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>

docs/mkdocs/docs/api/basic_json/accept.md

@@ -8,8 +8,8 @@ static bool accept(InputType&& i,
                    const bool ignore_trailing_commas = false);
 
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static bool accept(IteratorType first, IteratorType last,
+static bool accept(IteratorType first, SentinelType last,
                    const bool ignore_comments = false,
                    const bool ignore_trailing_commas = false);
 ```
@@ -17,10 +17,11 @@ static bool accept(IteratorType first, IteratorType last,
 Checks whether the input is valid JSON.
 
 1. Reads from a compatible input.
-2. Reads from a pair of character iterators
+2. 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.
+    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()`](parse.md) function, this function neither throws an exception in case of invalid JSON input
 (i.e., a parse error) nor creates diagnostic information.
@@ -44,6 +45,12 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
     - 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)
@@ -61,7 +68,7 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
 :   iterator to the start of the character range
 
 `last` (in)
-:   iterator to the end of the character range
+:   iterator to the end of the character range, or a sentinel value that compares equal to the end iterator with `operator!=`
 
 ## Return value
 
@@ -112,6 +119,7 @@ A UTF-8 byte order mark is silently ignored.
 - Changed [runtime assertion](../../features/assertions.md) 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"
 

docs/mkdocs/docs/api/basic_json/from_bjdata.md

@@ -7,8 +7,8 @@ static basic_json from_bjdata(InputType&& i,
                               const bool strict = true,
                               const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static basic_json from_bjdata(IteratorType first, IteratorType last,
+static basic_json from_bjdata(IteratorType first, SentinelType last,
                               const bool strict = true,
                               const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_bjdata(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the BJData (Binary JData) serialization format.
 
 1. Reads from a compatible input.
-2. Reads from an iterator range.
+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](../../features/binary_formats/bjdata.md).
 
@@ -35,6 +35,12 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 `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)
@@ -44,7 +50,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input
 
 `last` (in)
-:   iterator to the end of the input
+:   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)
@@ -103,3 +109,4 @@ Linear in the size of the input.
 
 - Added in version 3.11.0.
 - 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.

docs/mkdocs/docs/api/basic_json/from_bson.md

@@ -7,8 +7,8 @@ static basic_json from_bson(InputType&& i,
                             const bool strict = true,
                             const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static basic_json from_bson(IteratorType first, IteratorType last,
+static basic_json from_bson(IteratorType first, SentinelType last,
                             const bool strict = true,
                             const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_bson(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the BSON (Binary JSON) serialization format.
 
 1. Reads from a compatible input.
-2. Reads from an iterator range.
+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](../../features/binary_formats/bson.md).
 
@@ -35,6 +35,12 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 `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)
@@ -44,7 +50,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input
 
 `last` (in)
-:   iterator to the end of the input
+:   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)
@@ -103,6 +109,7 @@ Linear in the size of the input.
 
 - Added in version 3.4.0.
 - 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"
 

docs/mkdocs/docs/api/basic_json/from_cbor.md

@@ -9,8 +9,8 @@ static basic_json from_cbor(InputType&& i,
                             const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);
 
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static basic_json from_cbor(IteratorType first, IteratorType last,
+static basic_json from_cbor(IteratorType first, SentinelType last,
                             const bool strict = true,
                             const bool allow_exceptions = true,
                             const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);
@@ -19,7 +19,7 @@ static basic_json from_cbor(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the CBOR (Concise Binary Object Representation) serialization format.
 
 1. Reads from a compatible input.
-2. Reads from an iterator range.
+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](../../features/binary_formats/cbor.md).
 
@@ -38,6 +38,12 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 `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)
@@ -47,7 +53,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input
 
 `last` (in)
-:   iterator to the end of the input
+:   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)
@@ -113,6 +119,7 @@ Linear in the size of the input.
 - Added `allow_exceptions` parameter in version 3.2.0.
 - Added `tag_handler` parameter in version 3.9.0.
 - 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"
 

docs/mkdocs/docs/api/basic_json/from_msgpack.md

@@ -7,8 +7,8 @@ static basic_json from_msgpack(InputType&& i,
                                const bool strict = true,
                                const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static basic_json from_msgpack(IteratorType first, IteratorType last,
+static basic_json from_msgpack(IteratorType first, SentinelType last,
                                const bool strict = true,
                                const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_msgpack(IteratorType first, IteratorType last,
 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.
+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](../../features/binary_formats/messagepack.md).
 
@@ -35,6 +35,12 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 `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)
@@ -44,7 +50,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input
 
 `last` (in)
-:   iterator to the end of the input
+:   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)
@@ -105,6 +111,7 @@ Linear in the size of the input.
 - 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"
 

docs/mkdocs/docs/api/basic_json/from_ubjson.md

@@ -7,8 +7,8 @@ static basic_json from_ubjson(InputType&& i,
                               const bool strict = true,
                               const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static basic_json from_ubjson(IteratorType first, IteratorType last,
+static basic_json from_ubjson(IteratorType first, SentinelType last,
                               const bool strict = true,
                               const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_ubjson(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the UBJSON (Universal Binary JSON) serialization format.
 
 1. Reads from a compatible input.
-2. Reads from an iterator range.
+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](../../features/binary_formats/ubjson.md).
 
@@ -35,6 +35,12 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 `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)
@@ -44,7 +50,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input
 
 `last` (in)
-:   iterator to the end of the input
+:   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)
@@ -104,6 +110,7 @@ Linear in the size of the input.
 - Added in version 3.1.0.
 - 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"
 

docs/mkdocs/docs/api/basic_json/parse.md

@@ -10,8 +10,8 @@ static basic_json parse(InputType&& i,
                         const bool ignore_trailing_commas = false);
 
 // (2)
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType = IteratorType>
-static basic_json parse(IteratorType first, IteratorType last,
+static basic_json parse(IteratorType first, SentinelType last,
                         const parser_callback_t cb = nullptr,
                         const bool allow_exceptions = true,
                         const bool ignore_comments = false,
@@ -19,10 +19,11 @@ static basic_json parse(IteratorType first, IteratorType last,
 ```
 
