Add ignore_trailing_commas option (#4609)

Added examples and modified the corresponding documents and unit tests.

Signed-off-by: chirsz-ever <chirsz-ever@outlook.com>
Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

README.md

@@ -1801,7 +1801,17 @@ This library does not support comments by default. It does so for three reasons:
   
 3. It is dangerous for interoperability if some libraries would add comment support while others don't. Please check [The Harmful Consequences of the Robustness Principle](https://tools.ietf.org/html/draft-iab-protocol-maintenance-01) on this.
 
-However, you can pass set parameter `ignore_comments` to true in the `parse` function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace.
+However, you can set set parameter `ignore_comments` to true in the `parse` function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace.
+
+### Trailing commas
+
+The JSON specification does not allow trailing commas in arrays and objects, and hence this library is treating them as parsing errors by default.
+
+Like comments, you can set parameter `ignore_trailing_commas` to true in the `parse` function to ignore trailing commas in arrays and objects. Note that a single comma as the only content of the array or object (`[,]` or `{,}`) is not allowed, and multiple trailing commas (`[1,,]`) are not allowed either.
+
+This library does not add trailing commas when serializing JSON data.
+
+For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
 
 ### Order of object keys
 

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

@@ -4,12 +4,14 @@
 // (1)
 template<typename InputType>
 static bool accept(InputType&& i,
-                   const bool ignore_comments = false);
+                   const bool ignore_comments = false,
+                   const bool ignore_trailing_commas = false);
 
 // (2)
 template<typename IteratorType>
 static bool accept(IteratorType first, IteratorType last,
-                   const bool ignore_comments = false);
+                   const bool ignore_comments = false,
+                   const bool ignore_trailing_commas = false);
 ```
 
 Checks whether the input is valid JSON.
@@ -50,6 +52,10 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
 :   whether comments should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
     (`#!cpp false`); (optional, `#!cpp false` by default)
 
+`ignore_trailing_commas` (in)
+:   whether trailing commas in arrays or objects should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
+    (`#!cpp false`); (optional, `#!cpp false` by default)
+
 `first` (in)
 :   iterator to the start of the character range
 
@@ -102,6 +108,7 @@ A UTF-8 byte order mark is silently ignored.
 - Added in version 3.0.0.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Changed [runtime assertion](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0.
+- Added `ignore_trailing_commas` in version 3.12.1.
 
 !!! warning "Deprecation"
 

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

@@ -6,14 +6,16 @@ template<typename InputType>
 static basic_json parse(InputType&& i,
                         const parser_callback_t cb = nullptr,
                         const bool allow_exceptions = true,
-                        const bool ignore_comments = false);
+                        const bool ignore_comments = false,
+                        const bool ignore_trailing_commas = false);
 
 // (2)
 template<typename IteratorType>
 static basic_json parse(IteratorType first, IteratorType last,
                         const parser_callback_t cb = nullptr,
                         const bool allow_exceptions = true,
-                        const bool ignore_comments = false);
+                        const bool ignore_comments = false,
+                        const bool ignore_trailing_commas = false);
 ```
 
 1. Deserialize from a compatible input.
@@ -56,6 +58,10 @@ static basic_json parse(IteratorType first, IteratorType last,
 :   whether comments should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
     (`#!cpp false`); (optional, `#!cpp false` by default)
 
+`ignore_trailing_commas` (in)
+:   whether trailing commas in arrays or objects should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
+    (`#!cpp false`); (optional, `#!cpp false` by default)
+
 `first` (in)
 :   iterator to the start of a character range
 
@@ -189,6 +195,34 @@ A UTF-8 byte order mark is silently ignored.
     --8<-- "examples/parse__allow_exceptions.output"
     ```
 
+??? example "Effect of `ignore_comments` parameter"
+
+    The example below demonstrates the effect of the `ignore_comments` parameter in the `parse()` function.
+
+    ```cpp
+    --8<-- "examples/comments.cpp"
+    ```
+
+    Output:
+
+    ```
+    --8<-- "examples/comments.output"
+    ```
+
+??? example "Effect of `ignore_trailing_commas` parameter"
+
+    The example below demonstrates the effect of the `ignore_trailing_commas` parameter in the `parse()` function.
+
+    ```cpp
+    --8<-- "examples/trailing_commas.cpp"
+    ```
+
+    Output:
+
+    ```
+    --8<-- "examples/trailing_commas.output"
+    ```
+
 ## See also
 
 - [accept](accept.md) - check if the input is valid JSON
@@ -200,6 +234,7 @@ A UTF-8 byte order mark is silently ignored.
 - Overload for contiguous containers (1) added in version 2.0.3.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Changed [runtime assertion](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0.
+- Added `ignore_trailing_commas` in version 3.12.1.
 
