Fix ADL leak of nlohmann::detail through basic_json's default base class (#5238)

the standard only specifies that the first elements are sorted.

this caused my experimental C++ standard library implementation to fail.

Signed-off-by: Paul Dreik <github@pauldreik.se>

include/nlohmann/detail/input/binary_reader.hpp

@@ -12,7 +12,7 @@
 #include <array> // array
 #include <cmath> // ldexp
 #include <cstddef> // size_t
-#include <cstdint> // uint8_t, uint16_t, uint32_t, uint64_t
+#include <cstdint> // uint8_t, uint16_t, uint32_t, uint64_t, uintmax_t
 #include <cstdio> // snprintf
 #include <cstring> // memcpy
 #include <iterator> // back_inserter
@@ -2908,18 +2908,7 @@ class binary_reader
                     const NumberType len,
                     string_t& result)
     {
-        bool success = true;
-        for (NumberType i = 0; i < len; i++)
-        {
-            get();
-            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(format, "string")))
-            {
-                success = false;
-                break;
-            }
-            result.push_back(static_cast<typename string_t::value_type>(current));
-        }
-        return success;
+        return get_bytes(format, len, "string", result);
     }
 
     /*!
@@ -2941,18 +2930,66 @@ class binary_reader
                     const NumberType len,
                     binary_t& result)
     {
-        bool success = true;
-        for (NumberType i = 0; i < len; i++)
+        return get_bytes(format, len, "binary", result);
+    }
+
+    /*!
+    @brief read @a len bytes from the input into a string or byte container
+
+    @tparam NumberType    the type of the length
+    @tparam ContainerType the destination container (string_t or binary_t)
+    @param[in] format   the current format (for diagnostics)
+    @param[in] len      number of bytes to read
+    @param[in] context  further context information (for diagnostics)
+    @param[out] result  container the bytes are appended to
+
+    @return whether reading completed
+
+    @note We cannot reserve @a len bytes for the result up front, because
+          @a len may be far larger than the actual input. Instead we read in
+          bounded chunks, so the peak allocation is capped regardless of the
+          claimed length while the per-byte loop is replaced by block copies
+          (a std::memcpy for contiguous inputs). @ref unexpect_eof() still
+          detects a premature end of input.
+    */
+    template<typename NumberType, typename ContainerType>
+    bool get_bytes(const input_format_t format,
+                   NumberType len,
+                   const char* context,
+                   ContainerType& result)
+    {
+        // upper bound on the number of bytes read (and allocated) per chunk
+        constexpr std::size_t chunk_size = 4096;
+
+        while (len > 0)
         {
-            get();
-            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(format, "binary")))
+            // number of bytes to read this iteration: min(chunk_size, len),
+            // computed without truncating chunk_size to a narrow NumberType
+            const std::size_t wanted = (static_cast<std::uintmax_t>(len) < static_cast<std::uintmax_t>(chunk_size))
+                                       ? static_cast<std::size_t>(len)
+                                       : chunk_size;
+            const std::size_t old_size = result.size();
+            result.resize(old_size + wanted);
+            // resize() is required to make size() exactly old_size + wanted;
+            // that is the room get_elements() is allowed to write into
+            JSON_ASSERT(result.size() == old_size + wanted);
+            const std::size_t bytes_read = ia.get_elements(&result[old_size], wanted);
+            chars_read += bytes_read;
+            if (JSON_HEDLEY_UNLIKELY(bytes_read < wanted))
             {
-                success = false;
-                break;
+                // premature end of input: shrink to what was actually read and
+                // report the failure at the first missing byte (same position
+                // accounting as get_to() for partial number reads)
+                result.resize(old_size + bytes_read);
+                ++chars_read;
+                current = char_traits<char_type>::eof();
+                return unexpect_eof(format, context);
             }
-            result.push_back(static_cast<typename binary_t::value_type>(current));
+            // a full chunk was read; get_elements() never returns more than requested
+            JSON_ASSERT(bytes_read == wanted);
+            len = static_cast<NumberType>(len - static_cast<NumberType>(wanted));
         }
-        return success;
+        return true;
     }
 
     /*!

include/nlohmann/detail/input/input_adapters.hpp

@@ -161,8 +161,18 @@ class iterator_input_adapter
   public:
     using char_type = typename std::iterator_traits<IteratorType>::value_type;
 
+    // Whether the lexer may reconstruct already-consumed input on demand (for
+    // diagnostics) instead of copying every scanned character eagerly. This is
+    // only sound for multi-pass, randomly-addressable byte input: the iterator
+    // must be random-access (so the consumed prefix can be revisited in O(1))
+    // and each element must map 1:1 to an input byte (wide inputs are wrapped
+    // 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
+        && sizeof(char_type) == 1;
+
     iterator_input_adapter(IteratorType first, IteratorType last)
-        : current(std::move(first)), end(std::move(last))
+        : begin(first), current(std::move(first)), end(std::move(last))
     {}
 
     typename char_traits<char_type>::int_type get_character()
@@ -177,9 +187,67 @@ class iterator_input_adapter
         return char_traits<char_type>::eof();
     }
 