 1. Deserialize from a compatible input.
-2. Deserialize from a pair of character iterators
+2. Deserialize 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 size of 1, 2, or 4 bytes, which will be interpreted
-    respectively as UTF-8, UTF-16, and UTF-32.
+    respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to
+    the iterator type with `operator!=`.
 
 ## Template parameters
 
@@ -43,6 +44,12 @@ static basic_json parse(IteratorType first, IteratorType last,
     - 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)
@@ -67,7 +74,7 @@ static basic_json parse(IteratorType first, IteratorType last,
 :   iterator to the start of a character range
 
 `last` (in)
-:   iterator to the end of a character range
+:   iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with `operator!=`
 
 ## Return value
 
@@ -238,6 +245,7 @@ Invalid Unicode escapes and unpaired surrogates in the input are reported as
 - Changed [runtime assertion](../../features/assertions.md) 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"
 

docs/mkdocs/docs/api/basic_json/sax_parse.md

@@ -11,8 +11,8 @@ static bool sax_parse(InputType&& i,
                       const bool ignore_trailing_commas = false);
 
 // (2)
-template<class IteratorType, class SAX>
+template<class IteratorType, class SAX, class SentinelType = IteratorType>
-static bool sax_parse(IteratorType first, IteratorType last,
+static bool sax_parse(IteratorType first, SentinelType last,
                       SAX* sax,
                       input_format_t format = input_format_t::json,
                       const bool strict = true,
@@ -23,10 +23,11 @@ static bool sax_parse(IteratorType first, IteratorType last,
 Read from input and generate SAX events
 
 1. Read from a compatible input.
-2. Read from a pair of character iterators
+2. Read 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.
+    respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to
+    the iterator type with `operator!=`.
 
 The SAX event lister must follow the interface of [`json_sax`](../json_sax/index.md).
 
@@ -46,6 +47,12 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
 :   a compatible iterator type for overload (2); a pair of character iterators whose `value_type` is an integral type
     with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32)
 
+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for overload (2), for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
+
 `SAX`
 :   a class fulfilling the SAX event listener interface; see [`json_sax`](../json_sax/index.md)
 
@@ -76,7 +83,7 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
 :   iterator to the start of a character range
 
 `last` (in)
-:   iterator to the end of a character range
+:   iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with `operator!=`
 
 ## Return value
 
@@ -128,6 +135,7 @@ A UTF-8 byte order mark is silently ignored.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - 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"
 

include/nlohmann/detail/input/input_adapters.hpp

@@ -155,7 +155,9 @@ class input_stream_adapter
 
 // General-purpose iterator-based adapter. It might not be as fast as
 // theoretically possible for some containers, but it is extremely versatile.
-template<typename IteratorType>
+// SentinelType defaults to IteratorType for backward compatibility, but may
+// be a different type (e.g., a C++20 sentinel or counted_iterator).
+template<typename IteratorType, typename SentinelType = IteratorType>
 class iterator_input_adapter
 {
   public:
@@ -169,9 +171,10 @@ class iterator_input_adapter
     // in wide_string_input_adapter, which does not expose this).
     static constexpr bool supports_seek =
         std::is_same<typename std::iterator_traits<IteratorType>::iterator_category, std::random_access_iterator_tag>::value
+        && std::is_same<IteratorType, SentinelType>::value
         && sizeof(char_type) == 1;
 