 !!! warning "Deprecation"
 

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

@@ -7,7 +7,8 @@ static bool sax_parse(InputType&& i,
                       SAX* sax,
                       input_format_t format = input_format_t::json,
                       const bool strict = true,
-                      const bool ignore_comments = false);
+                      const bool ignore_comments = false,
+                      const bool ignore_trailing_commas = false);
 
 // (2)
 template<class IteratorType, class SAX>
@@ -15,7 +16,8 @@ static bool sax_parse(IteratorType first, IteratorType last,
                       SAX* sax,
                       input_format_t format = input_format_t::json,
                       const bool strict = true,
-                      const bool ignore_comments = false);
+                      const bool ignore_comments = false,
+                      const bool ignore_trailing_commas = false);
 ```
 
 Read from input and generate SAX events
@@ -65,6 +67,10 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
 :   whether comments should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
     (`#!cpp false`); (optional, `#!cpp false` by default)
 
+`ignore_trailing_commas` (in)
+:   whether trailing commas in arrays or objects should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
+    (`#!cpp false`); (optional, `#!cpp false` by default)
+
 `first` (in)
 :   iterator to the start of a character range
 
@@ -107,6 +113,7 @@ A UTF-8 byte order mark is silently ignored.
 
 - Added in version 3.2.0.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
+- Added `ignore_trailing_commas` in version 3.12.1.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/examples/comments.cpp

@@ -0,0 +1,31 @@
+
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::string s = R"(
+    {
+        // update in 2006: removed Pluto
+        "planets": ["Mercury", "Venus", "Earth", "Mars",
+                    "Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
+    }
+    )";
+
+    try
+    {
+        json j = json::parse(s);
+    }
+    catch (json::exception& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    json j = json::parse(s,
+                         /* callback */ nullptr,
+                         /* allow exceptions */ true,
+                         /* ignore_comments */ true);
+    std::cout << j.dump(2) << '\n';
+}

docs/mkdocs/docs/examples/comments.output

@@ -0,0 +1,12 @@
+[json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A>    {<U+000A>        /'; expected string literal
+{
+  "planets": [
+    "Mercury",
+    "Venus",
+    "Earth",
+    "Mars",
+    "Jupiter",
+    "Uranus",
+    "Neptune"
+  ]
+}

docs/mkdocs/docs/examples/trailing_commas.cpp

@@ -0,0 +1,37 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::string s = R"(
+    {
+        "planets": [
+            "Mercury",
+            "Venus",
+            "Earth",
+            "Mars",
+            "Jupiter",
+            "Uranus",
+            "Neptune",
+        ]
+    }
+    )";
+
+    try
+    {
+        json j = json::parse(s);
+    }
+    catch (json::exception& e)
+    {
+        std::cout << e.what() << std::endl;
+    }
+
+    json j = json::parse(s,
+                         /* callback */ nullptr,
+                         /* allow exceptions */ true,
+                         /* ignore_comments */ false,
+                         /* ignore_trailing_commas */ true);
+    std::cout << j.dump(2) << '\n';
+}

docs/mkdocs/docs/examples/trailing_commas.output

@@ -0,0 +1,12 @@
+[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
+{
+  "planets": [
+    "Mercury",
+    "Venus",
+    "Earth",
+    "Mars",
+    "Jupiter",
+    "Uranus",
+    "Neptune"
+  ]
+}

docs/mkdocs/docs/features/comments.md

@@ -11,7 +11,9 @@ This library does not support comments *by default*. It does so for three reason
 
 3. It is dangerous for interoperability if some libraries add comment support while others do not. Please check [The Harmful Consequences of the Robustness Principle](https://tools.ietf.org/html/draft-iab-protocol-maintenance-01) on this.
 
-However, you can pass set parameter `ignore_comments` to `#!c true` in the parse function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace.
+However, you can set parameter `ignore_comments` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace.
+
+For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
 
 !!! example
 
@@ -28,56 +30,11 @@ However, you can pass set parameter `ignore_comments` to `#!c true` in the parse
     When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_comments` is set to `#! true`, the comments are ignored during parsing:
 
     ```cpp
-    #include <iostream>
+    --8<-- "examples/comments.cpp"
-    #include "json.hpp"
-    
-    using json = nlohmann::json;
-    
-    int main()
-    {
-        std::string s = R"(
-        {
-            // update in 2006: removed Pluto
-            "planets": ["Mercury", "Venus", "Earth", "Mars",
-                        "Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
-        }
-        )";
-        
-        try
-        {
-            json j = json::parse(s);
-        }
-        catch (json::exception &e)
-        {
-            std::cout << e.what() << std::endl;
