Fix destroy() for ObjectTypes without reverse iteration

The non-recursive destroy walk from #5762 picked an object's last child
via object_t::rbegin() and std::prev(end()). Neither is available for
every ObjectType: no_key_compare_map in unit-custom-object-type.cpp has
no rbegin(), so develop no longer compiles that test, and hash maps such
as std::unordered_map only have forward iterators.

The walk can take an object's children in any order, as long as it
finds the same child again while the object is not modified in between.
So objects with bidirectional iterators keep using their last child
(O(1) to remove from vector-based maps like ordered_map), and objects
with forward-only iterators use begin() instead. No reverse iteration
or rbegin() is needed any more, and the walk stays allocation-free.

Adds a forward-only ObjectType to the tests, destroyed both with mixed
nesting and 100000 levels deep.

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

docs/mkdocs/docs/features/binary_formats/cbor.md

@@ -131,7 +131,6 @@ The library maps CBOR types to JSON value types as follows:
 | Byte string            | binary          | 0x59       |
 | Byte string            | binary          | 0x5A       |
 | Byte string            | binary          | 0x5B       |
-| Byte string            | binary          | 0x5F       |
 | UTF-8 string           | string          | 0x60..0x77 |
 | UTF-8 string           | string          | 0x78       |
 | UTF-8 string           | string          | 0x79       |
@@ -157,9 +156,6 @@ The library maps CBOR types to JSON value types as follows:
 | Single-Precision Float | number_float    | 0xFA       |
 | Double-Precision Float | number_float    | 0xFB       |
 
-Indefinite-length UTF-8 strings (0x7F) and byte strings (0x5F) are supported. Each chunk must be a definite-length
-string of the same major type, as required by [RFC 8949, Section 3.2.3](https://www.rfc-editor.org/rfc/rfc8949.html#section-3.2.3).
-
 !!! warning "Incomplete mapping"
 
     The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:

include/nlohmann/detail/input/binary_reader.hpp

@@ -1072,20 +1072,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reports a nested indefinite-length CBOR string or byte array
-    @param[in] type_name  name of the rejected string type
-    @param[in] context  parsing context for the error message
-    @return whether the SAX consumer accepts the parse error
-    */
-    bool cbor_indefinite_string_error(const char* type_name, const char* context)
-    {
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(concat("indefinite-length ", type_name,
-                                        " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", last_token), context), nullptr));
-    }
-
     /*!
     @brief reads a definite-length CBOR string
 
@@ -1095,13 +1081,12 @@ class binary_reader
     into the same string.
 
     @param[out] result  string the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether string creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_string_chunk(string_t& result, const bool inside_indefinite)
+    bool get_cbor_string_chunk(string_t& result)
     {
         switch (current)
         {
@@ -1162,7 +1147,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", last_token), "string"), nullptr));
+                                        exception_message(concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -1180,9 +1165,13 @@ class binary_reader
     */
     bool get_cbor_string(string_t& result, const char* context = "string")
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;
 
         while (true)
         {
@@ -1193,28 +1182,29 @@ class binary_reader
 
             if (current == 0x7F) // UTF-8 string (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("string", "string");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
             {
-                return check_string_utf8(result, context);
+                if (--open == 0)
+                {
+                    return check_string_utf8(result, context);
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return check_string_utf8(result, context);
             }
@@ -1306,13 +1296,12 @@ class binary_reader
     read into the same byte array.
 
     @param[out] result  byte array the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether byte array creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_binary_chunk(binary_t& result, const bool inside_indefinite)
+    bool get_cbor_binary_chunk(binary_t& result)
     {
         switch (current)
         {
@@ -1377,7 +1366,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", last_token), "binary"), nullptr));
+                                        exception_message(concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
             }
         }
     }
@@ -1395,9 +1384,9 @@ class binary_reader
     */
     bool get_cbor_binary(binary_t& result)
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;
 
         while (true)
         {
@@ -1408,28 +1397,29 @@ class binary_reader
 
             if (current == 0x5F) // Binary data (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("binary array", "binary");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
             {
-                return true;
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return true;
             }

include/nlohmann/detail/input/lexer.hpp

@@ -2021,6 +2021,39 @@ scan_number_done:
         // read the next character and ignore whitespace
         skip_whitespace();
 