-    // for general iterators, we cannot really do something better than falling back to processing the range one-by-one
+    // number of characters consumed from the input so far
+    std::size_t get_consumed_count() const
+    {
+        return static_cast<std::size_t>(std::distance(begin, current));
+    }
+
+    // append the already-consumed characters in the half-open range
+    // [first_index, last_index) to @a out; only valid when supports_seek
+    template<typename ContainerType>
+    void copy_consumed_range(std::size_t first_index, std::size_t last_index, ContainerType& out) const
+    {
+        const auto from = std::next(begin, static_cast<typename std::iterator_traits<IteratorType>::difference_type>(first_index));
+        const auto to = std::next(begin, static_cast<typename std::iterator_traits<IteratorType>::difference_type>(last_index));
+        out.insert(out.end(), from, to);
+    }
+
+    // Copy up to count * sizeof(T) bytes into dest, returning the number of
+    // bytes actually read. For contiguous iterators (e.g. pointers) this is a
+    // single std::memcpy; for general iterators we fall back to processing the
+    // range one-by-one.
     template<class T>
     std::size_t get_elements(T* dest, std::size_t count = 1)
+    {
+        return get_elements_impl(dest, count, std::integral_constant<bool, iterator_is_contiguous> {});
+    }
+
+  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)
+    static constexpr bool iterator_is_contiguous =
+#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
+        std::contiguous_iterator<IteratorType> ||
+#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);
+        const std::size_t available = static_cast<std::size_t>(std::distance(current, end)) * sizeof(char_type);
+        const std::size_t copied = (std::min)(wanted, available);
+        if (JSON_HEDLEY_LIKELY(copied != 0))
+        {
+            // the copy must stay within both buffers: the caller-provided
+            // destination holds `wanted` bytes and the remaining input range
+            // holds `available` bytes, and `copied` is the minimum of the two
+            JSON_ASSERT(copied <= wanted);    // does not overrun the destination
+            JSON_ASSERT(copied <= available); // does not read past the input end
+            // &*current yields the raw address for both raw pointers and
+            // non-pointer contiguous iterators (e.g. std::vector's iterator)
+            std::memcpy(dest, &*current, copied);
+            std::advance(current, static_cast<typename std::iterator_traits<IteratorType>::difference_type>(copied / sizeof(char_type)));
+        }
+        return copied;
+    }
+
+    // general fallback: copy the range one element at a time
+    template<class T>
+    std::size_t get_elements_impl(T* dest, std::size_t count, std::false_type /*contiguous*/)
     {
         auto* ptr = reinterpret_cast<char*>(dest);
         for (std::size_t read_index = 0; read_index < count * sizeof(T); ++read_index)
@@ -197,7 +265,7 @@ class iterator_input_adapter
         return count * sizeof(T);
     }
 
-  private:
+    IteratorType begin;
     IteratorType current;
     IteratorType end;
 

include/nlohmann/detail/input/lexer.hpp

@@ -103,6 +103,28 @@ class lexer_base
         }
     }
 };
+
+// Detect whether an input adapter can reconstruct already-consumed input on
+// demand (see iterator_input_adapter::supports_seek). Adapters that do not
+// expose the flag - e.g. file, stream, wide-string, and user-defined adapters -
+// are treated as non-seekable streaming input, for which the lexer keeps
+// copying every scanned character eagerly. The value is read via tag dispatch
+// on is_detected so the flag is only referenced for adapters that provide it.
+template<typename InputAdapterType>
+using detect_supports_seek = decltype(InputAdapterType::supports_seek);
+
+template<typename InputAdapterType>
+constexpr bool input_adapter_supports_seek(std::true_type /*detected*/)
+{
+    return InputAdapterType::supports_seek;
+}
+
+template<typename InputAdapterType>
+constexpr bool input_adapter_supports_seek(std::false_type /*detected*/)
+{
+    return false;
+}
+
 /*!
 @brief lexical analysis
 
@@ -118,6 +140,12 @@ class lexer : public lexer_base<BasicJsonType>
     using char_type = typename InputAdapterType::char_type;
     using char_int_type = typename char_traits<char_type>::int_type;
 
+    /// whether the last read token can be reconstructed from the input adapter
+    /// on demand (in error paths) instead of being copied on every scanned
+    /// character; see input_adapter_supports_seek
+    static constexpr bool lazy_token_string =
+        input_adapter_supports_seek<InputAdapterType>(is_detected<detect_supports_seek, InputAdapterType> {});
+
   public:
     using token_type = typename lexer_base<BasicJsonType>::token_type;
 
@@ -1327,8 +1355,24 @@ scan_number_done:
     void reset() noexcept
     {
         token_buffer.clear();
-        token_string.clear();
         decimal_point_position = std::string::npos;
+
+        note_token_start(std::integral_constant<bool, lazy_token_string> {});
+    }
+
+    /// seekable adapter: remember where the current token starts so it can be
+    /// reconstructed from the input on error; current has already been
+    /// consumed, hence the -1
+    void note_token_start(std::true_type /*lazy*/) noexcept
+    {
+        token_string_start = ia.get_consumed_count() - 1;
+    }
+
+    /// streaming adapter: start copying the token eagerly, beginning with the
+    /// already-read first character
+    void note_token_start(std::false_type /*lazy*/) noexcept
+    {
+        token_string.clear();
         token_string.push_back(char_traits<char_type>::to_char_type(current));
     }
 
@@ -1357,10 +1401,9 @@ scan_number_done:
             current = ia.get_character();
         }
 