-        }
-        
-        json j = json::parse(s,
-                             /* callback */ nullptr,
-                             /* allow exceptions */ true,
-                             /* ignore_comments */ true);
-        std::cout << j.dump(2) << '\n';
-    }
     ```
 
     Output:
     
     ```
-    [json.exception.parse_error.101] parse error at line 3, column 9:
+    --8<-- "examples/comments.output"
-    syntax error while parsing object key - invalid literal;
-    last read: '<U+000A>    {<U+000A>        /'; expected string literal
-    ```
-    
-    ```json
-    {
-      "planets": [
-        "Mercury",
-        "Venus",
-        "Earth",
-        "Mars",
-        "Jupiter",
-        "Uranus",
-        "Neptune"
-      ]
-    }
     ```

docs/mkdocs/docs/features/trailing_commas.md

@@ -0,0 +1,39 @@
+# Trailing Commas
+
+Like [comments](comments.md), this library does not support trailing commas in arrays and objects *by default*.
+
+You can set parameter `ignore_trailing_commas` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to allow trailing commas in arrays and objects. Note that a single comma as the only content of the array or object (`[,]` or `{,}`) is not allowed, and multiple trailing commas (`[1,,]`) are not allowed either.
+
+This library does not add trailing commas when serializing JSON data.
+
+For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
+
+!!! example
+
+    Consider the following JSON with trailing commas.
+
+    ```json
+    {
+        "planets": [
+            "Mercury",
+            "Venus",
+            "Earth",
+            "Mars",
+            "Jupiter",
+            "Uranus",
+            "Neptune",
+        ]
+    }
+    ```
+    
+    When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_trailing_commas` is set to `#! true`, the trailing commas are ignored during parsing:
+
+    ```cpp
+    --8<-- "examples/trailing_commas.cpp"
+    ```
+
+    Output:
+    
+    ```
+    --8<-- "examples/trailing_commas.output"
+    ```

docs/mkdocs/mkdocs.yml

@@ -67,6 +67,7 @@ nav:
           - features/binary_formats/ubjson.md
       - features/binary_values.md
       - features/comments.md
+      - features/trailing_commas.md
       - Element Access:
           - features/element_access/index.md
           - features/element_access/unchecked_access.md

include/nlohmann/detail/input/parser.hpp

@@ -71,10 +71,12 @@ class parser
     explicit parser(InputAdapterType&& adapter,
                     parser_callback_t<BasicJsonType> cb = nullptr,
                     const bool allow_exceptions_ = true,
-                    const bool skip_comments = false)
+                    const bool ignore_comments = false,
+                    const bool ignore_trailing_commas_ = false)
         : callback(std::move(cb))
-        , m_lexer(std::move(adapter), skip_comments)
+        , m_lexer(std::move(adapter), ignore_comments)
         , allow_exceptions(allow_exceptions_)
+        , ignore_trailing_commas(ignore_trailing_commas_)
     {
         // read first token
         get_token();
@@ -384,11 +386,17 @@ class parser
             if (states.back())  // array
             {
                 // comma -> next value
+                // or end of array (ignore_trailing_commas = true)
                 if (get_token() == token_type::value_separator)
                 {
                     // parse a new value
                     get_token();
-                    continue;
+
+                    // if ignore_trailing_commas and last_token is ], we can continue to "closing ]"
+                    if (!(ignore_trailing_commas && last_token == token_type::end_array))
+                    {
+                        continue;
+                    }
                 }
 
                 // closing ]
@@ -417,32 +425,39 @@ class parser
             // states.back() is false -> object
 
             // comma -> next value
+            // or end of object (ignore_trailing_commas = true)
             if (get_token() == token_type::value_separator)
             {
-                // parse key
-                if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::value_string))
-                {
-                    return sax->parse_error(m_lexer.get_position(),
-                                            m_lexer.get_token_string(),
-                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
-                }
-
-                if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
-                {
-                    return false;
-                }
-
-                // parse separator (:)
-                if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
-                {
-                    return sax->parse_error(m_lexer.get_position(),
-                                            m_lexer.get_token_string(),
-                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
-                }
-
-                // parse values
                 get_token();
-                continue;
+
+                // if ignore_trailing_commas and last_token is }, we can continue to "closing }"
+                if (!(ignore_trailing_commas && last_token == token_type::end_object))
+                {
+                    // parse key
