json/api/basic_json/from_msgpack.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, typename SentinelType = IteratorType>
static basic_json from_msgpack(IteratorType first, SentinelType 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, or an iterator and a sentinel of a different type (C++20 ranges support).

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
• a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)

IteratorType

a compatible iterator type

SentinelType

defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance a custom sentinel type for C++20 ranges

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, or a sentinel value that compares equal to the end iterator with operator!=

strict (in)

whether to expect the input to be consumed until EOF (#!cpp true by default)

allow_exceptions (in)

whether to throw exceptions in case of a parse error (optional, #!cpp true by default)

Return value

deserialized JSON value; in case of a parse error and allow_exceptions set to #!cpp 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.
 
```cpp
--8<-- "examples/from_msgpack.cpp"
```

Output:

```json
--8<-- "examples/from_msgpack.output"
```

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.
• Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0.
• Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

!!! warning "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
  `#!cpp from_msgpack(ptr, len, ...);` with `#!cpp 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
  `#!cpp from_msgpack({ptr, ptr+len}, ...);` with `#!cpp from_msgpack(ptr, ptr+len, ...);`.

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