Re-template json_pointer on string type (#3415)

* Make exception context optional

Change exception context parameter to pointer and replace context with
nullptr where appropriate.

* Support escaping other string types

* Add string concatenation function

Add variadic concat() function for concatenating char *, char, and
string types.

* Replace string concatenations using + with concat()

* Template json_pointer on string type

Change json_pointer from being templated on basic_json to being
templated on string type.

* Add unit test for #3388

Closes #3388.

* Fix regression test for #2958

* Add backwards compatibility with json_pointer<basic_json>

* Update json_pointer docs

* Allow comparing different json_pointers

* Update version numbers

doc/mkdocs/docs/api/json_pointer/back.md

 # <small>nlohmann::json_pointer::</small>back
 
 ```cpp
-const std::string& back() const;
+const string_t& back() const;
 ```
 
 Return last reference token.
@@ -36,4 +36,5 @@ Constant.
 
 ## Version history
 
-Added in version 3.6.0.
+- Added in version 3.6.0.
+- Changed return type to `string_t` in version 3.11.0.

doc/mkdocs/docs/api/json_pointer/index.md

 # <small>nlohmann::</small>json_pointer
 
 ```cpp
-template<typename BasicJsonType>
+template<typename RefStringType>
 class json_pointer;
 ```
 
@@ -11,14 +11,18 @@ are the base for JSON patches.
 
 ## Template parameters
 
-`BasicJsonType`
-:   a specialization of [`basic_json`](../basic_json/index.md)
+`RefStringType`
+:   the string type used for the reference tokens making up the JSON pointer
+
+## Notes
+
+For backwards compatibility `RefStringType` may also be a specialization of [`basic_json`](../basic_json/index.md) in which case `string_t` will be deduced as [`basic_json::string_t`](../basic_json/string_t.md). This feature is deprecated and may be removed in a future major version.
 
 ## Member functions
 
 - [(constructor)](json_pointer.md)
 - [**to_string**](to_string.md) - return a string representation of the JSON pointer
-- [**operator std::string**](operator_string.md) - return a string representation of the JSON pointer
+- [**operator string_t**](operator_string.md) - return a string representation of the JSON pointer
 - [**operator/=**](operator_slasheq.md) - append to the end of the JSON pointer
 - [**operator/**](operator_slash.md) - create JSON Pointer by appending
 - [**parent_pointer**](parent_pointer.md) - returns the parent of this JSON pointer
@@ -27,6 +31,10 @@ are the base for JSON patches.
 - [**push_back**](push_back.md) - append an unescaped token at the end of the pointer
 - [**empty**](empty.md) - return whether pointer points to the root document
 
+## Member types
+
+- [**string_t**](string_t.md) - the string type used for the reference tokens
+
 ## See also
 
 - [operator""_json_pointer](../basic_json/operator_literal_json_pointer.md) - user-defined string literal for JSON pointers
@@ -34,4 +42,5 @@ are the base for JSON patches.
 
 ## Version history
 
-Added in version 2.0.0.
+- Added in version 2.0.0.
+- Changed template parameter from `basic_json` to string type in version 3.11.0.

doc/mkdocs/docs/api/json_pointer/json_pointer.md

 # <small>nlohmann::json_pointer::</small>json_pointer
 
 ```cpp
-explicit json_pointer(const std::string& s = "");
+explicit json_pointer(const string_t& s = "");
 ```
 
 Create a JSON pointer according to the syntax described in
@@ -37,4 +37,5 @@ Create a JSON pointer according to the syntax described in
 
 ## Version history
 
-Added in version 2.0.0.
+- Added in version 2.0.0.
+- Changed type of `s` to `string_t` in version 3.11.0.

doc/mkdocs/docs/api/json_pointer/operator_slash.md

@@ -5,7 +5,7 @@
 json_pointer operator/(const json_pointer& lhs, const json_pointer& rhs);
 
 // (2)
-json_pointer operator/(const json_pointer& lhs, std::string token);
+json_pointer operator/(const json_pointer& lhs, string_t token);
 
 // (3)
 json_pointer operator/(const json_pointer& lhs, std::size_t array_idx);
@@ -60,5 +60,5 @@ json_pointer operator/(const json_pointer& lhs, std::size_t array_idx);
 ## Version history
 
 1. Added in version 3.6.0.
-2. Added in version 3.6.0.
+2. Added in version 3.6.0. Changed type of `token` to `string_t` in version 3.11.0.
 3. Added in version 3.6.0.

doc/mkdocs/docs/api/json_pointer/operator_slasheq.md

@@ -5,7 +5,7 @@
 json_pointer& operator/=(const json_pointer& ptr);
 
 // (2)
-json_pointer& operator/=(std::string token);
+json_pointer& operator/=(string_t token);
 
 // (3)
 json_pointer& operator/=(std::size_t array_idx)
@@ -57,5 +57,5 @@ json_pointer& operator/=(std::size_t array_idx)
 ## Version history
 
 1. Added in version 3.6.0.
-2. Added in version 3.6.0.
+2. Added in version 3.6.0. Changed type of `token` to `string_t` in version 3.11.0.
 3. Added in version 3.6.0.

doc/mkdocs/docs/api/json_pointer/operator_string.md

-# <small>nlohmann::json_pointer::</small>operator std::string
+# <small>nlohmann::json_pointer::</small>operator string_t
 
 ```cpp
-operator std::string() const
+operator string_t() const
 ```
 