+        return scan_after_whitespace();
+    }
+
+    /*!
+    @brief scan the next token when the caller expects a separator (':' or
+           ',') most of the time
+
+    After an object key the next token is almost always ':', after a value
+    inside an object or array almost always ','. Testing for that character
+    first is a compare and a well-predicted branch, where the switch in
+    scan_after_whitespace() is an indirect jump through a table. Anything else
+    goes through the switch, so the result is the same as scan()'s.
+
+    May only be called after scan() has run once (the BOM check is skipped).
+    */
+    token_type scan_expecting(token_type expected_type)
+    {
+        JSON_ASSERT(expected_type == token_type::name_separator || expected_type == token_type::value_separator);
+        JSON_ASSERT(position.chars_read_total > 0);
+        const char_int_type expected_char = static_cast<unsigned char>((expected_type == token_type::name_separator) ? ':' : ',');
+        skip_whitespace();
+        if (JSON_HEDLEY_LIKELY(current == expected_char))
+        {
+            return expected_type;
+        }
+        return scan_after_whitespace();
+    }
+
+  private:
+    /// the part of scan() after the leading whitespace: skip comments and
+    /// scan the token that starts with current
+    token_type scan_after_whitespace()
+    {
         // ignore comments
         while (ignore_comments && current == '/')
         {
@@ -2100,7 +2133,6 @@ scan_number_done:
         }
     }
 
-  private:
     /// input adapter
     InputAdapterType ia;
 

include/nlohmann/detail/input/parser.hpp

@@ -260,7 +260,7 @@ class parser
                         }
 
                         // parse separator (:)
-                        if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                        if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                         {
                             return sax->parse_error(m_lexer.get_position(),
                                                     m_lexer.get_token_string(),
@@ -423,7 +423,7 @@ class parser
             {
                 // comma -> next value
                 // or end of array (ignore_trailing_commas = true)
-                if (get_token() == token_type::value_separator)
+                if (get_token_expecting(token_type::value_separator))
                 {
                     // parse a new value
                     get_token();
@@ -463,7 +463,7 @@ class parser
 
             // comma -> next value
             // or end of object (ignore_trailing_commas = true)
-            if (get_token() == token_type::value_separator)
+            if (get_token_expecting(token_type::value_separator))
             {
                 get_token();
 
@@ -484,7 +484,7 @@ class parser
                     }
 
                     // parse separator (:)
-                    if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                    if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                     {
                         return sax->parse_error(m_lexer.get_position(),
                                                 m_lexer.get_token_string(),
@@ -528,6 +528,13 @@ class parser
         return last_token = m_lexer.scan();
     }
 
+    /// get next token from lexer; true if it is the separator @a expected_type
+    /// (name_separator or value_separator), which it usually is
+    bool get_token_expecting(token_type expected_type)
+    {
+        return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
+    }
+
     std::string exception_message(const token_type expected, const std::string& context)
     {
         std::string error_msg = "syntax error ";

include/nlohmann/json.hpp

@@ -638,21 +638,49 @@ private:
             }
         }
 
-        static basic_json& last_child(basic_json& v)
+        // The walk in destroy_container() may take the children of a
+        // container in any order, as long as it picks the same child again
+        // while that container is not modified in between. Arrays and
+        // objects with bidirectional iterators (std::map, ordered_map, ...)
+        // use their last child, which a vector-based container can remove
+        // in O(1). ObjectType only needs forward iterators, though (e.g.
+        // std::unordered_map), so other objects use their first child.
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o, std::bidirectional_iterator_tag /*unused*/)
+        {
+            return std::prev(o.end());
+        }
+
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o, std::forward_iterator_tag /*unused*/)
+        {
+            return o.begin();
+        }
+
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o)
+        {
+            JSON_ASSERT(!o.empty());
+            return walk_child_it(o, typename std::iterator_traits<typename ObjectType_::iterator>::iterator_category());
+        }
+
+        // the child of a non-empty array/object v that the walk in
+        // destroy_container() continues with (see walk_child_it() above)
+        static basic_json& walk_child(basic_json& v)
         {
             if (v.m_data.m_type == value_t::array)
             {
                 return v.m_data.m_value.array->back();
             }
             JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
+            return walk_child_it(*v.m_data.m_value.object)->second;
         }
 
-        // removes the last child of a non-empty array/object v; this never
+        // removes walk_child(v) from a non-empty array/object v; this never
         // allocates, and since it is only ever called when that child is a
         // scalar or an already-empty array/object, destroying it never
         // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
+        static void pop_walk_child(basic_json& v)
         {
             if (v.m_data.m_type == value_t::array)
             {
@@ -661,9 +689,7 @@ private:
             else
             {
                 JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
+                v.m_data.m_value.object->erase(walk_child_it(*v.m_data.m_value.object));
             }
         }
 