+                    if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
+                    {
+                        return sax->parse_error(m_lexer.get_position(),
+                                                m_lexer.get_token_string(),
+                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
+                    }
+
+                    if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
+                    {
+                        return false;
+                    }
+
+                    // parse separator (:)
+                    if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                    {
+                        return sax->parse_error(m_lexer.get_position(),
+                                                m_lexer.get_token_string(),
+                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
+                    }
+
+                    // parse values
+                    get_token();
+                    continue;
+                }
             }
 
             // closing }
@@ -513,6 +528,8 @@ class parser
     lexer_t m_lexer;
     /// whether to throw exceptions in case of errors
     const bool allow_exceptions = true;
+    /// whether trailing commas in objects and arrays should be ignored (true) or signaled as errors (false)
+    const bool ignore_trailing_commas = false;
 };
 
 }  // namespace detail

include/nlohmann/json.hpp

@@ -134,11 +134,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         InputAdapterType adapter,
         detail::parser_callback_t<basic_json>cb = nullptr,
         const bool allow_exceptions = true,
-        const bool ignore_comments = false
+        const bool ignore_comments = false,
+        const bool ignore_trailing_commas = false
     )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
-               std::move(cb), allow_exceptions, ignore_comments);
+               std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
     }
 
   private:
@@ -4045,10 +4046,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static basic_json parse(InputType&& i,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
-                            const bool ignore_comments = false)
+                            const bool ignore_comments = false,
+                            const bool ignore_trailing_commas = false)
     {
         basic_json result;
-        parser(detail::input_adapter(std::forward<InputType>(i)), std::move(cb), allow_exceptions, ignore_comments).parse(true, result); // cppcheck-suppress[accessMoved,accessForwarded]
+        parser(detail::input_adapter(std::forward<InputType>(i)), std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas).parse(true, result); // cppcheck-suppress[accessMoved,accessForwarded]
         return result;
     }
 
@@ -4060,10 +4062,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                             IteratorType last,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
-                            const bool ignore_comments = false)
+                            const bool ignore_comments = false,
+                            const bool ignore_trailing_commas = false)
     {
         basic_json result;
-        parser(detail::input_adapter(std::move(first), std::move(last)), std::move(cb), allow_exceptions, ignore_comments).parse(true, result); // cppcheck-suppress[accessMoved]
+        parser(detail::input_adapter(std::move(first), std::move(last)), std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas).parse(true, result); // cppcheck-suppress[accessMoved]
         return result;
     }
 
@@ -4072,10 +4075,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static basic_json parse(detail::span_input_adapter&& i,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
-                            const bool ignore_comments = false)
+                            const bool ignore_comments = false,
+                            const bool ignore_trailing_commas = false)
     {
         basic_json result;
-        parser(i.get(), std::move(cb), allow_exceptions, ignore_comments).parse(true, result); // cppcheck-suppress[accessMoved]
+        parser(i.get(), std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas).parse(true, result); // cppcheck-suppress[accessMoved]
         return result;
     }
 
@@ -4083,26 +4087,29 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename InputType>
     static bool accept(InputType&& i,
-                       const bool ignore_comments = false)
+                       const bool ignore_comments = false,
+                       const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments).accept(true);
+        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
     /// @brief check if the input is valid JSON
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename IteratorType>
     static bool accept(IteratorType first, IteratorType last,
-                       const bool ignore_comments = false)
+                       const bool ignore_comments = false,
+                       const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments).accept(true);
+        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
     JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_DEPRECATED_FOR(3.8.0, accept(ptr, ptr + len))
     static bool accept(detail::span_input_adapter&& i,
-                       const bool ignore_comments = false)
+                       const bool ignore_comments = false,
+                       const bool ignore_trailing_commas = false)
     {
-        return parser(i.get(), nullptr, false, ignore_comments).accept(true);
+        return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
     /// @brief generate SAX events
@@ -4112,11 +4119,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static bool sax_parse(InputType&& i, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
-                          const bool ignore_comments = false)
+                          const bool ignore_comments = false,
+                          const bool ignore_trailing_commas = false)
     {
         auto ia = detail::input_adapter(std::forward<InputType>(i));
         return format == input_format_t::json
-               ? parser(std::move(ia), nullptr, true, ignore_comments).sax_parse(sax, strict)
+               ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }
 
@@ -4127,11 +4135,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static bool sax_parse(IteratorType first, IteratorType last, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
-                          const bool ignore_comments = false)
+                          const bool ignore_comments = false,
+                          const bool ignore_trailing_commas = false)
     {
         auto ia = detail::input_adapter(std::move(first), std::move(last));
         return format == input_format_t::json
-               ? parser(std::move(ia), nullptr, true, ignore_comments).sax_parse(sax, strict)
+               ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }
 
@@ -4146,12 +4155,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static bool sax_parse(detail::span_input_adapter&& i, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
-                          const bool ignore_comments = false)
+                          const bool ignore_comments = false,
+                          const bool ignore_trailing_commas = false)
     {
         auto ia = i.get();
         return format == input_format_t::json
                // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
-               ? parser(std::move(ia), nullptr, true, ignore_comments).sax_parse(sax, strict)
+               ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }

single_include/nlohmann/json.hpp

@@ -12987,10 +12987,12 @@ class parser
     explicit parser(InputAdapterType&& adapter,
                     parser_callback_t<BasicJsonType> cb = nullptr,
                     const bool allow_exceptions_ = true,
-                    const bool skip_comments = false)
+                    const bool ignore_comments = false,
+                    const bool ignore_trailing_commas_ = false)
         : callback(std::move(cb))
-        , m_lexer(std::move(adapter), skip_comments)
+        , m_lexer(std::move(adapter), ignore_comments)
         , allow_exceptions(allow_exceptions_)
+        , ignore_trailing_commas(ignore_trailing_commas_)
     {
         // read first token
         get_token();
@@ -13300,11 +13302,17 @@ class parser
             if (states.back())  // array
             {
                 // comma -> next value
+                // or end of array (ignore_trailing_commas = true)
                 if (get_token() == token_type::value_separator)
                 {
                     // parse a new value
                     get_token();
-                    continue;
+
+                    // if ignore_trailing_commas and last_token is ], we can continue to "closing ]"
+                    if (!(ignore_trailing_commas && last_token == token_type::end_array))
+                    {
+                        continue;
+                    }
                 }
 
                 // closing ]
@@ -13333,32 +13341,39 @@ class parser
             // states.back() is false -> object
 
             // comma -> next value
+            // or end of object (ignore_trailing_commas = true)
             if (get_token() == token_type::value_separator)
             {
-                // parse key
-                if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::value_string))
-                {
-                    return sax->parse_error(m_lexer.get_position(),
-                                            m_lexer.get_token_string(),
-                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
-                }
-
-                if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
-                {
-                    return false;
-                }
-
-                // parse separator (:)
-                if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
-                {
-                    return sax->parse_error(m_lexer.get_position(),
-                                            m_lexer.get_token_string(),
-                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
-                }
-
-                // parse values
                 get_token();
-                continue;
+
+                // if ignore_trailing_commas and last_token is }, we can continue to "closing }"