-    iterator_input_adapter(IteratorType first, IteratorType last)
+    iterator_input_adapter(IteratorType first, SentinelType last)
         : begin(first), current(std::move(first)), end(std::move(last))
     {}
 
@@ -216,19 +219,30 @@ class iterator_input_adapter
   private:
     // whether IteratorType refers to a contiguous range and therefore supports
     // a std::memcpy fast path (pointers always do; in C++20 we can also detect
-    // library iterators such as those of std::vector and std::string)
+    // library iterators such as those of std::vector and std::string).
+    // Computing the available element count needs either same-type iterators
+    // (plain std::distance) or, in C++20, a sized sentinel (std::ranges::distance),
+    // e.g. std::counted_iterator paired with std::default_sentinel_t.
     static constexpr bool iterator_is_contiguous =
 #if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
-        std::contiguous_iterator<IteratorType> ||
+        (std::is_same<IteratorType, SentinelType>::value || std::sized_sentinel_for<SentinelType, IteratorType>)
+        && (std::contiguous_iterator<IteratorType> || std::is_pointer<IteratorType>::value);
+#else
+        std::is_same<IteratorType, SentinelType>::value && std::is_pointer<IteratorType>::value;
 #endif
-        std::is_pointer<IteratorType>::value;
 
     // contiguous fast path: bulk copy the remaining range with std::memcpy
     template<class T>
     std::size_t get_elements_impl(T* dest, std::size_t count, std::true_type /*contiguous*/)
     {
         const std::size_t wanted = count * sizeof(T);
+#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
+        // std::ranges::distance also supports sized sentinels of a different
+        // type (e.g. std::counted_iterator + std::default_sentinel_t)
+        const std::size_t available = static_cast<std::size_t>(std::ranges::distance(current, end)) * sizeof(char_type);
+#else
         const std::size_t available = static_cast<std::size_t>(std::distance(current, end)) * sizeof(char_type);
+#endif
         const std::size_t copied = (std::min)(wanted, available);
         if (JSON_HEDLEY_LIKELY(copied != 0))
         {
@@ -267,7 +281,7 @@ class iterator_input_adapter
 
     IteratorType begin;
     IteratorType current;
-    IteratorType end;
+    SentinelType end;
 
     template<typename BaseInputAdapter, size_t T>
     friend struct wide_string_input_helper;
@@ -453,19 +467,54 @@ class wide_string_input_adapter
     std::size_t utf8_bytes_filled = 0;
 };
 
-template<typename IteratorType, typename Enable = void>
+template<typename IteratorType, typename SentinelType = IteratorType, typename Enable = void>
 struct iterator_input_adapter_factory
 {
     using iterator_type = IteratorType;
+    using sentinel_type = SentinelType;
     using char_type = typename std::iterator_traits<iterator_type>::value_type;
-    using adapter_type = iterator_input_adapter<iterator_type>;
+    using adapter_type = iterator_input_adapter<iterator_type, sentinel_type>;
 
-    static adapter_type create(IteratorType first, IteratorType last)
+    static adapter_type create(IteratorType first, SentinelType last)
     {
         return adapter_type(std::move(first), std::move(last));
     }
 };
 
+// Detection: whether IteratorType and SentinelType can be compared with !=
+template<typename IteratorType, typename SentinelType, typename = void>
+struct can_compare_ne_impl : std::false_type {};
+
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne_impl < IteratorType, SentinelType,
+       void_t < decltype(std::declval<IteratorType>() != std::declval<SentinelType>()) >>
+           : std::true_type {};
+
+// Workaround for reversed operator order
+template<typename IteratorType, typename SentinelType, typename = void>
+struct can_compare_ne_reversed : std::false_type {};
+
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne_reversed < IteratorType, SentinelType,
+       void_t < decltype(std::declval<SentinelType>() != std::declval<IteratorType>()) >>
+           : std::true_type {};
+
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne_either_order : std::integral_constant < bool,
+    can_compare_ne_impl<IteratorType, SentinelType>::value ||
+    can_compare_ne_reversed<IteratorType, SentinelType>::value > {};
+
+// std::nullptr_t is excluded explicitly: a literal `nullptr` passed as a
+// trailing default argument (e.g. parse(s, nullptr, ...)) must never be
+// mistaken for a sentinel, and some compilers (e.g. GCC 4.8) unreliably
+// SFINAE the `operator!=` detection above for std::nullptr_t against
+// container/string types, which would otherwise make such calls ambiguous
+// with the compatible-input overload.
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne : std::integral_constant < bool,
+    !std::is_same<SentinelType, std::nullptr_t>::value &&
+    can_compare_ne_either_order<IteratorType, SentinelType>::value > {};
+
 template<typename T>
 struct is_iterator_of_multibyte
 {
@@ -476,25 +525,31 @@ struct is_iterator_of_multibyte
     };
 };
 