@@ -734,14 +760,17 @@ public:
             // bad_alloc, which would escape this noexcept destructor and
             // terminate the program (#5135).
             //
-            // Instead, walk down the "last child" chain, reversing links
-            // as we go: cur is the container currently being emptied,
-            // and prev is its parent (value_t::null when there is none).
-            // Each parent's last child slot doubles as storage for that
-            // parent's own parent link while we are below it, so no
-            // extra memory is needed. We only ever remove a child once
-            // it is a scalar or an empty array/object, which neither
-            // allocates nor recurses more than one level deep.
+            // Instead, walk down a chain of children (always the one
+            // walk_child() picks), reversing links as we go: cur is the
+            // container currently being emptied, and prev is its parent
+            // (value_t::null when there is none). Each parent's
+            // walk_child() slot doubles as storage for that parent's own
+            // parent link while we are below it, so no extra memory is
+            // needed; the parent is not modified meanwhile, so
+            // walk_child() finds that same slot again on the way up. We
+            // only ever remove a child once it is a scalar or an empty
+            // array/object, which neither allocates nor recurses more
+            // than one level deep.
             //
             // This json_value is not itself a basic_json, so the
             // top-level container is first moved into a local stand-in
@@ -767,11 +796,11 @@ public:
                     }
 
                     // ascend: detach the grandparent link from prev's
-                    // last slot, drop that (now null) slot, free cur
+                    // walk_child() slot, drop that (now null) slot, free cur
                     // (it is empty), then move up one level
                     basic_json gp;
-                    take(gp, last_child(prev));
-                    pop_last_child(prev);
+                    take(gp, walk_child(prev));
+                    pop_walk_child(prev);
 
                     free_container(cur);
 
@@ -780,21 +809,21 @@ public:
                     continue;
                 }
 
-                basic_json& cur_last_ref = last_child(cur);
+                basic_json& cur_child_ref = walk_child(cur);
 
-                if (has_no_children(cur_last_ref))
+                if (has_no_children(cur_child_ref))
                 {
                     // scalar, or already-empty array/object
-                    pop_last_child(cur);
+                    pop_walk_child(cur);
                     continue;
                 }
 
-                // descend into the non-empty last child, reversing the
+                // descend into the non-empty child, reversing the
                 // link: its slot takes over prev, and the child becomes
                 // the new cur
                 basic_json tmp;
-                take(tmp, cur_last_ref);
-                take(cur_last_ref, prev);
+                take(tmp, cur_child_ref);
+                take(cur_child_ref, prev);
                 take(prev, cur);
                 take(cur, tmp);
             }

single_include/nlohmann/json.hpp

@@ -12313,6 +12313,39 @@ scan_number_done:
         // read the next character and ignore whitespace
         skip_whitespace();
 
+        return scan_after_whitespace();
+    }
+
+    /*!
+    @brief scan the next token when the caller expects a separator (':' or
+           ',') most of the time
+
+    After an object key the next token is almost always ':', after a value
+    inside an object or array almost always ','. Testing for that character
+    first is a compare and a well-predicted branch, where the switch in
+    scan_after_whitespace() is an indirect jump through a table. Anything else
+    goes through the switch, so the result is the same as scan()'s.
+
+    May only be called after scan() has run once (the BOM check is skipped).
+    */
+    token_type scan_expecting(token_type expected_type)
+    {
+        JSON_ASSERT(expected_type == token_type::name_separator || expected_type == token_type::value_separator);
+        JSON_ASSERT(position.chars_read_total > 0);
+        const char_int_type expected_char = static_cast<unsigned char>((expected_type == token_type::name_separator) ? ':' : ',');
+        skip_whitespace();
+        if (JSON_HEDLEY_LIKELY(current == expected_char))
+        {
+            return expected_type;
+        }
+        return scan_after_whitespace();
+    }
+
+  private:
+    /// the part of scan() after the leading whitespace: skip comments and
+    /// scan the token that starts with current
+    token_type scan_after_whitespace()
+    {
         // ignore comments
         while (ignore_comments && current == '/')
         {
@@ -12392,7 +12425,6 @@ scan_number_done:
         }
     }
 
-  private:
     /// input adapter
     InputAdapterType ia;
 
@@ -14789,20 +14821,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reports a nested indefinite-length CBOR string or byte array
-    @param[in] type_name  name of the rejected string type
-    @param[in] context  parsing context for the error message
-    @return whether the SAX consumer accepts the parse error
-    */
-    bool cbor_indefinite_string_error(const char* type_name, const char* context)
-    {
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(concat("indefinite-length ", type_name,
-                                        " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", last_token), context), nullptr));
-    }
-
     /*!
     @brief reads a definite-length CBOR string
 
@@ -14812,13 +14830,12 @@ class binary_reader
     into the same string.
 