+                if (!(ignore_trailing_commas && last_token == token_type::end_object))
+                {
+                    // parse key
+                    if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
+                    {
+                        return sax->parse_error(m_lexer.get_position(),
+                                                m_lexer.get_token_string(),
+                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
+                    }
+
+                    if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
+                    {
+                        return false;
+                    }
+
+                    // parse separator (:)
+                    if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+                    {
+                        return sax->parse_error(m_lexer.get_position(),
+                                                m_lexer.get_token_string(),
+                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
+                    }
+
+                    // parse values
+                    get_token();
+                    continue;
+                }
             }
 
             // closing }
@@ -13429,6 +13444,8 @@ class parser
     lexer_t m_lexer;
     /// whether to throw exceptions in case of errors
     const bool allow_exceptions = true;
+    /// whether trailing commas in objects and arrays should be ignored (true) or signaled as errors (false)
+    const bool ignore_trailing_commas = false;
 };
 
 }  // namespace detail
@@ -20219,11 +20236,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         InputAdapterType adapter,
         detail::parser_callback_t<basic_json>cb = nullptr,
         const bool allow_exceptions = true,
-        const bool ignore_comments = false
+        const bool ignore_comments = false,
+        const bool ignore_trailing_commas = false
                                  )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
-            std::move(cb), allow_exceptions, ignore_comments);
+            std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
     }
 
   private:
@@ -24130,10 +24148,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static basic_json parse(InputType&& i,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
-                            const bool ignore_comments = false)
+                            const bool ignore_comments = false,
+                            const bool ignore_trailing_commas = false)
     {
         basic_json result;
-        parser(detail::input_adapter(std::forward<InputType>(i)), std::move(cb), allow_exceptions, ignore_comments).parse(true, result); // cppcheck-suppress[accessMoved,accessForwarded]
+        parser(detail::input_adapter(std::forward<InputType>(i)), std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas).parse(true, result); // cppcheck-suppress[accessMoved,accessForwarded]
         return result;
     }
 
@@ -24145,10 +24164,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                             IteratorType last,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
-                            const bool ignore_comments = false)
+                            const bool ignore_comments = false,
+                            const bool ignore_trailing_commas = false)
     {
         basic_json result;
-        parser(detail::input_adapter(std::move(first), std::move(last)), std::move(cb), allow_exceptions, ignore_comments).parse(true, result); // cppcheck-suppress[accessMoved]
+        parser(detail::input_adapter(std::move(first), std::move(last)), std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas).parse(true, result); // cppcheck-suppress[accessMoved]
         return result;
     }
 
@@ -24157,10 +24177,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static basic_json parse(detail::span_input_adapter&& i,
                             parser_callback_t cb = nullptr,
                             const bool allow_exceptions = true,
-                            const bool ignore_comments = false)
+                            const bool ignore_comments = false,
+                            const bool ignore_trailing_commas = false)
     {
         basic_json result;
-        parser(i.get(), std::move(cb), allow_exceptions, ignore_comments).parse(true, result); // cppcheck-suppress[accessMoved]
+        parser(i.get(), std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas).parse(true, result); // cppcheck-suppress[accessMoved]
         return result;
     }
 
@@ -24168,26 +24189,29 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename InputType>
     static bool accept(InputType&& i,
-                       const bool ignore_comments = false)
+                       const bool ignore_comments = false,
+                       const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments).accept(true);
+        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
     /// @brief check if the input is valid JSON
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename IteratorType>
     static bool accept(IteratorType first, IteratorType last,
-                       const bool ignore_comments = false)
