename IteratorType>
static basic_json from_msgpack(IteratorType first, IteratorType last,
                               const bool strict = true,
                               const bool allow_exceptions = true);

Deserializes a given input to a JSON value using the MessagePack serialization format.

1. Reads from a compatible input.
2. Reads from an iterator range.

The exact mapping and its limitations are described on a dedicated page.

Template parameters

InputType : A compatible input, for instance:

- an `std::istream` object
- a `FILE` pointer
- a C-style array of characters
- a pointer to a null-terminated string of single byte characters
- an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.

IteratorType : a compatible iterator type

Parameters

i (in) : an input in MessagePack format convertible to an input adapter

first (in) : iterator to the start of the input

last (in) : iterator to the end of the input

strict (in) : whether to expect the input to be consumed until EOF (true by default)

allow_exceptions (in) : whether to throw exceptions in case of a parse error (optional, true by default)

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.

Exception safety

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

Exceptions

• Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
• Throws parse_error.112 if unsupported features from MessagePack were used in the given input or if the input is not valid MessagePack
• Throws parse_error.113 if a string was expected as a map key, but not found

Complexity

Linear in the size of the input.

Examples

Example

The example shows the deserialization of a byte vector in MessagePack format to a JSON value.

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

using json = nlohmann::json;

int main()
{
    // create byte vector
    std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,
                                   0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,
                                   0x61, 0x00
                                  };

    // deserialize it with MessagePack
    json j = json::from_msgpack(v);

    // print the deserialized JSON value
    std::cout << std::setw(2) << j << std::endl;
}

Output:

{
  "compact": true,
  "schema": 0
}

See also

• to_msgpack create a MessagePack serialization of a JSON value
• from_cbor create a JSON value from an input in CBOR format
• from_bson create a JSON value from an input in BSON format
• from_ubjson create a JSON value from an input in UBJSON format
• from_bjdata create a JSON value from an input in BJData format

Version history

• Added in version 2.0.9.
• Parameter start_index since version 2.1.1.
• Changed to consume input adapters, removed start_index parameter, and added strict parameter in version 3.0.0.
• Added allow_exceptions parameter in version 3.2.0.

Deprecation

• Overload (2) replaces calls to from_msgpack with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_msgpack(ptr, len, ...); with from_msgpack(ptr, ptr+len, ...);.
• Overload (2) replaces calls to from_msgpack 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 from_msgpack({ptr, ptr+len}, ...); with from_msgpack(ptr, ptr+len, ...);.

You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.