     @param[out] result  string the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether string creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_string_chunk(string_t& result, const bool inside_indefinite)
+    bool get_cbor_string_chunk(string_t& result)
     {
         switch (current)
         {
@@ -14879,7 +14896,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", last_token), "string"), nullptr));
+                                        exception_message(concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -14897,9 +14914,13 @@ class binary_reader
     */
     bool get_cbor_string(string_t& result, const char* context = "string")
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;
 
         while (true)
         {
@@ -14910,28 +14931,29 @@ class binary_reader
 
             if (current == 0x7F) // UTF-8 string (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("string", "string");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
             {
-                return check_string_utf8(result, context);
+                if (--open == 0)
+                {
+                    return check_string_utf8(result, context);
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return check_string_utf8(result, context);
             }
@@ -15023,13 +15045,12 @@ class binary_reader
     read into the same byte array.
 
     @param[out] result  byte array the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether byte array creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_binary_chunk(binary_t& result, const bool inside_indefinite)
+    bool get_cbor_binary_chunk(binary_t& result)
     {
         switch (current)
         {
@@ -15094,7 +15115,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", last_token), "binary"), nullptr));
+                                        exception_message(concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
             }
         }
     }
@@ -15112,9 +15133,9 @@ class binary_reader
     */
     bool get_cbor_binary(binary_t& result)
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;
 
         while (true)
         {
@@ -15125,28 +15146,29 @@ class binary_reader
 
             if (current == 0x5F) // Binary data (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("binary array", "binary");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
             {
-                return true;
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return true;
             }
@@ -18432,7 +18454,7 @@ class parser
                         }
 
                         // parse separator (:)
-                        if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                        if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                         {
                             return sax->parse_error(m_lexer.get_position(),
                                                     m_lexer.get_token_string(),
@@ -18595,7 +18617,7 @@ class parser
             {
                 // comma -> next value
                 // or end of array (ignore_trailing_commas = true)
-                if (get_token() == token_type::value_separator)
+                if (get_token_expecting(token_type::value_separator))
                 {
                     // parse a new value
                     get_token();
@@ -18635,7 +18657,7 @@ class parser
 
             // comma -> next value
             // or end of object (ignore_trailing_commas = true)
-            if (get_token() == token_type::value_separator)
+            if (get_token_expecting(token_type::value_separator))
             {
                 get_token();
 
@@ -18656,7 +18678,7 @@ class parser
                     }
 
                     // parse separator (:)
-                    if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                    if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                     {
                         return sax->parse_error(m_lexer.get_position(),
                                                 m_lexer.get_token_string(),
@@ -18700,6 +18722,13 @@ class parser
         return last_token = m_lexer.scan();
     }
 
+    /// get next token from lexer; true if it is the separator @a expected_type
+    /// (name_separator or value_separator), which it usually is
+    bool get_token_expecting(token_type expected_type)
+    {
+        return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
+    }
+
     std::string exception_message(const token_type expected, const std::string& context)
     {
         std::string error_msg = "syntax error ";
@@ -27848,21 +27877,49 @@ private:
             }
         }
 
-        static basic_json& last_child(basic_json& v)
+        // The walk in destroy_container() may take the children of a
+        // container in any order, as long as it picks the same child again
+        // while that container is not modified in between. Arrays and
+        // objects with bidirectional iterators (std::map, ordered_map, ...)
+        // use their last child, which a vector-based container can remove
+        // in O(1). ObjectType only needs forward iterators, though (e.g.
+        // std::unordered_map), so other objects use their first child.
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o, std::bidirectional_iterator_tag /*unused*/)
+        {
+            return std::prev(o.end());
+        }
+
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o, std::forward_iterator_tag /*unused*/)
+        {
+            return o.begin();
+        }
+
+        template<typename ObjectType_>
+        static typename ObjectType_::iterator walk_child_it(ObjectType_& o)
+        {
+            JSON_ASSERT(!o.empty());
+            return walk_child_it(o, typename std::iterator_traits<typename ObjectType_::iterator>::iterator_category());
+        }
+
+        // the child of a non-empty array/object v that the walk in
+        // destroy_container() continues with (see walk_child_it() above)
+        static basic_json& walk_child(basic_json& v)
         {
             if (v.m_data.m_type == value_t::array)
             {
                 return v.m_data.m_value.array->back();
             }
             JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
+            return walk_child_it(*v.m_data.m_value.object)->second;
         }
 