-        if (JSON_HEDLEY_LIKELY(current != char_traits<char_type>::eof()))
-        {
-            token_string.push_back(char_traits<char_type>::to_char_type(current));
-        }
+        // seekable adapters reconstruct the token lazily on error (see
+        // get_token_string), so the eager per-character copy is skipped
+        capture_char(std::integral_constant<bool, lazy_token_string> {});
 
         if (current == '\n')
         {
@@ -1371,6 +1414,18 @@ scan_number_done:
         return current;
     }
 
+    /// seekable adapter: nothing to capture, the token is rebuilt on error
+    void capture_char(std::true_type /*lazy*/) const noexcept {}
+
+    /// streaming adapter: copy the scanned character into token_string
+    void capture_char(std::false_type /*lazy*/)
+    {
+        if (JSON_HEDLEY_LIKELY(current != char_traits<char_type>::eof()))
+        {
+            token_string.push_back(char_traits<char_type>::to_char_type(current));
+        }
+    }
+
     /*!
     @brief unget current character (read it again on next get)
 
@@ -1398,6 +1453,15 @@ scan_number_done:
             --position.chars_read_current_line;
         }
 
+        uncapture_char(std::integral_constant<bool, lazy_token_string> {});
+    }
+
+    /// seekable adapter: nothing was captured, so nothing to undo
+    void uncapture_char(std::true_type /*lazy*/) const noexcept {}
+
+    /// streaming adapter: drop the character copied by the matching get()
+    void uncapture_char(std::false_type /*lazy*/)
+    {
         if (JSON_HEDLEY_LIKELY(current != char_traits<char_type>::eof()))
         {
             JSON_ASSERT(!token_string.empty());
@@ -1455,14 +1519,38 @@ scan_number_done:
         return position;
     }
 
+    /// seekable adapter: rebuild the last read token from the input on demand
+    const std::vector<char_type>& collect_token_chars(std::vector<char_type>& out, std::true_type /*lazy*/) const
+    {
+        // a pending unget of a real (non-EOF) character means that character
+        // was consumed from the input but is not part of the token; EOF is
+        // never consumed, so it must not be subtracted (mirrors unget())
+        const bool pending_real_unget = next_unget && current != char_traits<char_type>::eof();
+        const std::size_t stop = ia.get_consumed_count() - (pending_real_unget ? 1u : 0u);
+        if (JSON_HEDLEY_LIKELY(stop >= token_string_start))
+        {
+            ia.copy_consumed_range(token_string_start, stop, out);
+        }
+        return out;
+    }
+
+    /// streaming adapter: the token was copied eagerly while scanning
+    const std::vector<char_type>& collect_token_chars(std::vector<char_type>& /*out*/, std::false_type /*lazy*/) const
+    {
+        return token_string;
+    }
+
     /// return the last read token (for errors only).  Will never contain EOF
     /// (an arbitrary value that is not a valid char value, often -1), because
     /// 255 may legitimately occur.  May contain NUL, which should be escaped.
     std::string get_token_string() const
     {
+        std::vector<char_type> reconstructed;
+        const std::vector<char_type>& chars = collect_token_chars(reconstructed, std::integral_constant<bool, lazy_token_string> {});
+
         // escape control characters
         std::string result;
-        for (const auto c : token_string)
+        for (const auto c : chars)
         {
             if (static_cast<unsigned char>(c) <= '\x1F')
             {
@@ -1623,9 +1711,14 @@ scan_number_done:
     /// the start position of the current token
     position_t position {};
 
-    /// raw input token string (for error messages)
+    /// raw input token string for error messages; only populated for streaming
+    /// adapters (seekable adapters reconstruct it lazily via token_string_start)
     std::vector<char_type> token_string {};
 
+    /// start offset of the current token within the input, used to reconstruct
+    /// the last read token on error for seekable adapters (see collect_token_chars)
+    std::size_t token_string_start = 0;
+
     /// buffer for variable-length tokens (numbers, strings)
     string_t token_buffer {};
 

include/nlohmann/detail/json_custom_base_class.hpp

@@ -13,8 +13,6 @@
 #include <nlohmann/detail/abi_macros.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
-namespace detail
-{
 
 /*!
 @brief Default base class of the @ref basic_json class.
@@ -25,13 +23,27 @@ of @ref basic_json do not require complex case distinctions
 @ref basic_json always has a base class.
 By default, this class is used because it is empty and thus has no effect
 on the behavior of @ref basic_json.
+
+@note This class intentionally lives in namespace @ref nlohmann rather than
+      @ref nlohmann::detail. Every @ref basic_json specialization derives from
+      it (via @ref detail::json_base_class) unless a custom base class is
+      supplied, which makes its namespace an associated namespace of
+      @ref basic_json for the purpose of argument-dependent lookup (ADL). If
+      it lived in `nlohmann::detail`, that namespace - and with it the
+      library's internal `to_json`/`from_json` overloads - would leak into
+      ADL for any unqualified `to_json`/`from_json` call a user makes
+      involving a @ref basic_json argument, silently shadowing the user's own
+      overloads in some cases.
 */
 struct json_default_base {};
 