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType>
-struct iterator_input_adapter_factory<IteratorType, enable_if_t<is_iterator_of_multibyte<IteratorType>::value>>
+struct iterator_input_adapter_factory<IteratorType, SentinelType, enable_if_t<is_iterator_of_multibyte<IteratorType>::value>>
 {
     using iterator_type = IteratorType;
+    using sentinel_type = SentinelType;
     using char_type = typename std::iterator_traits<iterator_type>::value_type;
-    using base_adapter_type = iterator_input_adapter<iterator_type>;
+    using base_adapter_type = iterator_input_adapter<iterator_type, sentinel_type>;
     using adapter_type = wide_string_input_adapter<base_adapter_type, char_type>;
 
-    static adapter_type create(IteratorType first, IteratorType last)
+    static adapter_type create(IteratorType first, SentinelType last)
     {
         return adapter_type(base_adapter_type(std::move(first), std::move(last)));
     }
 };
 
-// General purpose iterator-based input
+// General purpose iterator-based input (iterator+sentinel pair; SentinelType
-template<typename IteratorType>
+// defaults to IteratorType for the common same-type case, but may differ for
-typename iterator_input_adapter_factory<IteratorType>::adapter_type input_adapter(IteratorType first, IteratorType last)
+// C++20 ranges-style iterator+sentinel pairs). Only enable for types that can
+// be compared with !=.
+template < typename IteratorType, typename SentinelType = IteratorType,
+           typename = typename std::enable_if <
+               can_compare_ne<IteratorType, SentinelType>::value >::type >
+typename iterator_input_adapter_factory<IteratorType, SentinelType>::adapter_type input_adapter(IteratorType first, SentinelType last)
 {
-    using factory_type = iterator_input_adapter_factory<IteratorType>;
+    using factory_type = iterator_input_adapter_factory<IteratorType, SentinelType>;
     return factory_type::create(first, last);
 }
 

include/nlohmann/json.hpp

@@ -4083,12 +4083,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return result;
     }
 
-    /// @brief deserialize from a pair of character iterators
+    /// @brief deserialize from a pair of character iterators (or an iterator+sentinel pair, C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/parse/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
     static basic_json parse(IteratorType first,
-                            IteratorType last,
+                            SentinelType last,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
                             const bool ignore_comments = false,
@@ -4122,10 +4123,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
-    /// @brief check if the input is valid JSON
+    /// @brief check if the input is valid JSON (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
-    static bool accept(IteratorType first, IteratorType last,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
+    static bool accept(IteratorType first, SentinelType last,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
@@ -4157,11 +4159,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }
 
-    /// @brief generate SAX events
+    /// @brief generate SAX events (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/sax_parse/
-    template<class IteratorType, class SAX>
+    template<class IteratorType, class SAX, class SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_NON_NULL(3)
-    static bool sax_parse(IteratorType first, IteratorType last, SAX* sax,
+    static bool sax_parse(IteratorType first, SentinelType last, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
                           const bool ignore_comments = false,
@@ -4461,11 +4464,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in CBOR format
+    /// @brief create a JSON value from an input in CBOR format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_cbor/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_cbor(IteratorType first, IteratorType last,
+    static basic_json from_cbor(IteratorType first, SentinelType last,
                                 const bool strict = true,
                                 const bool allow_exceptions = true,
                                 const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error)
@@ -4518,11 +4522,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in MessagePack format
+    /// @brief create a JSON value from an input in MessagePack format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_msgpack/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_msgpack(IteratorType first, IteratorType last,
+    static basic_json from_msgpack(IteratorType first, SentinelType last,
                                    const bool strict = true,
                                    const bool allow_exceptions = true)
     {
@@ -4572,11 +4577,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in UBJSON format
+    /// @brief create a JSON value from an input in UBJSON format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_ubjson/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_ubjson(IteratorType first, IteratorType last,
+    static basic_json from_ubjson(IteratorType first, SentinelType last,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
     {
@@ -4626,11 +4632,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in BJData format
+    /// @brief create a JSON value from an input in BJData format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_bjdata/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_bjdata(IteratorType first, IteratorType last,
+    static basic_json from_bjdata(IteratorType first, SentinelType last,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
     {
@@ -4656,11 +4663,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in BSON format
+    /// @brief create a JSON value from an input in BSON format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_bson/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_bson(IteratorType first, IteratorType last,
+    static basic_json from_bson(IteratorType first, SentinelType last,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
     {