-        // removes the last child of a non-empty array/object v; this never
+        // removes walk_child(v) from a non-empty array/object v; this never
         // allocates, and since it is only ever called when that child is a
         // scalar or an already-empty array/object, destroying it never
         // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
+        static void pop_walk_child(basic_json& v)
         {
             if (v.m_data.m_type == value_t::array)
             {
@@ -27871,9 +27928,7 @@ private:
             else
             {
                 JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
+                v.m_data.m_value.object->erase(walk_child_it(*v.m_data.m_value.object));
             }
         }
 
@@ -27944,14 +27999,17 @@ public:
             // bad_alloc, which would escape this noexcept destructor and
             // terminate the program (#5135).
             //
-            // Instead, walk down the "last child" chain, reversing links
-            // as we go: cur is the container currently being emptied,
-            // and prev is its parent (value_t::null when there is none).
-            // Each parent's last child slot doubles as storage for that
-            // parent's own parent link while we are below it, so no
-            // extra memory is needed. We only ever remove a child once
-            // it is a scalar or an empty array/object, which neither
-            // allocates nor recurses more than one level deep.
+            // Instead, walk down a chain of children (always the one
+            // walk_child() picks), reversing links as we go: cur is the
+            // container currently being emptied, and prev is its parent
+            // (value_t::null when there is none). Each parent's
+            // walk_child() slot doubles as storage for that parent's own
+            // parent link while we are below it, so no extra memory is
+            // needed; the parent is not modified meanwhile, so
+            // walk_child() finds that same slot again on the way up. We
+            // only ever remove a child once it is a scalar or an empty
+            // array/object, which neither allocates nor recurses more
+            // than one level deep.
             //
             // This json_value is not itself a basic_json, so the
             // top-level container is first moved into a local stand-in
@@ -27977,11 +28035,11 @@ public:
                     }
 
                     // ascend: detach the grandparent link from prev's
-                    // last slot, drop that (now null) slot, free cur
+                    // walk_child() slot, drop that (now null) slot, free cur
                     // (it is empty), then move up one level
                     basic_json gp;
-                    take(gp, last_child(prev));
-                    pop_last_child(prev);
+                    take(gp, walk_child(prev));
+                    pop_walk_child(prev);
 
                     free_container(cur);
 
@@ -27990,21 +28048,21 @@ public:
                     continue;
                 }
 
-                basic_json& cur_last_ref = last_child(cur);
+                basic_json& cur_child_ref = walk_child(cur);
 
-                if (has_no_children(cur_last_ref))
+                if (has_no_children(cur_child_ref))
                 {
                     // scalar, or already-empty array/object
-                    pop_last_child(cur);
+                    pop_walk_child(cur);
                     continue;
                 }
 
-                // descend into the non-empty last child, reversing the
+                // descend into the non-empty child, reversing the
                 // link: its slot takes over prev, and the child becomes
                 // the new cur
                 basic_json tmp;
-                take(tmp, cur_last_ref);
-                take(cur_last_ref, prev);
+                take(tmp, cur_child_ref);
+                take(cur_child_ref, prev);
                 take(prev, cur);
                 take(cur, tmp);
             }

tests/src/unit-cbor.cpp

@@ -1699,7 +1699,7 @@ TEST_CASE("CBOR")
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xBF, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x41})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
 
             CHECK(json::from_cbor(std::vector<uint8_t>({0x18}), true, false).is_discarded());
@@ -2305,21 +2305,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
 {
     // Reading an indefinite-length string or byte array used to call itself
     // once per chunk, so a payload of repeated 0x7F (or 0x5F) bytes exhausted
-    // the call stack before any of the input was rejected. Nested indefinite
-    // chunks are now rejected at the second byte, without recursing.
+    // the call stack before any of the input was rejected. The open levels are
+    // counted now, and the levels below prove the reader still reads the same
+    // values and reports the same errors at the same byte offsets.
     json _;
 
