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Niels Lohmann 424a88b4fb Extend memcpy fast path to sized sentinels (e.g. std::counted_iterator)

Extend the memcpy fast path in iterator_input_adapter to sized
sentinels of a different type, not just same-type iterator pairs.
std::counted_iterator paired with std::default_sentinel_t already
satisfies std::contiguous_iterator and std::sized_sentinel_for, so
std::ranges::distance (C++20) lets that combination reach the fast
path too, instead of silently falling back to the byte-by-byte path.

- iterator_is_contiguous now also allows std::sized_sentinel_for<SentinelType,
  IteratorType> under C++20, gated the same way as the existing
  std::contiguous_iterator detection.
- get_elements_impl's fast path uses std::ranges::distance under C++20
  (works for both same-type and sized-sentinel pairs) and falls back to
  std::distance pre-C++20, where SentinelType is always IteratorType.
- Add a C++20-only test exercising json::parse/accept with
  std::counted_iterator + std::default_sentinel_t.
- Document std::default_sentinel_t + std::counted_iterator as a
  SentinelType example across parse.md, accept.md, sax_parse.md, and
  the five from_*.md pages, replacing an earlier ambiguously worded
  bullet.

Addresses review feedback: https://github.com/nlohmann/json/pull/5265#discussion_r3564237584

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-07-11 16:27:56 +02:00

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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.

Reads from a compatible input.
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
std::default_sentinel_t, when IteratorType is std::counted_iterator

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"
```

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.

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