 Return a string representation of the JSON pointer.
@@ -13,7 +13,7 @@ A string representation of the JSON pointer
 ## Possible implementation
 
 ```cpp
-operator std::string() const
+operator string_t() const
 {
     return to_string();
 }
@@ -21,4 +21,5 @@ operator std::string() const
 
 ## Version history
 
-Since version 2.0.0.
+- Since version 2.0.0.
+- Changed type to `string_t` in version 3.11.0.

doc/mkdocs/docs/api/json_pointer/push_back.md

 # <small>nlohmann::json_pointer::</small>push_back
 
 ```cpp
-void push_back(const std::string& token);
+void push_back(const string_t& token);
 
-void push_back(std::string&& token);
+void push_back(string_t&& token);
 ```
 
 Append an unescaped token at the end of the reference pointer.
@@ -35,4 +35,5 @@ Amortized constant.
 
 ## Version history
 
-Added in version 3.6.0.
+- Added in version 3.6.0.
+- Changed type of `token` to `string_t` in version 3.11.0.

doc/mkdocs/docs/api/json_pointer/string_t.md

+# <small>nlohmann::json_pointer::</small>string_t
+```cpp
+using string_t = RefStringType;
+```
+
+The string type used for the reference tokens making up the JSON pointer.
+
+See [`basic_json::string_t`](../basic_json/string_t.md) for more information.
+
+## Version history
+
+- Added in version 3.11.0.

doc/mkdocs/docs/api/json_pointer/to_string.md

 # <small>nlohmann::json_pointer::</small>to_string
 