-    SECTION("nested levels are rejected, not crashed on")
+    SECTION("many open levels are reported, not crashed on")
     {
         const std::vector<uint8_t> input(200000, 0x7F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
         CHECK(json::from_cbor(input, true, false).is_discarded());
     }
 
-    SECTION("nested levels are rejected, not crashed on (binary)")
+    SECTION("many open levels are reported, not crashed on (binary)")
     {
         const std::vector<uint8_t> input(200000, 0x5F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
         CHECK(json::from_cbor(input, true, false).is_discarded());
     }
 
@@ -2327,22 +2328,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
     {
         CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0xFF})) == json(""));
         CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 0x61, 0xFF})) == json("a"));
-        // empty and nonempty definite-length chunks concatenate in order
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 'a', 0x60, 0x61, 'b', 0x61, 'c', 0xFF})) == json("abc"));
+        // nested indefinite-length strings are concatenated across levels
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0x61, 0x62, 0xFF})) == json("ab"));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x7F, 0x61, 0x7A, 0xFF, 0xFF, 0xFF})) == json("z"));
         CHECK(json::from_cbor(std::vector<uint8_t>({0xA1, 0x7F, 0x61, 0x61, 0xFF, 0x01})) == json({{"a", 1}}));
     }
 
     SECTION("chunks are still concatenated (binary)")
     {
         CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0xFF})) == json::binary({0x61}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0xFF})) == json::binary({}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0x40, 0x41, 0x62, 0x41, 0x63, 0xFF})) == json::binary({0x61, 0x62, 0x63}));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0x41, 0x62, 0xFF})) == json::binary({0x61, 0x62}));
     }
 
     SECTION("a chunk that is not a string is still rejected")
     {
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B); last byte: 0x00", json::parse_error&);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
     }
 
     SECTION("a break marker outside an indefinite-length string is not a string")
@@ -2895,17 +2896,9 @@ TEST_CASE("examples from RFC 8949 Appendix A")
     {
         const auto packed = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.cbor");
         json j;
-        // the fixture's tail contains nested indefinite-length byte strings.
-        CHECK_THROWS_WITH_AS(j = json::from_cbor(packed), "[json.exception.parse_error.113] parse error at byte 513: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
+        CHECK_NOTHROW(j = json::from_cbor(packed));
 
-        // keep the byte-for-byte decoding check for its valid prefix: the first
-        // 512 encoded bytes contain 468 payload bytes in definite-length chunks.
-        auto valid_prefix = packed;
-        valid_prefix.resize(512);
-        valid_prefix.push_back(0xFF);
-        auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
-        expected.resize(468);
-        CHECK_NOTHROW(j = json::from_cbor(valid_prefix));
+        const auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
         CHECK(j == json::binary(expected));
 
         // 0xd8

tests/src/unit-class_parser.cpp

@@ -2317,6 +2317,58 @@ TEST_CASE("parser class")
 #endif
     }
 
+    SECTION("comments before separators")
+    {
+        // The parser first checks for the expected ':' or ',' and only then
+        // falls back to the full token switch, which skips comments. A comment
+        // directly before a separator takes that fallback.
+        json _;
+
+        SECTION("ignored")
+        {
+            const std::vector<std::pair<std::string, json>> inputs =
+            {
+                {"{\"a\" /* c */ : 1}", {{"a", 1}}},
+                {"{\"a\" // c\n: 1}", {{"a", 1}}},
+                {R"({"a": 1, "b" /* c */ : 2})", {{"a", 1}, {"b", 2}}},
+                {R"({"a": 1 /* c */ , "b": 2})", {{"a", 1}, {"b", 2}}},
+                {"{\"a\": 1 // c\n, \"b\": 2}", {{"a", 1}, {"b", 2}}},
+                {"[1 /* c */ , 2]", {1, 2}},
+                {"[1 // c\n, 2]", {1, 2}},
+                {"{\"a\" /* c */ /* d */ : [1 // c\n , 2 /**/ ] /**/ , \"b\" : 3}", {{"a", {1, 2}}, {"b", 3}}}
+            };
+            for (const auto& input : inputs)
+            {
+                CAPTURE(input.first)
+                CHECK(json::parse(input.first, nullptr, true, true) == input.second);
+                CHECK(json::accept(input.first, true));
+            }
+        }
+
+        SECTION("ignored, with trailing commas")
+        {
+            CHECK(json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true, true) == json({1}));
+            CHECK(json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true, true) == json({{"a", 1}}));
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 19: syntax error while parsing object key - unexpected '}'; expected string literal", json::parse_error);
+        }
+
+        SECTION("not ignored")
+        {
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\" /* c */ : 1}")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing object separator - invalid literal; last read: '\"a\" /'; expected ':'", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1, \"b\" /* c */ : 2}")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing object separator - invalid literal; last read: '\"b\" /'; expected ':'", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , \"b\": 2}")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 9: syntax error while parsing object - invalid literal; last read: '1 /'; expected '}'", json::parse_error);
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , 2]")),
+                                 "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing array - invalid literal; last read: '1 /'; expected ']'", json::parse_error);
+            CHECK(!json::accept(std::string("[1 /* c */ , 2]")));
+        }
+    }
+
 #if JSON_DIAGNOSTIC_POSITIONS
     // Macro for all test cases for start_pos and end_pos
 #define SETUP_TESTCASES() \