single_include/nlohmann/json.hpp

@@ -6992,7 +6992,9 @@ class input_stream_adapter
 
 // General-purpose iterator-based adapter. It might not be as fast as
 // theoretically possible for some containers, but it is extremely versatile.
-template<typename IteratorType>
+// SentinelType defaults to IteratorType for backward compatibility, but may
+// be a different type (e.g., a C++20 sentinel or counted_iterator).
+template<typename IteratorType, typename SentinelType = IteratorType>
 class iterator_input_adapter
 {
   public:
@@ -7006,9 +7008,10 @@ class iterator_input_adapter
     // in wide_string_input_adapter, which does not expose this).
     static constexpr bool supports_seek =
         std::is_same<typename std::iterator_traits<IteratorType>::iterator_category, std::random_access_iterator_tag>::value
+        && std::is_same<IteratorType, SentinelType>::value
         && sizeof(char_type) == 1;
 
-    iterator_input_adapter(IteratorType first, IteratorType last)
+    iterator_input_adapter(IteratorType first, SentinelType last)
         : begin(first), current(std::move(first)), end(std::move(last))
     {}
 
@@ -7053,19 +7056,30 @@ class iterator_input_adapter
   private:
     // whether IteratorType refers to a contiguous range and therefore supports
     // a std::memcpy fast path (pointers always do; in C++20 we can also detect
-    // library iterators such as those of std::vector and std::string)
+    // library iterators such as those of std::vector and std::string).
+    // Computing the available element count needs either same-type iterators
+    // (plain std::distance) or, in C++20, a sized sentinel (std::ranges::distance),
+    // e.g. std::counted_iterator paired with std::default_sentinel_t.
     static constexpr bool iterator_is_contiguous =
 #if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
-        std::contiguous_iterator<IteratorType> ||
+        (std::is_same<IteratorType, SentinelType>::value || std::sized_sentinel_for<SentinelType, IteratorType>)
+        && (std::contiguous_iterator<IteratorType> || std::is_pointer<IteratorType>::value);
+#else
+        std::is_same<IteratorType, SentinelType>::value && std::is_pointer<IteratorType>::value;
 #endif
-        std::is_pointer<IteratorType>::value;
 
     // contiguous fast path: bulk copy the remaining range with std::memcpy
     template<class T>
     std::size_t get_elements_impl(T* dest, std::size_t count, std::true_type /*contiguous*/)
     {
         const std::size_t wanted = count * sizeof(T);
+#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
+        // std::ranges::distance also supports sized sentinels of a different
+        // type (e.g. std::counted_iterator + std::default_sentinel_t)
+        const std::size_t available = static_cast<std::size_t>(std::ranges::distance(current, end)) * sizeof(char_type);
+#else
         const std::size_t available = static_cast<std::size_t>(std::distance(current, end)) * sizeof(char_type);
+#endif
         const std::size_t copied = (std::min)(wanted, available);
         if (JSON_HEDLEY_LIKELY(copied != 0))
         {
@@ -7104,7 +7118,7 @@ class iterator_input_adapter
 
     IteratorType begin;
     IteratorType current;
-    IteratorType end;
+    SentinelType end;
 
     template<typename BaseInputAdapter, size_t T>
     friend struct wide_string_input_helper;
@@ -7290,19 +7304,54 @@ class wide_string_input_adapter
     std::size_t utf8_bytes_filled = 0;
 };
 
-template<typename IteratorType, typename Enable = void>
+template<typename IteratorType, typename SentinelType = IteratorType, typename Enable = void>
 struct iterator_input_adapter_factory
 {
     using iterator_type = IteratorType;
+    using sentinel_type = SentinelType;
     using char_type = typename std::iterator_traits<iterator_type>::value_type;
-    using adapter_type = iterator_input_adapter<iterator_type>;
+    using adapter_type = iterator_input_adapter<iterator_type, sentinel_type>;
 
-    static adapter_type create(IteratorType first, IteratorType last)
+    static adapter_type create(IteratorType first, SentinelType last)
     {
         return adapter_type(std::move(first), std::move(last));
     }
 };
 