+namespace detail
+{
+
 template<class T>
 using json_base_class = typename std::conditional <
                         std::is_same<T, void>::value,
-                        json_default_base,
+                        ::nlohmann::json_default_base,
                         T
                         >::type;
 

single_include/nlohmann/json.hpp

@@ -6811,7 +6811,7 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <array> // array
 #include <cmath> // ldexp
 #include <cstddef> // size_t
-#include <cstdint> // uint8_t, uint16_t, uint32_t, uint64_t
+#include <cstdint> // uint8_t, uint16_t, uint32_t, uint64_t, uintmax_t
 #include <cstdio> // snprintf
 #include <cstring> // memcpy
 #include <iterator> // back_inserter
@@ -6993,8 +6993,18 @@ class iterator_input_adapter
   public:
     using char_type = typename std::iterator_traits<IteratorType>::value_type;
 
+    // Whether the lexer may reconstruct already-consumed input on demand (for
+    // diagnostics) instead of copying every scanned character eagerly. This is
+    // only sound for multi-pass, randomly-addressable byte input: the iterator
+    // must be random-access (so the consumed prefix can be revisited in O(1))
+    // and each element must map 1:1 to an input byte (wide inputs are wrapped
+    // 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
+        && sizeof(char_type) == 1;
+
     iterator_input_adapter(IteratorType first, IteratorType last)
-        : current(std::move(first)), end(std::move(last))
+        : begin(first), current(std::move(first)), end(std::move(last))
     {}
 
     typename char_traits<char_type>::int_type get_character()
@@ -7009,9 +7019,67 @@ class iterator_input_adapter
         return char_traits<char_type>::eof();
     }
 
-    // for general iterators, we cannot really do something better than falling back to processing the range one-by-one
+    // number of characters consumed from the input so far
+    std::size_t get_consumed_count() const
+    {
+        return static_cast<std::size_t>(std::distance(begin, current));
+    }
+
+    // append the already-consumed characters in the half-open range
+    // [first_index, last_index) to @a out; only valid when supports_seek
+    template<typename ContainerType>
+    void copy_consumed_range(std::size_t first_index, std::size_t last_index, ContainerType& out) const
+    {
+        const auto from = std::next(begin, static_cast<typename std::iterator_traits<IteratorType>::difference_type>(first_index));
+        const auto to = std::next(begin, static_cast<typename std::iterator_traits<IteratorType>::difference_type>(last_index));
+        out.insert(out.end(), from, to);
+    }
+
+    // Copy up to count * sizeof(T) bytes into dest, returning the number of
+    // bytes actually read. For contiguous iterators (e.g. pointers) this is a
+    // single std::memcpy; for general iterators we fall back to processing the
+    // range one-by-one.
     template<class T>
     std::size_t get_elements(T* dest, std::size_t count = 1)
+    {
+        return get_elements_impl(dest, count, std::integral_constant<bool, iterator_is_contiguous> {});
+    }
+
+  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)
+    static constexpr bool iterator_is_contiguous =
+#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
+        std::contiguous_iterator<IteratorType> ||
+#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);
+        const std::size_t available = static_cast<std::size_t>(std::distance(current, end)) * sizeof(char_type);
+        const std::size_t copied = (std::min)(wanted, available);
+        if (JSON_HEDLEY_LIKELY(copied != 0))
+        {
+            // the copy must stay within both buffers: the caller-provided
+            // destination holds `wanted` bytes and the remaining input range
+            // holds `available` bytes, and `copied` is the minimum of the two
+            JSON_ASSERT(copied <= wanted);    // does not overrun the destination
+            JSON_ASSERT(copied <= available); // does not read past the input end
+            // &*current yields the raw address for both raw pointers and
+            // non-pointer contiguous iterators (e.g. std::vector's iterator)
+            std::memcpy(dest, &*current, copied);
+            std::advance(current, static_cast<typename std::iterator_traits<IteratorType>::difference_type>(copied / sizeof(char_type)));
+        }
+        return copied;
+    }
+
+    // general fallback: copy the range one element at a time
+    template<class T>
+    std::size_t get_elements_impl(T* dest, std::size_t count, std::false_type /*contiguous*/)
     {
         auto* ptr = reinterpret_cast<char*>(dest);
         for (std::size_t read_index = 0; read_index < count * sizeof(T); ++read_index)
@@ -7029,7 +7097,7 @@ class iterator_input_adapter
         return count * sizeof(T);
     }
 
-  private:
+    IteratorType begin;
     IteratorType current;
     IteratorType end;
 
@@ -7510,6 +7578,28 @@ class lexer_base
         }
     }
 };
+
+// Detect whether an input adapter can reconstruct already-consumed input on
+// demand (see iterator_input_adapter::supports_seek). Adapters that do not
+// expose the flag - e.g. file, stream, wide-string, and user-defined adapters -
+// are treated as non-seekable streaming input, for which the lexer keeps
+// copying every scanned character eagerly. The value is read via tag dispatch
+// on is_detected so the flag is only referenced for adapters that provide it.
+template<typename InputAdapterType>
+using detect_supports_seek = decltype(InputAdapterType::supports_seek);
+
+template<typename InputAdapterType>
+constexpr bool input_adapter_supports_seek(std::true_type /*detected*/)
+{
+    return InputAdapterType::supports_seek;
+}
+
+template<typename InputAdapterType>
+constexpr bool input_adapter_supports_seek(std::false_type /*detected*/)
+{
+    return false;
+}
+
 /*!
 @brief lexical analysis
 