tests/src/unit-custom-object-type.cpp

@@ -10,7 +10,9 @@
 
 #include <nlohmann/json.hpp>
 
+#include <cstddef>
 #include <cstdint>
+#include <iterator>
 #include <map>
 #include <string>
 #include <type_traits>
@@ -196,6 +198,198 @@ struct void_erase_map : std::map<Key, T, Compare, Allocator>
 
 using void_erase_json = nlohmann::basic_json<void_erase_map>;
 
+// wraps an iterator, but only offers the LegacyForwardIterator operations,
+// like the iterators of std::unordered_map and other hash maps
+template<class BaseIterator>
+class forward_only_iterator
+{
+    BaseIterator m_it{};
+
+  public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = typename std::iterator_traits<BaseIterator>::value_type;
+    using difference_type = typename std::iterator_traits<BaseIterator>::difference_type;
+    using pointer = typename std::iterator_traits<BaseIterator>::pointer;
+    using reference = typename std::iterator_traits<BaseIterator>::reference;
+
+    forward_only_iterator() = default;
+    explicit forward_only_iterator(BaseIterator it) : m_it(it) {}
+
+    BaseIterator base() const
+    {
+        return m_it;
+    }
+
+    reference operator*() const
+    {
+        return *m_it;
+    }
+    pointer operator->() const
+    {
+        return &*m_it;
+    }
+    forward_only_iterator& operator++()
+    {
+        ++m_it;
+        return *this;
+    }
+    forward_only_iterator operator++(int)
+    {
+        auto result = *this;
+        ++m_it;
+        return result;
+    }
+
+    friend bool operator==(const forward_only_iterator& lhs, const forward_only_iterator& rhs)
+    {
+        return lhs.m_it == rhs.m_it;
+    }
+    friend bool operator!=(const forward_only_iterator& lhs, const forward_only_iterator& rhs)
+    {
+        return lhs.m_it != rhs.m_it;
+    }
+};
+
+// An ObjectType whose iterators are forward-only, as those of hash maps are;
+// it has no rbegin() and its iterators no operator--. A hash map is not used
+// directly for the same reason as in no_key_compare_map above.
+template<class Key, class T, class Compare, class Allocator>
+class forward_only_map
+{
+    using map_t = std::map<Key, T, Compare, Allocator>;
+    map_t data;
+
+  public:
+    using key_type = typename map_t::key_type;
+    using mapped_type = typename map_t::mapped_type;
+    using value_type = typename map_t::value_type;
+    using size_type = typename map_t::size_type;
+    using allocator_type = typename map_t::allocator_type;
+    using iterator = forward_only_iterator<typename map_t::iterator>;
+    using const_iterator = forward_only_iterator<typename map_t::const_iterator>;
+
+    forward_only_map() noexcept(std::is_nothrow_default_constructible<map_t>::value) : data() {}
+
+    template<class InputIt>
+    forward_only_map(InputIt first, InputIt last) : data(first, last) {}
+
+    iterator begin() noexcept
+    {
+        return iterator(data.begin());
+    }
+    iterator end() noexcept
+    {
+        return iterator(data.end());
+    }
+    const_iterator begin() const noexcept
+    {
+        return const_iterator(data.begin());
+    }
+    const_iterator end() const noexcept
+    {
+        return const_iterator(data.end());
+    }
+    const_iterator cbegin() const noexcept
+    {
+        return const_iterator(data.cbegin());
+    }
+    const_iterator cend() const noexcept
+    {
+        return const_iterator(data.cend());
+    }
+
+    bool empty() const noexcept
+    {
+        return data.empty();
+    }
+    size_type size() const noexcept
+    {
+        return data.size();
+    }
+    size_type max_size() const noexcept
+    {
+        return data.max_size();
+    }
+    void clear() noexcept
+    {
+        data.clear();
+    }
+
+    iterator find(const key_type& key)
+    {
+        return iterator(data.find(key));
+    }
+    const_iterator find(const key_type& key) const
+    {
+        return const_iterator(data.find(key));
+    }
+    size_type count(const key_type& key) const
+    {
+        return data.count(key);
+    }
+
+    std::pair<iterator, bool> emplace(const key_type& key, const mapped_type& value)
+    {
+        const auto result = data.emplace(key, value);
+        return {iterator(result.first), result.second};
+    }
+
+    std::pair<iterator, bool> insert(const value_type& value)
+    {
+        const auto result = data.insert(value);
+        return {iterator(result.first), result.second};
+    }
+
+    template<class InputIt>
+    void insert(InputIt first, InputIt last)
+    {
+        data.insert(first, last);
+    }
+
+    mapped_type& operator[](const key_type& key)
+    {
+        return data[key];
+    }
+
+    mapped_type& at(const key_type& key)
+    {
+        return data.at(key);
+    }
+    const mapped_type& at(const key_type& key) const
+    {
+        return data.at(key);
+    }
+
+    iterator erase(iterator pos)
+    {
+        return iterator(data.erase(pos.base()));
+    }
+    iterator erase(iterator first, iterator last)
+    {
+        return iterator(data.erase(first.base(), last.base()));
+    }
+    size_type erase(const key_type& key)
+    {
+        return data.erase(key);
+    }
+
+    void swap(forward_only_map& other) noexcept(noexcept(data.swap(other.data)))
+    {
+        data.swap(other.data);
+    }
+
+    friend bool operator==(const forward_only_map& lhs, const forward_only_map& rhs)
+    {
+        return lhs.data == rhs.data;
+    }
+    friend bool operator<(const forward_only_map& lhs, const forward_only_map& rhs)
+    {
+        return lhs.data < rhs.data;
+    }
+};
+
+using forward_only_json = nlohmann::basic_json<forward_only_map>;
+
 } // namespace
 