+// Detection: whether IteratorType and SentinelType can be compared with !=
+template<typename IteratorType, typename SentinelType, typename = void>
+struct can_compare_ne_impl : std::false_type {};
+
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne_impl < IteratorType, SentinelType,
+       void_t < decltype(std::declval<IteratorType>() != std::declval<SentinelType>()) >>
+           : std::true_type {};
+
+// Workaround for reversed operator order
+template<typename IteratorType, typename SentinelType, typename = void>
+struct can_compare_ne_reversed : std::false_type {};
+
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne_reversed < IteratorType, SentinelType,
+       void_t < decltype(std::declval<SentinelType>() != std::declval<IteratorType>()) >>
+           : std::true_type {};
+
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne_either_order : std::integral_constant < bool,
+    can_compare_ne_impl<IteratorType, SentinelType>::value ||
+    can_compare_ne_reversed<IteratorType, SentinelType>::value > {};
+
+// std::nullptr_t is excluded explicitly: a literal `nullptr` passed as a
+// trailing default argument (e.g. parse(s, nullptr, ...)) must never be
+// mistaken for a sentinel, and some compilers (e.g. GCC 4.8) unreliably
+// SFINAE the `operator!=` detection above for std::nullptr_t against
+// container/string types, which would otherwise make such calls ambiguous
+// with the compatible-input overload.
+template<typename IteratorType, typename SentinelType>
+struct can_compare_ne : std::integral_constant < bool,
+    !std::is_same<SentinelType, std::nullptr_t>::value &&
+    can_compare_ne_either_order<IteratorType, SentinelType>::value > {};
+
 template<typename T>
 struct is_iterator_of_multibyte
 {
@@ -7313,25 +7362,31 @@ struct is_iterator_of_multibyte
     };
 };
 
-template<typename IteratorType>
+template<typename IteratorType, typename SentinelType>
-struct iterator_input_adapter_factory<IteratorType, enable_if_t<is_iterator_of_multibyte<IteratorType>::value>>
+struct iterator_input_adapter_factory<IteratorType, SentinelType, enable_if_t<is_iterator_of_multibyte<IteratorType>::value>>
 {
     using iterator_type = IteratorType;
+    using sentinel_type = SentinelType;
     using char_type = typename std::iterator_traits<iterator_type>::value_type;
-    using base_adapter_type = iterator_input_adapter<iterator_type>;
+    using base_adapter_type = iterator_input_adapter<iterator_type, sentinel_type>;
     using adapter_type = wide_string_input_adapter<base_adapter_type, char_type>;
 
-    static adapter_type create(IteratorType first, IteratorType last)
+    static adapter_type create(IteratorType first, SentinelType last)
     {
         return adapter_type(base_adapter_type(std::move(first), std::move(last)));
     }
 };
 
-// General purpose iterator-based input
+// General purpose iterator-based input (iterator+sentinel pair; SentinelType
-template<typename IteratorType>
+// defaults to IteratorType for the common same-type case, but may differ for
-typename iterator_input_adapter_factory<IteratorType>::adapter_type input_adapter(IteratorType first, IteratorType last)
+// C++20 ranges-style iterator+sentinel pairs). Only enable for types that can
+// be compared with !=.
+template < typename IteratorType, typename SentinelType = IteratorType,
+           typename = typename std::enable_if <
+               can_compare_ne<IteratorType, SentinelType>::value >::type >
+typename iterator_input_adapter_factory<IteratorType, SentinelType>::adapter_type input_adapter(IteratorType first, SentinelType last)
 {
-    using factory_type = iterator_input_adapter_factory<IteratorType>;
+    using factory_type = iterator_input_adapter_factory<IteratorType, SentinelType>;
     return factory_type::create(first, last);
 }
 
@@ -24934,12 +24989,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return result;
     }
 
-    /// @brief deserialize from a pair of character iterators
+    /// @brief deserialize from a pair of character iterators (or an iterator+sentinel pair, C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/parse/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
     static basic_json parse(IteratorType first,
-                            IteratorType last,
+                            SentinelType last,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
                             const bool ignore_comments = false,
@@ -24973,10 +25029,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
-    /// @brief check if the input is valid JSON
+    /// @brief check if the input is valid JSON (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
-    static bool accept(IteratorType first, IteratorType last,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
+    static bool accept(IteratorType first, SentinelType last,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
@@ -25008,11 +25065,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }
 
-    /// @brief generate SAX events
+    /// @brief generate SAX events (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/sax_parse/
-    template<class IteratorType, class SAX>
+    template<class IteratorType, class SAX, class SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_NON_NULL(3)
-    static bool sax_parse(IteratorType first, IteratorType last, SAX* sax,
+    static bool sax_parse(IteratorType first, SentinelType last, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
                           const bool ignore_comments = false,
@@ -25312,11 +25370,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in CBOR format
+    /// @brief create a JSON value from an input in CBOR format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_cbor/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_cbor(IteratorType first, IteratorType last,
+    static basic_json from_cbor(IteratorType first, SentinelType last,
                                 const bool strict = true,
                                 const bool allow_exceptions = true,
                                 const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error)
@@ -25369,11 +25428,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in MessagePack format
+    /// @brief create a JSON value from an input in MessagePack format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_msgpack/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_msgpack(IteratorType first, IteratorType last,
+    static basic_json from_msgpack(IteratorType first, SentinelType last,
                                    const bool strict = true,
                                    const bool allow_exceptions = true)
     {
@@ -25423,11 +25483,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in UBJSON format
+    /// @brief create a JSON value from an input in UBJSON format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_ubjson/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_ubjson(IteratorType first, IteratorType last,
+    static basic_json from_ubjson(IteratorType first, SentinelType last,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
     {
@@ -25477,11 +25538,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in BJData format
+    /// @brief create a JSON value from an input in BJData format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_bjdata/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_bjdata(IteratorType first, IteratorType last,
+    static basic_json from_bjdata(IteratorType first, SentinelType last,
                                   const bool strict = true,
                                   const bool allow_exceptions = true)
     {
@@ -25507,11 +25569,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return res ? result : basic_json(value_t::discarded);
     }
 
-    /// @brief create a JSON value from an input in BSON format
+    /// @brief create a JSON value from an input in BSON format (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/from_bson/
-    template<typename IteratorType>
+    template<typename IteratorType, typename SentinelType = IteratorType,
+             detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
     JSON_HEDLEY_WARN_UNUSED_RESULT
-    static basic_json from_bson(IteratorType first, IteratorType last,
+    static basic_json from_bson(IteratorType first, SentinelType last,
                                 const bool strict = true,
                                 const bool allow_exceptions = true)
     {

tests/src/test_utils.hpp

@@ -30,4 +30,17 @@ inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
     return byte_vector;
 }
 
+// sentinel for istreambuf_iterator; compares != true until EOF is reached
+// lets tests read a file directly via the new iterator+sentinel overloads
+// instead of buffering the whole file into a vector first.
+// Only the iterator-first direction (it != sentinel) is ever evaluated by
+// the library's parse loop, so no reversed-order overload is needed.
+struct istreambuf_sentinel
+{
+    friend bool operator!=(const std::istreambuf_iterator<char>& it, const istreambuf_sentinel& /*unused*/) noexcept
+    {
+        return it != std::istreambuf_iterator<char>();
+    }
+};
+
 } // namespace utils

tests/src/unit-bjdata.cpp

@@ -3699,6 +3699,15 @@ TEST_CASE("Universal Binary JSON Specification Examples 1")
     }
 }
 
+TEST_CASE("Parse BJData directly from a file using iterator and sentinel")
+{
+    std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.bjdata";
+    std::ifstream file(filename, std::ios::binary);
+    const std::istreambuf_iterator<char> first(file);
+    const json parsed = json::from_bjdata(first, utils::istreambuf_sentinel{});
+    CHECK((parsed.is_object() || parsed.is_array()));
+}
+
 #if !defined(JSON_NOEXCEPTION)
 TEST_CASE("all BJData first bytes")
 {

tests/src/unit-bson.cpp

@@ -1208,6 +1208,19 @@ TEST_CASE("BSON numerical data")
     }
 }
 
+TEST_CASE("Parse BSON directly from a file using iterator and sentinel")
+{
+    std::string const filename = TEST_DATA_DIRECTORY "/json.org/1.json";
+
+    std::ifstream f_json(filename);
+    const json expected = json::parse(f_json);
+
+    std::ifstream file(filename + ".bson", std::ios::binary);
+    const std::istreambuf_iterator<char> first(file);
+    const json parsed = json::from_bson(first, utils::istreambuf_sentinel{});
+    CHECK(parsed == expected);
+}
+
 TEST_CASE("BSON roundtrips" * doctest::skip())
 {
     SECTION("reference files")

tests/src/unit-cbor.cpp

@@ -1921,6 +1921,15 @@ TEST_CASE("single CBOR roundtrip")
     }
 }
 
+TEST_CASE("Parse CBOR directly from a file using iterator and sentinel")
+{
+    std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.cbor";
+    std::ifstream file(filename, std::ios::binary);
+    const std::istreambuf_iterator<char> first(file);
+    const json parsed = json::from_cbor(first, utils::istreambuf_sentinel{});
+    CHECK((parsed.is_object() || parsed.is_array()));
+}
+
 #if !defined(JSON_NOEXCEPTION)
 TEST_CASE("CBOR regressions")
 {

tests/src/unit-msgpack.cpp

@@ -1641,6 +1641,15 @@ TEST_CASE("single MessagePack roundtrip")
     }
 }
 
+TEST_CASE("Parse MessagePack directly from a file using iterator and sentinel")
+{
+    std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.msgpack";
+    std::ifstream file(filename, std::ios::binary);
+    const std::istreambuf_iterator<char> first(file);
+    const json parsed = json::from_msgpack(first, utils::istreambuf_sentinel{});
+    CHECK((parsed.is_object() || parsed.is_array()));
+}
+
 TEST_CASE("MessagePack roundtrips" * doctest::skip())
 {
     SECTION("input from msgpack-python")

tests/src/unit-ubjson.cpp

@@ -2393,6 +2393,15 @@ TEST_CASE("Universal Binary JSON Specification Examples 1")
     }
 }
 
+TEST_CASE("Parse UBJSON directly from a file using iterator and sentinel")
+{
+    std::string const filename = TEST_DATA_DIRECTORY "/json_testsuite/sample.json.ubjson";
+    std::ifstream file(filename, std::ios::binary);
+    const std::istreambuf_iterator<char> first(file);
+    const json parsed = json::from_ubjson(first, utils::istreambuf_sentinel{});
+    CHECK((parsed.is_object() || parsed.is_array()));
+}
+
 #if !defined(JSON_NOEXCEPTION)
 TEST_CASE("all UBJSON first bytes")
 {

tests/src/unit-user_defined_input.cpp

@@ -6,6 +6,13 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
+// cmake/test.cmake selects the C++ standard versions with which to build a
+// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
+// When using macros that are only defined for particular versions of the standard
+// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
+// version macro in a comment close by, like this:
+// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
@@ -13,6 +20,10 @@ using nlohmann::json;
 
 #include <list>
 
+#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
+    #include <iterator>
+#endif
+
 namespace
 {
 TEST_CASE("Use arbitrary stdlib container")
@@ -168,4 +179,54 @@ TEST_CASE("Custom iterator")
     CHECK(as_json.at(3) == 4);
 }
 
+// Custom sentinel type for testing heterogeneous iterator+sentinel support
+struct CustomSentinel
+{
+    const char* end_ptr;
+
+    // only the iterator-first direction (it != sentinel) is ever evaluated by
+    // the library's parse loop; a reversed-order overload would go unused and
+    // trip -Wunneeded-internal-declaration under -Weverything
+    friend bool operator!=(const char* it, const CustomSentinel& sentinel)
+    {
+        return it != sentinel.end_ptr;
+    }
+};
+
+TEST_CASE("Parse with heterogeneous iterator and sentinel types")
+{
+    const std::string json_str = R"({"key":"value"})";
+    const char* end_ptr = json_str.data() + json_str.size();
+
+    // Parse using pointer and sentinel (different types)
+    json j = json::parse(json_str.data(), CustomSentinel{end_ptr});
+    CHECK(j["key"] == "value");
+
+    // Accept using pointer and sentinel
+    CHECK(json::accept(json_str.data(), CustomSentinel{end_ptr}));
+
+    // Test that the same-type case still works
+    std::string raw_data = R"([1,2,3])";
+    std::list<char> data(raw_data.begin(), raw_data.end());
+    json j2 = json::parse(data.begin(), data.end());
+    CHECK(j2.at(0) == 1);
+}
+
+#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
+// JSON_HAS_CPP_20 (do not remove; see note at top of file)
+TEST_CASE("Parse with std::counted_iterator and std::default_sentinel_t")
+{
+    const std::string json_str = R"({"key":"value","array":[1,2,3]})";
+    const auto len = static_cast<std::iter_difference_t<std::string::const_iterator>>(json_str.size());
+
+    std::counted_iterator first(json_str.begin(), len);
+    const json j = json::parse(first, std::default_sentinel);
+    CHECK(j["key"] == "value");
+    CHECK(j["array"].size() == 3);
+
+    std::counted_iterator first2(json_str.begin(), len);
+    CHECK(json::accept(first2, std::default_sentinel));
+}
+#endif
+
 } // namespace