@@ -7525,6 +7615,12 @@ class lexer : public lexer_base<BasicJsonType>
     using char_type = typename InputAdapterType::char_type;
     using char_int_type = typename char_traits<char_type>::int_type;
 
+    /// whether the last read token can be reconstructed from the input adapter
+    /// on demand (in error paths) instead of being copied on every scanned
+    /// character; see input_adapter_supports_seek
+    static constexpr bool lazy_token_string =
+        input_adapter_supports_seek<InputAdapterType>(is_detected<detect_supports_seek, InputAdapterType> {});
+
   public:
     using token_type = typename lexer_base<BasicJsonType>::token_type;
 
@@ -8734,8 +8830,24 @@ scan_number_done:
     void reset() noexcept
     {
         token_buffer.clear();
-        token_string.clear();
         decimal_point_position = std::string::npos;
+
+        note_token_start(std::integral_constant<bool, lazy_token_string> {});
+    }
+
+    /// seekable adapter: remember where the current token starts so it can be
+    /// reconstructed from the input on error; current has already been
+    /// consumed, hence the -1
+    void note_token_start(std::true_type /*lazy*/) noexcept
+    {
+        token_string_start = ia.get_consumed_count() - 1;
+    }
+
+    /// streaming adapter: start copying the token eagerly, beginning with the
+    /// already-read first character
+    void note_token_start(std::false_type /*lazy*/) noexcept
+    {
+        token_string.clear();
         token_string.push_back(char_traits<char_type>::to_char_type(current));
     }
 
@@ -8764,10 +8876,9 @@ scan_number_done:
             current = ia.get_character();
         }
 
-        if (JSON_HEDLEY_LIKELY(current != char_traits<char_type>::eof()))
-        {
-            token_string.push_back(char_traits<char_type>::to_char_type(current));
-        }
+        // seekable adapters reconstruct the token lazily on error (see
+        // get_token_string), so the eager per-character copy is skipped
+        capture_char(std::integral_constant<bool, lazy_token_string> {});
 
         if (current == '\n')
         {
@@ -8778,6 +8889,18 @@ scan_number_done:
         return current;
     }
 
+    /// seekable adapter: nothing to capture, the token is rebuilt on error
+    void capture_char(std::true_type /*lazy*/) const noexcept {}
+
+    /// streaming adapter: copy the scanned character into token_string
+    void capture_char(std::false_type /*lazy*/)
+    {
+        if (JSON_HEDLEY_LIKELY(current != char_traits<char_type>::eof()))
+        {
+            token_string.push_back(char_traits<char_type>::to_char_type(current));
+        }
+    }
+
     /*!
     @brief unget current character (read it again on next get)
 
@@ -8805,6 +8928,15 @@ scan_number_done:
             --position.chars_read_current_line;
         }
 
+        uncapture_char(std::integral_constant<bool, lazy_token_string> {});
+    }
+
+    /// seekable adapter: nothing was captured, so nothing to undo
+    void uncapture_char(std::true_type /*lazy*/) const noexcept {}
+
+    /// streaming adapter: drop the character copied by the matching get()
+    void uncapture_char(std::false_type /*lazy*/)
+    {
         if (JSON_HEDLEY_LIKELY(current != char_traits<char_type>::eof()))
         {
             JSON_ASSERT(!token_string.empty());
@@ -8862,14 +8994,38 @@ scan_number_done:
         return position;
     }
 
+    /// seekable adapter: rebuild the last read token from the input on demand
+    const std::vector<char_type>& collect_token_chars(std::vector<char_type>& out, std::true_type /*lazy*/) const
+    {
+        // a pending unget of a real (non-EOF) character means that character
+        // was consumed from the input but is not part of the token; EOF is
+        // never consumed, so it must not be subtracted (mirrors unget())
+        const bool pending_real_unget = next_unget && current != char_traits<char_type>::eof();
+        const std::size_t stop = ia.get_consumed_count() - (pending_real_unget ? 1u : 0u);
+        if (JSON_HEDLEY_LIKELY(stop >= token_string_start))
+        {
+            ia.copy_consumed_range(token_string_start, stop, out);
+        }
+        return out;
+    }
+
+    /// streaming adapter: the token was copied eagerly while scanning
+    const std::vector<char_type>& collect_token_chars(std::vector<char_type>& /*out*/, std::false_type /*lazy*/) const
+    {
+        return token_string;
+    }
+
     /// return the last read token (for errors only).  Will never contain EOF
     /// (an arbitrary value that is not a valid char value, often -1), because
     /// 255 may legitimately occur.  May contain NUL, which should be escaped.
     std::string get_token_string() const
     {
+        std::vector<char_type> reconstructed;
+        const std::vector<char_type>& chars = collect_token_chars(reconstructed, std::integral_constant<bool, lazy_token_string> {});
+
         // escape control characters
         std::string result;
-        for (const auto c : token_string)
+        for (const auto c : chars)
         {
             if (static_cast<unsigned char>(c) <= '\x1F')
             {
@@ -9030,9 +9186,14 @@ scan_number_done:
     /// the start position of the current token
     position_t position {};
 
-    /// raw input token string (for error messages)
+    /// raw input token string for error messages; only populated for streaming
+    /// adapters (seekable adapters reconstruct it lazily via token_string_start)
     std::vector<char_type> token_string {};
 
+    /// start offset of the current token within the input, used to reconstruct
+    /// the last read token on error for seekable adapters (see collect_token_chars)
+    std::size_t token_string_start = 0;
+
     /// buffer for variable-length tokens (numbers, strings)
     string_t token_buffer {};
 
@@ -13073,18 +13234,7 @@ class binary_reader
                     const NumberType len,
                     string_t& result)
     {
-        bool success = true;
-        for (NumberType i = 0; i < len; i++)
-        {
-            get();
-            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(format, "string")))
-            {
-                success = false;
-                break;
-            }
-            result.push_back(static_cast<typename string_t::value_type>(current));
-        }
-        return success;
+        return get_bytes(format, len, "string", result);
     }
 
     /*!
@@ -13106,18 +13256,66 @@ class binary_reader
                     const NumberType len,
                     binary_t& result)
     {
-        bool success = true;
-        for (NumberType i = 0; i < len; i++)
+        return get_bytes(format, len, "binary", result);
+    }
+
+    /*!
+    @brief read @a len bytes from the input into a string or byte container
+
+    @tparam NumberType    the type of the length
+    @tparam ContainerType the destination container (string_t or binary_t)
+    @param[in] format   the current format (for diagnostics)
+    @param[in] len      number of bytes to read
+    @param[in] context  further context information (for diagnostics)
+    @param[out] result  container the bytes are appended to
+
+    @return whether reading completed
+
+    @note We cannot reserve @a len bytes for the result up front, because
+          @a len may be far larger than the actual input. Instead we read in
+          bounded chunks, so the peak allocation is capped regardless of the
+          claimed length while the per-byte loop is replaced by block copies
+          (a std::memcpy for contiguous inputs). @ref unexpect_eof() still
+          detects a premature end of input.
+    */
+    template<typename NumberType, typename ContainerType>
+    bool get_bytes(const input_format_t format,
+                   NumberType len,
+                   const char* context,
+                   ContainerType& result)
+    {
+        // upper bound on the number of bytes read (and allocated) per chunk
+        constexpr std::size_t chunk_size = 4096;
+
+        while (len > 0)
         {
-            get();
-            if (JSON_HEDLEY_UNLIKELY(!unexpect_eof(format, "binary")))
+            // number of bytes to read this iteration: min(chunk_size, len),
+            // computed without truncating chunk_size to a narrow NumberType
+            const std::size_t wanted = (static_cast<std::uintmax_t>(len) < static_cast<std::uintmax_t>(chunk_size))
+                                       ? static_cast<std::size_t>(len)
+                                       : chunk_size;
+            const std::size_t old_size = result.size();
+            result.resize(old_size + wanted);
+            // resize() is required to make size() exactly old_size + wanted;
+            // that is the room get_elements() is allowed to write into
+            JSON_ASSERT(result.size() == old_size + wanted);
+            const std::size_t bytes_read = ia.get_elements(&result[old_size], wanted);
+            chars_read += bytes_read;
+            if (JSON_HEDLEY_UNLIKELY(bytes_read < wanted))
             {
-                success = false;
-                break;
+                // premature end of input: shrink to what was actually read and
+                // report the failure at the first missing byte (same position
+                // accounting as get_to() for partial number reads)
+                result.resize(old_size + bytes_read);
+                ++chars_read;
+                current = char_traits<char_type>::eof();
+                return unexpect_eof(format, context);
             }
-            result.push_back(static_cast<typename binary_t::value_type>(current));
+            // a full chunk was read; get_elements() never returns more than requested
+            JSON_ASSERT(bytes_read == wanted);
+            len = static_cast<NumberType>(len - static_cast<NumberType>(wanted));
         }
-        return success;
+        return true;
     }
 
     /*!
@@ -14896,8 +15094,6 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
-namespace detail
-{
 
 /*!
 @brief Default base class of the @ref basic_json class.
@@ -14908,13 +15104,27 @@ of @ref basic_json do not require complex case distinctions
 @ref basic_json always has a base class.
 By default, this class is used because it is empty and thus has no effect
 on the behavior of @ref basic_json.
+
+@note This class intentionally lives in namespace @ref nlohmann rather than
+      @ref nlohmann::detail. Every @ref basic_json specialization derives from
+      it (via @ref detail::json_base_class) unless a custom base class is
+      supplied, which makes its namespace an associated namespace of
+      @ref basic_json for the purpose of argument-dependent lookup (ADL). If
+      it lived in `nlohmann::detail`, that namespace - and with it the
+      library's internal `to_json`/`from_json` overloads - would leak into
+      ADL for any unqualified `to_json`/`from_json` call a user makes
+      involving a @ref basic_json argument, silently shadowing the user's own
+      overloads in some cases.
 */
 struct json_default_base {};
 
+namespace detail
+{
+
 template<class T>
 using json_base_class = typename std::conditional <
                         std::is_same<T, void>::value,
-                        json_default_base,
+                        ::nlohmann::json_default_base,
                         T
                         >::type;
 

tests/src/unit-algorithms.cpp

@@ -229,7 +229,10 @@ TEST_CASE("algorithms")
         {
             json j = {13, 29, 3, {{"one", 1}, {"two", 2}}, true, false, {1, 2, 3}, "foo", "baz", nullptr};
             std::partial_sort(j.begin(), j.begin() + 4, j.end());
-            CHECK(j == json({nullptr, false, true, 3, {{"one", 1}, {"two", 2}}, 29, {1, 2, 3}, "foo", "baz", 13}));
+            // only the first four elements are expected to be sorted, the rest are
+            // unspecified by the standard
+            const json expected({nullptr, false, true, 3});
+            CHECK(std::equal(j.begin(), j.begin() + 4, begin(expected)));
         }
     }
 

tests/src/unit-cbor.cpp

@@ -2778,3 +2778,43 @@ TEST_CASE("Tagged values")
         CHECK(!jb["binary"].get_binary().has_subtype());
     }
 }
+
+TEST_CASE("CBOR large strings and binaries (chunked reader)")
+{
+    // The binary reader reads strings and byte arrays in bounded chunks; make
+    // sure roundtripping is correct for lengths around and beyond the internal
+    // chunk size (4096 bytes), for both vector (iterator) and pointer inputs.
+    for (const std::size_t len :
+            {
+                std::size_t{0}, std::size_t{1}, std::size_t{4095}, std::size_t{4096},
+                std::size_t{4097}, std::size_t{8192}, std::size_t{100000}
+            })
+    {
+        CAPTURE(len);
+
+        // text string
+        const json j_string = std::string(len, 'x');
+        const std::vector<std::uint8_t> v_string = json::to_cbor(j_string);
+        CHECK(json::from_cbor(v_string) == j_string);
+        // pointer input exercises the std::memcpy fast path
+        CHECK(json::from_cbor(reinterpret_cast<const char*>(v_string.data()),
+                              reinterpret_cast<const char*>(v_string.data()) + v_string.size()) == j_string);
+
+        // byte string
+        const json j_binary = json::binary(std::vector<std::uint8_t>(len, 0xCD));
+        const std::vector<std::uint8_t> v_binary = json::to_cbor(j_binary);
+        CHECK(json::from_cbor(v_binary) == j_binary);
+        CHECK(json::from_cbor(reinterpret_cast<const char*>(v_binary.data()),
+                              reinterpret_cast<const char*>(v_binary.data()) + v_binary.size()) == j_binary);
+
+        // a truncated payload must still be reported as an error, never crash
+        // or loop, regardless of the (large) announced length
+        if (len > 16)
+        {
+            std::vector<std::uint8_t> truncated = v_string;
+            truncated.resize(truncated.size() - 8);
+            json _;
+            CHECK_THROWS_AS(_ = json::from_cbor(truncated), json::parse_error);
+        }
+    }
+}

tests/src/unit-class_parser.cpp

@@ -16,6 +16,12 @@ using nlohmann::json;
 #endif
 
 #include <valarray>
+#include <algorithm>
+#include <list>
+#include <sstream>
+#include <string>
+#include <utility>
+#include <vector>
 
 namespace
 {
@@ -1725,3 +1731,110 @@ TEST_CASE("parser class")
         CHECK_THROWS_WITH_AS(_ = json::parse("/*", nullptr, true, true), "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid comment; missing closing '*/'; last read: '/*<U+0000>'", json::parse_error);
     }
 }
+
+// this test relies on parse errors being thrown, so it is skipped when
+// exceptions are disabled (json::parse aborts instead of throwing there)
+#if !defined(JSON_NOEXCEPTION)
+namespace
+{
+// Return the exception message from parsing @a input, or a "<no error ...>"
+// sentinel if the parse unexpectedly succeeds. json::parse is nodiscard, so the
+// result is consumed (via size()) to keep -Wunused-result / -Werror happy.
+template<typename InputType>
+std::string parse_error_message(InputType&& input)
+{
+    try
+    {
+        const json j = json::parse(std::forward<InputType>(input));
+        return "<no error, size " + std::to_string(j.size()) + ">";
+    }
+    catch (const json::exception& e)
+    {
+        return e.what();
+    }
+}
+
+template<typename IteratorType>
+std::string parse_error_message_range(IteratorType first, IteratorType last)
+{
+    try
+    {
+        const json j = json::parse(first, last);
+        return "<no error, size " + std::to_string(j.size()) + ">";
+    }
+    catch (const json::exception& e)
+    {
+        return e.what();
+    }
+}
+} // namespace
+
+TEST_CASE("last-read diagnostics are identical across input adapters")
+{
+    // The lexer reconstructs the "last read" token lazily for seekable adapters
+    // (contiguous byte input) and copies it eagerly for streaming adapters.
+    // Both strategies must yield byte-for-byte identical error messages.
+
+    // a selection of malformed inputs that exercise different token kinds,
+    // whitespace/structural accumulation, number overflow, and control-char
+    // escaping in the reconstructed "last read" token
+    const std::vector<std::string> inputs =
+    {
+        "[1,2,x]",
+        "  \n  @",
+        "{\"a\": }",
+        "1.18973e+4932",
+        "\"\t\"",
+        "tru",
+        "[1 2]",
+        "\xEF\xBB\xBF   nul",
+    };
+
+    for (const auto& s : inputs)
+    {
+        CAPTURE(s);
+
+        // reference: contiguous std::string -> seekable (lazy) path
+        const std::string reference = parse_error_message(s);
+        // every input is malformed, so parsing must fail (error messages start
+        // with '['; the success sentinel returned above starts with '<')
+        CHECK(reference.front() == '[');
+
+        // const char* -> also seekable
+        CHECK(parse_error_message(s.c_str()) == reference);
+
+        // std::vector<char> iterators -> seekable (random-access)
+        {
+            const std::vector<char> v(s.begin(), s.end());
+            CHECK(parse_error_message_range(v.begin(), v.end()) == reference);
+        }
+
+        // std::list iterators -> non-seekable (bidirectional) eager path
+        {
+            const std::list<char> l(s.begin(), s.end());
+            CHECK(parse_error_message_range(l.begin(), l.end()) == reference);
+        }
+
+        // std::istringstream -> non-seekable streaming eager path
+        {
+            std::istringstream ss(s);
+            CHECK(parse_error_message(ss) == reference);
+        }
+
+        // wide strings -> wide_string_input_adapter eager path; only comparable
+        // for ASCII input, as non-ASCII bytes are transcoded to different UTF-8
+        const bool is_ascii = std::all_of(s.begin(), s.end(), [](char c)
+        {
+            return static_cast<unsigned char>(c) < 0x80;
+        });
+        if (is_ascii)
+        {
+            const std::u16string w16(s.begin(), s.end());
+            CHECK(parse_error_message(w16) == reference);
+
+            const std::u32string w32(s.begin(), s.end());
+            CHECK(parse_error_message(w32) == reference);
+        }
+    }
+}
+#endif // !defined(JSON_NOEXCEPTION)