 TEST_CASE("object type whose erase() returns void")
@@ -322,3 +516,46 @@ TEST_CASE("object type without key_compare")
     }
 }
 
+
+TEST_CASE("object type with forward-only iterators")
+{
+    CHECK(std::is_same<std::iterator_traits<forward_only_json::object_t::iterator>::iterator_category,
+          std::forward_iterator_tag>::value);
+
+    SECTION("destroying nested objects and arrays")
+    {
+        forward_only_json j;
+        j["a"] = 1;
+        j["b"]["c"] = "x";
+        j["b"]["d"] = forward_only_json::array();
+        j["b"]["d"].push_back(forward_only_json::object());
+        j["b"]["d"].push_back(true);
+        j["b"]["e"]["f"]["g"] = nullptr;
+        j["h"] = forward_only_json::object();
+        j["i"]["j"] = 2;
+
+        CHECK(j.size() == 4);
+        CHECK(j["b"].size() == 3);
+        CHECK(j["b"]["d"].size() == 2);
+        CHECK(j["b"]["e"]["f"]["g"].is_null());
+
+        CHECK(j.erase("b") == 1);
+        CHECK(j.size() == 3);
+        j = 42;
+        CHECK(j == 42);
+    }
+
+    SECTION("destroying a deeply nested object")
+    {
+        constexpr std::size_t depth = 100000;
+        forward_only_json j;
+        forward_only_json* cur = &j;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            (*cur)["s"] = i;
+            cur = &(*cur)["o"];
+        }
+        CHECK(j["o"]["o"]["s"] == 2);
+        // destroyed at the end of scope without recursing per level
+    }
+}

tests/src/unit-regression3.cpp

@@ -920,12 +920,4 @@ TEST_CASE("regression test #5476 - array type without reserve()")
     }
 }
 
-TEST_CASE("issue #5317 - nested indefinite-length CBOR string chunks are rejected")
-{
-    json _;
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0xFF})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0xFF})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0xA1, 0x7F, 0x7F, 0xFF, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
-}
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP