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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

4.6 KiB

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nlohmann::basic_json::accept

// (1)
template<typename InputType>
static bool accept(InputType&& i,
                   const bool ignore_comments = false,
                   const bool ignore_trailing_commas = false);

// (2)
template<typename IteratorType, typename SentinelType = IteratorType>
static bool accept(IteratorType first, SentinelType last,
                   const bool ignore_comments = false,
                   const bool ignore_trailing_commas = false);

Checks whether the input is valid JSON.

Reads from a compatible input.
Reads from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

Unlike the parse() function, this function neither throws an exception in case of invalid JSON input (i.e., a parse error) nor creates diagnostic information.

Template parameters

InputType

A compatible input, for instance:

an std::istream object
a #!c FILE pointer (throws if null)
a C-style array of characters
a pointer to a null-terminated string of single byte characters (throws if null)
a std::string
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, for instance.

a pair of std::string::iterator or std::vector<std::uint8_t>::iterator
a pair of pointers such as ptr and ptr + len

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): Input to parse from.
ignore_comments (in): whether comments should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)
ignore_trailing_commas (in): whether trailing commas in arrays or objects should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)
first (in): iterator to the start of the character range
last (in): iterator to the end of the character range, or a sentinel value that compares equal to the end iterator with operator!=

Return value

Whether the input is valid JSON.

Exception safety

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

Exceptions

Throws parse_error.101 in case of an empty input like a null #!c FILE* or #!c char* pointer.

Complexity

Linear in the length of the input. The parser is a predictive LL(1) parser.

Notes

A UTF-8 byte order mark is silently ignored.

Examples

??? example

The example below demonstrates the `accept()` function reading from a string.

```cpp
--8<-- "examples/accept__string.cpp"
```

Output:

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

Version history

Added in version 3.0.0.
Ignoring comments via ignore_comments added in version 3.9.0.
Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
Added ignore_trailing_commas in version 3.13.0.
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 `accept` 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 accept({ptr, ptr+len}, ...);` with `#!cpp accept(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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