tests/src/unit-regression2.cpp

@@ -1358,4 +1358,100 @@ TEST_CASE("regression test #5122 - nlohmann::ordered_map move-assignment transfe
     CHECK(src.begin()->first == "after-move");
 }
 
+// Stand-in for a third-party library (e.g., Eigen as of 3.4, which added
+// STL-compatible begin()/end() to its vector types), living in its own
+// namespace with its own to_json overload for its vector type.
+namespace issue_4320_eigen
+{
+// "array-compatible" from the library's point of view (it has begin()/end()),
+// but for which this (fake) third-party namespace provides its own to_json.
+struct vector3
+{
+    double v[3]; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-use-default-member-init,modernize-use-default-member-init)
+    vector3(double x, double y, double z) : v{x, y, z} {} // NOLINT(hicpp-member-init,cppcoreguidelines-pro-type-member-init)
+    double x() const
+    {
+        return v[0];
+    }
+    double y() const
+    {
+        return v[1];
+    }
+    double z() const
+    {
+        return v[2];
+    }
+    double* begin()
+    {
+        return v;
+    }
+    double* end()
+    {
+        return v + 3;
+    }
+    const double* begin() const
+    {
+        return v;
+    }
+    const double* end() const
+    {
+        return v + 3;
+    }
+};
+
+inline void to_json(json& j, const vector3& v) // NOLINT(misc-use-internal-linkage)
+{
+    j = {{"x", v.x()}, {"y", v.y()}, {"z", v.z()}};
+}
+} // namespace issue_4320_eigen
+
+// The user's own namespace, using the (fake) Eigen type as an implementation
+// detail behind a payload type that has nothing to do with vectors/arrays.
+namespace issue_4320
+{
+// Publicly derives from issue_4320_eigen::vector3 but does *not* define its
+// own to_json - it is only ever used as a temporary to reach the base
+// class's to_json via ADL.
+struct vector3_wrapper : issue_4320_eigen::vector3
+{
+    using issue_4320_eigen::vector3::vector3;
+};
+
+struct payload
+{
+    double x, y, z;
+};
+
+inline vector3_wrapper to_eigen(const payload& p) // NOLINT(misc-use-internal-linkage)
+{
+    return {p.x, p.y, p.z};
+}
+
+inline void to_json(json& j, const payload& p) // NOLINT(misc-use-internal-linkage)
+{
+    // Unqualified call, passing a *derived* vector3_wrapper: relies on ADL
+    // finding issue_4320_eigen::to_json(json&, const vector3&) through the
+    // vector3 base class, via a derived-to-base conversion. Must NOT resolve
+    // to the library's own generic array-compatible to_json (an exact-match
+    // template for vector3_wrapper, since it also has begin()/end()), which
+    // would serialize this as [x, y, z] instead of {"x":x, "y":y, "z":z}.
+    to_json(j, to_eigen(p));
+}
+} // namespace issue_4320
+
+TEST_CASE("issue #4320 - custom base class must not leak nlohmann::detail into ADL")
+{
+    // Before the fix, basic_json unconditionally derived from a type living in
+    // nlohmann::detail (json_default_base), which made nlohmann::detail an
+    // associated namespace of every basic_json for ADL purposes. That leaked
+    // the library's internal generic-array to_json overload into unqualified
+    // to_json() calls made from user code, silently bypassing user-defined
+    // to_json overloads reached via a derived-to-base conversion.
+    const issue_4320::payload p{1.0, 2.0, 3.0};
+
+    json j;
+    to_json(j, p);
+    CHECK(j == json({{"x", 1.0}, {"y", 2.0}, {"z", 3.0}}));
+}
+
 DOCTEST_CLANG_SUPPRESS_WARNING_POP