+                       const bool ignore_comments = false,
+                       const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments).accept(true);
+        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
     JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_DEPRECATED_FOR(3.8.0, accept(ptr, ptr + len))
     static bool accept(detail::span_input_adapter&& i,
-                       const bool ignore_comments = false)
+                       const bool ignore_comments = false,
+                       const bool ignore_trailing_commas = false)
     {
-        return parser(i.get(), nullptr, false, ignore_comments).accept(true);
+        return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
     }
 
     /// @brief generate SAX events
@@ -24197,11 +24221,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static bool sax_parse(InputType&& i, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
-                          const bool ignore_comments = false)
+                          const bool ignore_comments = false,
+                          const bool ignore_trailing_commas = false)
     {
         auto ia = detail::input_adapter(std::forward<InputType>(i));
         return format == input_format_t::json
-               ? parser(std::move(ia), nullptr, true, ignore_comments).sax_parse(sax, strict)
+               ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }
 
@@ -24212,11 +24237,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static bool sax_parse(IteratorType first, IteratorType last, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
-                          const bool ignore_comments = false)
+                          const bool ignore_comments = false,
+                          const bool ignore_trailing_commas = false)
     {
         auto ia = detail::input_adapter(std::move(first), std::move(last));
         return format == input_format_t::json
-               ? parser(std::move(ia), nullptr, true, ignore_comments).sax_parse(sax, strict)
+               ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }
 
@@ -24231,12 +24257,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     static bool sax_parse(detail::span_input_adapter&& i, SAX* sax,
                           input_format_t format = input_format_t::json,
                           const bool strict = true,
-                          const bool ignore_comments = false)
+                          const bool ignore_comments = false,
+                          const bool ignore_trailing_commas = false)
     {
         auto ia = i.get();
         return format == input_format_t::json
                // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
-               ? parser(std::move(ia), nullptr, true, ignore_comments).sax_parse(sax, strict)
+               ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
                : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(format, sax, strict);
     }

tests/src/unit-class_parser.cpp

@@ -206,6 +206,7 @@ class SaxCountdown : public nlohmann::json::json_sax_t
 json parser_helper(const std::string& s);
 bool accept_helper(const std::string& s);
 void comments_helper(const std::string& s);
+void trailing_comma_helper(const std::string& s);
 
 json parser_helper(const std::string& s)
 {
@@ -225,6 +226,8 @@ json parser_helper(const std::string& s)
 
     comments_helper(s);
 
+    trailing_comma_helper(s);
+
     return j;
 }
 
@@ -259,10 +262,11 @@ bool accept_helper(const std::string& s)
     // 6. check if this approach came to the same result
     CHECK(ok_noexcept == ok_noexcept_cb);
 
-    // 7. check if comments are properly ignored
+    // 7. check if comments or trailing commas are properly ignored
     if (ok_accept)
     {
         comments_helper(s);
+        trailing_comma_helper(s);
     }
 
     // 8. return result
@@ -302,6 +306,38 @@ void comments_helper(const std::string& s)
     }
 }
 
+void trailing_comma_helper(const std::string& s)
+{
+    json _;
+
+    // parse/accept with default parser
+    CHECK_NOTHROW(_ = json::parse(s));
+    CHECK(json::accept(s));
+
+    // parse/accept while allowing trailing commas
+    CHECK_NOTHROW(_ = json::parse(s, nullptr, false, false, true));
+    CHECK(json::accept(s, false, true));
+
+    // note: [,] and {,} are not allowed
+    if (s.size() > 1 && (s.back() == ']' || s.back() == '}') && !_.empty())
+    {
+        std::vector<std::string> json_with_trailing_commas;
+        json_with_trailing_commas.push_back(s.substr(0, s.size() - 1) + " ," + s.back());
+        json_with_trailing_commas.push_back(s.substr(0, s.size() - 1) + "," + s.back());
+        json_with_trailing_commas.push_back(s.substr(0, s.size() - 1) + ", " + s.back());
+
+        for (const auto& json_with_trailing_comma : json_with_trailing_commas)
+        {
+            CAPTURE(json_with_trailing_comma)
+            CHECK_THROWS_AS(_ = json::parse(json_with_trailing_comma), json::parse_error);
+            CHECK(!json::accept(json_with_trailing_comma));
+
+            CHECK_NOTHROW(_ = json::parse(json_with_trailing_comma, nullptr, true, false, true));
+            CHECK(json::accept(json_with_trailing_comma, false, true));
+        }
+    }
+}
+
 } // namespace
 
 TEST_CASE("parser class")