 ```cpp
-std::string to_string() const;
+string_t to_string() const;
 ```
 
 Return a string representation of the JSON pointer.
@@ -36,4 +36,5 @@ ptr == json_pointer(ptr.to_string());
 
 ## Version history
 
-Since version 2.0.0.
+- Since version 2.0.0.
+- Changed return type to `string_t` in version 3.11.0.

doc/mkdocs/mkdocs.yml

@@ -211,6 +211,7 @@ nav:
         - 'parent_pointer': api/json_pointer/parent_pointer.md
         - 'pop_back': api/json_pointer/pop_back.md
         - 'push_back': api/json_pointer/push_back.md
+        - 'string_t': api/json_pointer/string_t.md
         - 'to_string': api/json_pointer/to_string.md
       - json_sax:
         - 'Overview': api/json_sax/index.md

include/nlohmann/detail/conversions/from_json.hpp

@@ -17,6 +17,7 @@
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/meta/identity_tag.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
 #if JSON_HAS_EXPERIMENTAL_FILESYSTEM
@@ -42,7 +43,7 @@ void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_null()))
     {
-        JSON_THROW(type_error::create(302, "type must be null, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be null, but is ", j.type_name()), &j));
     }
     n = nullptr;
 }
@@ -80,7 +81,7 @@ void get_arithmetic_value(const BasicJsonType& j, ArithmeticType& val)
         case value_t::binary:
         case value_t::discarded:
         default:
-            JSON_THROW(type_error::create(302, "type must be number, but is " + std::string(j.type_name()), j));
+            JSON_THROW(type_error::create(302, concat("type must be number, but is ", j.type_name()), &j));
     }
 }
 
@@ -89,7 +90,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::boolean_t& b)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_boolean()))
     {
-        JSON_THROW(type_error::create(302, "type must be boolean, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be boolean, but is ", j.type_name()), &j));
     }
     b = *j.template get_ptr<const typename BasicJsonType::boolean_t*>();
 }
@@ -99,7 +100,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
-        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
 }
@@ -114,7 +115,7 @@ void from_json(const BasicJsonType& j, StringType& s)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
-        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
 
     s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
@@ -154,7 +155,7 @@ void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
     l.clear();
     std::transform(j.rbegin(), j.rend(),
@@ -171,7 +172,7 @@ void from_json(const BasicJsonType& j, std::valarray<T>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
     l.resize(j.size());
     std::transform(j.begin(), j.end(), std::begin(l),
@@ -268,7 +269,7 @@ void())
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
 
     from_json_array_impl(j, arr, priority_tag<3> {});
@@ -287,7 +288,7 @@ auto from_json(BasicJsonType&& j, identity_tag<std::array<T, N>> tag)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
 
     return from_json_inplace_array_impl(std::forward<BasicJsonType>(j), tag, make_index_sequence<N> {});
@@ -298,7 +299,7 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::binary_t& bin)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_binary()))
     {
-        JSON_THROW(type_error::create(302, "type must be binary, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be binary, but is ", j.type_name()), &j));
     }
 
     bin = *j.template get_ptr<const typename BasicJsonType::binary_t*>();
@@ -310,7 +311,7 @@ void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_object()))
     {
-        JSON_THROW(type_error::create(302, "type must be object, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
     ConstructibleObjectType ret;
@@ -370,7 +371,7 @@ void from_json(const BasicJsonType& j, ArithmeticType& val)
         case value_t::binary:
         case value_t::discarded:
         default:
-            JSON_THROW(type_error::create(302, "type must be number, but is " + std::string(j.type_name()), j));
+            JSON_THROW(type_error::create(302, concat("type must be number, but is ", j.type_name()), &j));
     }
 }
 
@@ -411,7 +412,7 @@ auto from_json(BasicJsonType&& j, TupleRelated&& t)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
 
     return from_json_tuple_impl(std::forward<BasicJsonType>(j), std::forward<TupleRelated>(t), priority_tag<3> {});
@@ -424,14 +425,14 @@ void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>&
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
     m.clear();
     for (const auto& p : j)
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(p.type_name()), j));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -444,14 +445,14 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
-        JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be array, but is ", j.type_name()), &j));
     }
     m.clear();
     for (const auto& p : j)
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(p.type_name()), j));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -463,7 +464,7 @@ void from_json(const BasicJsonType& j, std_fs::path& p)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
     {
-        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
+        JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     p = *j.template get_ptr<const typename BasicJsonType::string_t*>();
 }

include/nlohmann/detail/exceptions.hpp

 #pragma once
 
+#include <cstddef> // nullptr_t
 #include <exception> // exception
 #include <stdexcept> // runtime_error
 #include <string> // to_string
@@ -9,6 +10,10 @@
 #include <nlohmann/detail/string_escape.hpp>
 #include <nlohmann/detail/input/position_t.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
+#include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/type_traits.hpp>
+#include <nlohmann/detail/string_concat.hpp>
+
 
 namespace nlohmann
 {
@@ -38,15 +43,20 @@ class exception : public std::exception
 
     static std::string name(const std::string& ename, int id_)
     {
-        return "[json.exception." + ename + "." + std::to_string(id_) + "] ";
+        return concat("[json.exception.", ename, '.', std::to_string(id_), "] ");
+    }
+
+    static std::string diagnostics(std::nullptr_t /*leaf_element*/)
+    {
+        return "";
     }
 
     template<typename BasicJsonType>
-    static std::string diagnostics(const BasicJsonType& leaf_element)
+    static std::string diagnostics(const BasicJsonType* leaf_element)
     {
 #if JSON_DIAGNOSTICS
         std::vector<std::string> tokens;
-        for (const auto* current = &leaf_element; current->m_parent != nullptr; current = current->m_parent)
+        for (const auto* current = leaf_element; current != nullptr && current->m_parent != nullptr; current = current->m_parent)
         {
             switch (current->m_parent->type())
             {
@@ -94,11 +104,12 @@ class exception : public std::exception
             return "";
         }
 
-        return "(" + std::accumulate(tokens.rbegin(), tokens.rend(), std::string{},
-                                     [](const std::string & a, const std::string & b)
+        auto str = std::accumulate(tokens.rbegin(), tokens.rend(), std::string{},
+                                   [](const std::string & a, const std::string & b)
         {
-            return a + "/" + detail::escape(b);
-        }) + ") ";
+            return concat(a, '/', detail::escape(b));
+        });
+        return concat('(', str, ") ");
 #else
         static_cast<void>(leaf_element);
         return "";
@@ -124,20 +135,20 @@ class parse_error : public exception
     @param[in] what_arg  the explanatory string
     @return parse_error object
     */
-    template<typename BasicJsonType>
-    static parse_error create(int id_, const position_t& pos, const std::string& what_arg, const BasicJsonType& context)
+    template<typename BasicJsonContext, enable_if_t<is_basic_json_context<BasicJsonContext>::value, int> = 0>
+    static parse_error create(int id_, const position_t& pos, const std::string& what_arg, BasicJsonContext context)
     {
-        std::string w = exception::name("parse_error", id_) + "parse error" +
-                        position_string(pos) + ": " + exception::diagnostics(context) + what_arg;
+        std::string w = concat(exception::name("parse_error", id_), "parse error",
+                               position_string(pos), ": ", exception::diagnostics(context), what_arg);
         return {id_, pos.chars_read_total, w.c_str()};
     }
 
-    template<typename BasicJsonType>
-    static parse_error create(int id_, std::size_t byte_, const std::string& what_arg, const BasicJsonType& context)
+    template<typename BasicJsonContext, enable_if_t<is_basic_json_context<BasicJsonContext>::value, int> = 0>
+    static parse_error create(int id_, std::size_t byte_, const std::string& what_arg, BasicJsonContext context)
     {
-        std::string w = exception::name("parse_error", id_) + "parse error" +
-                        (byte_ != 0 ? (" at byte " + std::to_string(byte_)) : "") +
-                        ": " + exception::diagnostics(context) + what_arg;
+        std::string w = concat(exception::name("parse_error", id_), "parse error",
+                               (byte_ != 0 ? (concat(" at byte ", std::to_string(byte_))) : ""),
+                               ": ", exception::diagnostics(context), what_arg);
         return {id_, byte_, w.c_str()};
     }
 
@@ -158,8 +169,8 @@ class parse_error : public exception
 
     static std::string position_string(const position_t& pos)
     {
-        return " at line " + std::to_string(pos.lines_read + 1) +
-               ", column " + std::to_string(pos.chars_read_current_line);
+        return concat(" at line ", std::to_string(pos.lines_read + 1),
+                      ", column ", std::to_string(pos.chars_read_current_line));
     }
 };
 
@@ -168,10 +179,10 @@ class parse_error : public exception
 class invalid_iterator : public exception
 {
   public:
-    template<typename BasicJsonType>
-    static invalid_iterator create(int id_, const std::string& what_arg, const BasicJsonType& context)
+    template<typename BasicJsonContext, enable_if_t<is_basic_json_context<BasicJsonContext>::value, int> = 0>
+    static invalid_iterator create(int id_, const std::string& what_arg, BasicJsonContext context)
     {
-        std::string w = exception::name("invalid_iterator", id_) + exception::diagnostics(context) + what_arg;
+        std::string w = concat(exception::name("invalid_iterator", id_), exception::diagnostics(context), what_arg);
         return {id_, w.c_str()};
     }
 
@@ -186,10 +197,10 @@ class invalid_iterator : public exception
 class type_error : public exception
 {
   public:
-    template<typename BasicJsonType>
-    static type_error create(int id_, const std::string& what_arg, const BasicJsonType& context)
+    template<typename BasicJsonContext, enable_if_t<is_basic_json_context<BasicJsonContext>::value, int> = 0>
+    static type_error create(int id_, const std::string& what_arg, BasicJsonContext context)
     {
-        std::string w = exception::name("type_error", id_) + exception::diagnostics(context) + what_arg;
+        std::string w = concat(exception::name("type_error", id_), exception::diagnostics(context), what_arg);
         return {id_, w.c_str()};
     }
 
@@ -203,10 +214,10 @@ class type_error : public exception
 class out_of_range : public exception
 {
   public:
-    template<typename BasicJsonType>
-    static out_of_range create(int id_, const std::string& what_arg, const BasicJsonType& context)
+    template<typename BasicJsonContext, enable_if_t<is_basic_json_context<BasicJsonContext>::value, int> = 0>
+    static out_of_range create(int id_, const std::string& what_arg, BasicJsonContext context)
     {
-        std::string w = exception::name("out_of_range", id_) + exception::diagnostics(context) + what_arg;
+        std::string w = concat(exception::name("out_of_range", id_), exception::diagnostics(context), what_arg);
         return {id_, w.c_str()};
     }
 
@@ -220,10 +231,10 @@ class out_of_range : public exception
 class other_error : public exception
 {
   public:
-    template<typename BasicJsonType>
-    static other_error create(int id_, const std::string& what_arg, const BasicJsonType& context)
+    template<typename BasicJsonContext, enable_if_t<is_basic_json_context<BasicJsonContext>::value, int> = 0>
+    static other_error create(int id_, const std::string& what_arg, BasicJsonContext context)
     {
-        std::string w = exception::name("other_error", id_) + exception::diagnostics(context) + what_arg;
+        std::string w = concat(exception::name("other_error", id_), exception::diagnostics(context), what_arg);
         return {id_, w.c_str()};
     }
 

include/nlohmann/detail/input/binary_reader.hpp

@@ -20,6 +20,7 @@
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/is_sax.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
 namespace nlohmann
@@ -139,8 +140,8 @@ class binary_reader
 
             if (JSON_HEDLEY_UNLIKELY(current != std::char_traits<char_type>::eof()))
             {
-                return sax->parse_error(chars_read, get_token_string(),
-                                        parse_error::create(110, chars_read, exception_message(format, "expected end of input; last byte: 0x" + get_token_string(), "value"), BasicJsonType()));
+                return sax->parse_error(chars_read, get_token_string(), parse_error::create(110, chars_read,
+                                        exception_message(format, concat("expected end of input; last byte: 0x", get_token_string()), "value"), nullptr));
             }
         }
 
@@ -216,7 +217,8 @@ class binary_reader
         if (JSON_HEDLEY_UNLIKELY(len < 1))
         {
             auto last_token = get_token_string();
-            return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::bson, "string length must be at least 1, is " + std::to_string(len), "string"), BasicJsonType()));
+            return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                    exception_message(input_format_t::bson, concat("string length must be at least 1, is ", std::to_string(len)), "string"), nullptr));
         }
 
         return get_string(input_format_t::bson, len - static_cast<NumberType>(1), result) && get() != std::char_traits<char_type>::eof();
@@ -237,7 +239,8 @@ class binary_reader
         if (JSON_HEDLEY_UNLIKELY(len < 0))
         {
             auto last_token = get_token_string();
-            return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::bson, "byte array length cannot be negative, is " + std::to_string(len), "binary"), BasicJsonType()));
+            return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                    exception_message(input_format_t::bson, concat("byte array length cannot be negative, is ", std::to_string(len)), "binary"), nullptr));
         }
 
         // All BSON binary values have a subtype
@@ -319,7 +322,9 @@ class binary_reader
             {
                 std::array<char, 3> cr{{}};
                 static_cast<void>((std::snprintf)(cr.data(), cr.size(), "%.2hhX", static_cast<unsigned char>(element_type))); // NOLINT(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
-                return sax->parse_error(element_type_parse_position, std::string(cr.data()), parse_error::create(114, element_type_parse_position, "Unsupported BSON record type 0x" + std::string(cr.data()), BasicJsonType()));
+                std::string cr_str{cr.data()};
+                return sax->parse_error(element_type_parse_position, cr_str,
+                                        parse_error::create(114, element_type_parse_position, concat("Unsupported BSON record type 0x", cr_str), nullptr));
             }
         }
     }
@@ -719,7 +724,8 @@ class binary_reader
                     case cbor_tag_handler_t::error:
                     {
                         auto last_token = get_token_string();
-                        return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::cbor, "invalid byte: 0x" + last_token, "value"), BasicJsonType()));
+                        return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                                exception_message(input_format_t::cbor, concat("invalid byte: 0x", last_token), "value"), nullptr));
                     }
 
                     case cbor_tag_handler_t::ignore:
@@ -876,7 +882,8 @@ class binary_reader
             default: // anything else (0xFF is handled inside the other types)
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::cbor, "invalid byte: 0x" + last_token, "value"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                        exception_message(input_format_t::cbor, concat("invalid byte: 0x", last_token), "value"), nullptr));
             }
         }
     }
@@ -971,7 +978,8 @@ class binary_reader
             default:
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read, exception_message(input_format_t::cbor, "expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x" + last_token, "string"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                        exception_message(input_format_t::cbor, concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -1070,7 +1078,8 @@ class binary_reader
             default:
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read, exception_message(input_format_t::cbor, "expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x" + last_token, "binary"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                        exception_message(input_format_t::cbor, concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
             }
         }
     }
@@ -1540,7 +1549,8 @@ class binary_reader
             default: // anything else
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::msgpack, "invalid byte: 0x" + last_token, "value"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                        exception_message(input_format_t::msgpack, concat("invalid byte: 0x", last_token), "value"), nullptr));
             }
         }
     }
@@ -1622,7 +1632,8 @@ class binary_reader
             default:
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read, exception_message(input_format_t::msgpack, "expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0x" + last_token, "string"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                        exception_message(input_format_t::msgpack, concat("expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -1872,7 +1883,8 @@ class binary_reader
 
             default:
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read, exception_message(input_format_t::ubjson, "expected length type specification (U, i, I, l, L); last byte: 0x" + last_token, "string"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                        exception_message(input_format_t::ubjson, concat("expected length type specification (U, i, I, l, L); last byte: 0x", last_token), "string"), nullptr));
         }
     }
 
@@ -1942,7 +1954,8 @@ class binary_reader
             default:
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read, exception_message(input_format_t::ubjson, "expected length type specification (U, i, I, l, L) after '#'; last byte: 0x" + last_token, "size"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                        exception_message(input_format_t::ubjson, concat("expected length type specification (U, i, I, l, L) after '#'; last byte: 0x", last_token), "size"), nullptr));
             }
         }
     }
@@ -1980,7 +1993,8 @@ class binary_reader
                     return false;
                 }
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::ubjson, "expected '#' after type information; last byte: 0x" + last_token, "size"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                        exception_message(input_format_t::ubjson, concat("expected '#' after type information; last byte: 0x", last_token), "size"), nullptr));
             }
 
             return get_ubjson_size_value(result.first);
@@ -2070,7 +2084,8 @@ class binary_reader
                 if (JSON_HEDLEY_UNLIKELY(current > 127))
                 {
                     auto last_token = get_token_string();
-                    return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read, exception_message(input_format_t::ubjson, "byte after 'C' must be in range 0x00..0x7F; last byte: 0x" + last_token, "char"), BasicJsonType()));
+                    return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                            exception_message(input_format_t::ubjson, concat("byte after 'C' must be in range 0x00..0x7F; last byte: 0x", last_token), "char"), nullptr));
                 }
                 string_t s(1, static_cast<typename string_t::value_type>(current));
                 return sax->string(s);
@@ -2091,7 +2106,8 @@ class binary_reader
             default: // anything else
             {
                 auto last_token = get_token_string();
-                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read, exception_message(input_format_t::ubjson, "invalid byte: 0x" + last_token, "value"), BasicJsonType()));
+                return sax->parse_error(chars_read, last_token, parse_error::create(112, chars_read,
+                                        exception_message(input_format_t::ubjson, concat("invalid byte: 0x", last_token), "value"), nullptr));
             }
         }
     }
@@ -2269,7 +2285,8 @@ class binary_reader
 
         if (JSON_HEDLEY_UNLIKELY(result_remainder != token_type::end_of_input))
         {
-            return sax->parse_error(chars_read, number_string, parse_error::create(115, chars_read, exception_message(input_format_t::ubjson, "invalid number text: " + number_lexer.get_token_string(), "high-precision number"), BasicJsonType()));
+            return sax->parse_error(chars_read, number_string, parse_error::create(115, chars_read,
+                                    exception_message(input_format_t::ubjson, concat("invalid number text: ", number_lexer.get_token_string()), "high-precision number"), nullptr));
         }
 
         switch (result_number)
@@ -2295,7 +2312,8 @@ class binary_reader
             case token_type::end_of_input:
             case token_type::literal_or_value:
             default:
-                return sax->parse_error(chars_read, number_string, parse_error::create(115, chars_read, exception_message(input_format_t::ubjson, "invalid number text: " + number_lexer.get_token_string(), "high-precision number"), BasicJsonType()));
+                return sax->parse_error(chars_read, number_string, parse_error::create(115, chars_read,
+                                        exception_message(input_format_t::ubjson, concat("invalid number text: ", number_lexer.get_token_string()), "high-precision number"), nullptr));
         }
     }
 
@@ -2451,7 +2469,7 @@ class binary_reader
         if (JSON_HEDLEY_UNLIKELY(current == std::char_traits<char_type>::eof()))
         {
             return sax->parse_error(chars_read, "<end of file>",
-                                    parse_error::create(110, chars_read, exception_message(format, "unexpected end of input", context), BasicJsonType()));
+                                    parse_error::create(110, chars_read, exception_message(format, "unexpected end of input", context), nullptr));
         }
         return true;
     }
@@ -2501,7 +2519,7 @@ class binary_reader
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
         }
 
-        return error_msg + " " + context + ": " + detail;
+        return concat(error_msg, ' ', context, ": ", detail);
     }
 
   private:

include/nlohmann/detail/input/json_sax.hpp

@@ -7,6 +7,7 @@
 
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 
 namespace nlohmann
 {
@@ -224,7 +225,7 @@ class json_sax_dom_parser
 
         if (JSON_HEDLEY_UNLIKELY(len != static_cast<std::size_t>(-1) && len > ref_stack.back()->max_size()))
         {
-            JSON_THROW(out_of_range::create(408, "excessive object size: " + std::to_string(len), *ref_stack.back()));
+            JSON_THROW(out_of_range::create(408, concat("excessive object size: ", std::to_string(len)), ref_stack.back()));
         }
 
         return true;
@@ -250,7 +251,7 @@ class json_sax_dom_parser
 
         if (JSON_HEDLEY_UNLIKELY(len != static_cast<std::size_t>(-1) && len > ref_stack.back()->max_size()))
         {
-            JSON_THROW(out_of_range::create(408, "excessive array size: " + std::to_string(len), *ref_stack.back()));
+            JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
         return true;
@@ -405,7 +406,7 @@ class json_sax_dom_callback_parser
         // check object limit
         if (ref_stack.back() && JSON_HEDLEY_UNLIKELY(len != static_cast<std::size_t>(-1) && len > ref_stack.back()->max_size()))
         {
-            JSON_THROW(out_of_range::create(408, "excessive object size: " + std::to_string(len), *ref_stack.back()));
+            JSON_THROW(out_of_range::create(408, concat("excessive object size: ", std::to_string(len)), ref_stack.back()));
         }
 
         return true;
@@ -475,7 +476,7 @@ class json_sax_dom_callback_parser
         // check array limit
         if (ref_stack.back() && JSON_HEDLEY_UNLIKELY(len != static_cast<std::size_t>(-1) && len > ref_stack.back()->max_size()))
         {
-            JSON_THROW(out_of_range::create(408, "excessive array size: " + std::to_string(len), *ref_stack.back()));
+            JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
         return true;

include/nlohmann/detail/input/parser.hpp

@@ -13,6 +13,7 @@
 #include <nlohmann/detail/input/lexer.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/is_sax.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
 namespace nlohmann
@@ -95,7 +96,7 @@ class parser
                 sdp.parse_error(m_lexer.get_position(),
                                 m_lexer.get_token_string(),
                                 parse_error::create(101, m_lexer.get_position(),
-                                                    exception_message(token_type::end_of_input, "value"), BasicJsonType()));
+                                                    exception_message(token_type::end_of_input, "value"), nullptr));
             }
 
             // in case of an error, return discarded value
@@ -122,7 +123,7 @@ class parser
             {
                 sdp.parse_error(m_lexer.get_position(),
                                 m_lexer.get_token_string(),
-                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), BasicJsonType()));
+                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
             }
 
             // in case of an error, return discarded value
@@ -160,7 +161,7 @@ class parser
         {
             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::end_of_input, "value"), BasicJsonType()));
+                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
         }
 
         return result;
@@ -206,7 +207,7 @@ class parser
                         {
                             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"), BasicJsonType()));
+                                                    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())))
                         {
@@ -218,7 +219,7 @@ class parser
                         {
                             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"), BasicJsonType()));
+                                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
                         }
 
                         // remember we are now inside an object
@@ -261,7 +262,7 @@ class parser
                         {
                             return sax->parse_error(m_lexer.get_position(),
                                                     m_lexer.get_token_string(),
-                                                    out_of_range::create(406, "number overflow parsing '" + m_lexer.get_token_string() + "'", BasicJsonType()));
+                                                    out_of_range::create(406, concat("number overflow parsing '", m_lexer.get_token_string(), '\''), nullptr));
                         }
 
                         if (JSON_HEDLEY_UNLIKELY(!sax->number_float(res, m_lexer.get_string())))
@@ -331,7 +332,7 @@ class parser
                         // using "uninitialized" to avoid "expected" message
                         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::uninitialized, "value"), BasicJsonType()));
+                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::uninitialized, "value"), nullptr));
                     }
 
                     case token_type::uninitialized:
@@ -345,7 +346,7 @@ class parser
                     {
                         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::literal_or_value, "value"), BasicJsonType()));
+                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr));
                     }
                 }
             }
@@ -391,7 +392,7 @@ class parser
 
                 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::end_array, "array"), BasicJsonType()));
+                                        parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_array, "array"), nullptr));
             }
 
             // states.back() is false -> object
@@ -404,7 +405,7 @@ class parser
                 {
                     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"), BasicJsonType()));
+                                            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())))
@@ -417,7 +418,7 @@ class parser
                 {
                     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"), BasicJsonType()));
+                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
                 }
 
                 // parse values
@@ -445,7 +446,7 @@ class parser
 
             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::end_object, "object"), BasicJsonType()));
+                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_object, "object"), nullptr));
         }
     }
 
@@ -461,24 +462,24 @@ class parser
 
         if (!context.empty())
         {
-            error_msg += "while parsing " + context + " ";
+            error_msg += concat("while parsing ", context, ' ');
         }
 
         error_msg += "- ";
 
         if (last_token == token_type::parse_error)
         {
-            error_msg += std::string(m_lexer.get_error_message()) + "; last read: '" +
-                         m_lexer.get_token_string() + "'";
+            error_msg += concat(m_lexer.get_error_message(), "; last read: '",
+                                m_lexer.get_token_string(), '\'');
         }
         else
         {
-            error_msg += "unexpected " + std::string(lexer_t::token_type_name(last_token));
+            error_msg += concat("unexpected ", lexer_t::token_type_name(last_token));
         }
 
         if (expected != token_type::uninitialized)
         {
-            error_msg += "; expected " + std::string(lexer_t::token_type_name(expected));
+            error_msg += concat("; expected ", lexer_t::token_type_name(expected));
         }
 
         return error_msg;

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -285,7 +285,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
             }
 
             case value_t::null:
-                JSON_THROW(invalid_iterator::create(214, "cannot get value", *m_object));
+                JSON_THROW(invalid_iterator::create(214, "cannot get value", m_object));
 
             case value_t::string:
             case value_t::boolean:
@@ -301,7 +301,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
                     return *m_object;
                 }
 
-                JSON_THROW(invalid_iterator::create(214, "cannot get value", *m_object));
+                JSON_THROW(invalid_iterator::create(214, "cannot get value", m_object));
             }
         }
     }
@@ -343,7 +343,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
                     return m_object;
                 }
 
-                JSON_THROW(invalid_iterator::create(214, "cannot get value", *m_object));
+                JSON_THROW(invalid_iterator::create(214, "cannot get value", m_object));
             }
         }
     }
@@ -460,7 +460,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         // if objects are not the same, the comparison is undefined
         if (JSON_HEDLEY_UNLIKELY(m_object != other.m_object))
         {
-            JSON_THROW(invalid_iterator::create(212, "cannot compare iterators of different containers", *m_object));
+            JSON_THROW(invalid_iterator::create(212, "cannot compare iterators of different containers", m_object));
         }
 
         JSON_ASSERT(m_object != nullptr);
@@ -505,7 +505,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         // if objects are not the same, the comparison is undefined
         if (JSON_HEDLEY_UNLIKELY(m_object != other.m_object))
         {
-            JSON_THROW(invalid_iterator::create(212, "cannot compare iterators of different containers", *m_object));
+            JSON_THROW(invalid_iterator::create(212, "cannot compare iterators of different containers", m_object));
         }
 
         JSON_ASSERT(m_object != nullptr);
@@ -513,7 +513,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(213, "cannot compare order of object iterators", *m_object));
+                JSON_THROW(invalid_iterator::create(213, "cannot compare order of object iterators", m_object));
 
             case value_t::array:
                 return (m_it.array_iterator < other.m_it.array_iterator);
@@ -569,7 +569,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(209, "cannot use offsets with object iterators", *m_object));
+                JSON_THROW(invalid_iterator::create(209, "cannot use offsets with object iterators", m_object));
 
             case value_t::array:
             {
@@ -648,7 +648,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(209, "cannot use offsets with object iterators", *m_object));
+                JSON_THROW(invalid_iterator::create(209, "cannot use offsets with object iterators", m_object));
 
             case value_t::array:
                 return m_it.array_iterator - other.m_it.array_iterator;
@@ -677,13 +677,13 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
         switch (m_object->m_type)
         {
             case value_t::object:
-                JSON_THROW(invalid_iterator::create(208, "cannot use operator[] for object iterators", *m_object));
+                JSON_THROW(invalid_iterator::create(208, "cannot use operator[] for object iterators", m_object));
 
             case value_t::array:
                 return *std::next(m_it.array_iterator, n);
 
             case value_t::null:
-                JSON_THROW(invalid_iterator::create(214, "cannot get value", *m_object));
+                JSON_THROW(invalid_iterator::create(214, "cannot get value", m_object));
 
             case value_t::string:
             case value_t::boolean:
@@ -699,7 +699,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
                     return *m_object;
                 }
 
-                JSON_THROW(invalid_iterator::create(214, "cannot get value", *m_object));
+                JSON_THROW(invalid_iterator::create(214, "cannot get value", m_object));
             }
         }
     }
@@ -717,7 +717,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
             return m_it.object_iterator->first;
         }
 
-        JSON_THROW(invalid_iterator::create(207, "cannot use key() for non-object iterators", *m_object));
+        JSON_THROW(invalid_iterator::create(207, "cannot use key() for non-object iterators", m_object));
     }
 
     /*!

include/nlohmann/detail/meta/type_traits.hpp

@@ -44,6 +44,21 @@ template<typename> struct is_basic_json : std::false_type {};
 NLOHMANN_BASIC_JSON_TPL_DECLARATION
 struct is_basic_json<NLOHMANN_BASIC_JSON_TPL> : std::true_type {};
 
+// used by exceptions create() member functions
+// true_type for pointer to possibly cv-qualified basic_json or std::nullptr_t
+// false_type otherwise
+template<typename BasicJsonContext>
+struct is_basic_json_context :
+    std::integral_constant < bool,
+    is_basic_json<typename std::remove_cv<typename std::remove_pointer<BasicJsonContext>::type>::type>::value
+    || std::is_same<BasicJsonContext, std::nullptr_t>::value >
+{};
+
+template<typename> struct is_json_pointer : std::false_type {};
+
+template<typename RefStringType>
+struct is_json_pointer<json_pointer<RefStringType>> : std::true_type {};
+
 //////////////////////
 // json_ref helpers //
 //////////////////////

include/nlohmann/detail/output/binary_writer.hpp

@@ -12,6 +12,7 @@
 #include <nlohmann/detail/input/binary_reader.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/output/output_adapters.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 
 namespace nlohmann
 {
@@ -67,7 +68,7 @@ class binary_writer
             case value_t::discarded:
             default:
             {
-                JSON_THROW(type_error::create(317, "to serialize to BSON, top-level type must be object, but is " + std::string(j.type_name()), j));
+                JSON_THROW(type_error::create(317, concat("to serialize to BSON, top-level type must be object, but is ", j.type_name()), &j));
             }
         }
     }
@@ -937,7 +938,7 @@ class binary_writer
         const auto it = name.find(static_cast<typename string_t::value_type>(0));
         if (JSON_HEDLEY_UNLIKELY(it != BasicJsonType::string_t::npos))
         {
-            JSON_THROW(out_of_range::create(409, "BSON key cannot contain code point U+0000 (at byte " + std::to_string(it) + ")", j));
+            JSON_THROW(out_of_range::create(409, concat("BSON key cannot contain code point U+0000 (at byte ", std::to_string(it), ")"), &j));
             static_cast<void>(j);
         }
 
@@ -1062,7 +1063,7 @@ class binary_writer
         }
         else
         {
-            JSON_THROW(out_of_range::create(407, "integer number " + std::to_string(j.m_value.number_unsigned) + " cannot be represented by BSON as it does not fit int64", j));
+            JSON_THROW(out_of_range::create(407, concat("integer number ", std::to_string(j.m_value.number_unsigned), " cannot be represented by BSON as it does not fit int64"), &j));
         }
     }
 

include/nlohmann/detail/output/serializer.hpp

@@ -19,6 +19,7 @@
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/output/binary_writer.hpp>
 #include <nlohmann/detail/output/output_adapters.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
 namespace nlohmann
@@ -500,7 +501,7 @@ class serializer
                     {
                         case error_handler_t::strict:
                         {
-                            JSON_THROW(type_error::create(316, "invalid UTF-8 byte at index " + std::to_string(i) + ": 0x" + hex_bytes(byte | 0), BasicJsonType()));
+                            JSON_THROW(type_error::create(316, concat("invalid UTF-8 byte at index ", std::to_string(i), ": 0x", hex_bytes(byte | 0)), nullptr));
                         }
 
                         case error_handler_t::ignore:
@@ -592,7 +593,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    JSON_THROW(type_error::create(316, "incomplete UTF-8 string; last byte: 0x" + hex_bytes(static_cast<std::uint8_t>(s.back() | 0)), BasicJsonType()));
+                    JSON_THROW(type_error::create(316, concat("incomplete UTF-8 string; last byte: 0x", hex_bytes(static_cast<std::uint8_t>(s.back() | 0))), nullptr));
                 }
 
                 case error_handler_t::ignore:

include/nlohmann/detail/string_concat.hpp

+#pragma once
+
+#include <cstring> // strlen
+#include <string> // string
+#include <utility> // forward
+
+#include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/detected.hpp>
+
+namespace nlohmann
+{
+namespace detail
+{
+
+inline std::size_t concat_length()
+{
+    return 0;
+}
+
+template<typename... Args>
+inline std::size_t concat_length(const char* cstr, Args&& ... rest);
+
+template<typename StringType, typename... Args>
+inline std::size_t concat_length(const StringType& str, Args&& ... rest);
+
+template<typename... Args>
+inline std::size_t concat_length(const char /*c*/, Args&& ... rest)
+{
+    return 1 + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename... Args>
+inline std::size_t concat_length(const char* cstr, Args&& ... rest)
+{
+    // cppcheck-suppress ignoredReturnValue
+    return ::strlen(cstr) + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename StringType, typename... Args>
+inline std::size_t concat_length(const StringType& str, Args&& ... rest)
+{
+    return str.size() + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename OutStringType>
+inline void concat_into(OutStringType& /*out*/)
+{}
+
+template<typename StringType, typename Arg>
+using string_can_append = decltype(std::declval<StringType&>().append(std::declval < Arg && > ()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append = is_detected<string_can_append, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_op = decltype(std::declval<StringType&>() += std::declval < Arg && > ());
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_op = is_detected<string_can_append_op, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_iter = decltype(std::declval<StringType&>().append(std::declval<const Arg&>().begin(), std::declval<const Arg&>().end()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_iter = is_detected<string_can_append_iter, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_data = decltype(std::declval<StringType&>().append(std::declval<const Arg&>().data(), std::declval<const Arg&>().size()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_data = is_detected<string_can_append_data, StringType, Arg>;
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && detect_string_can_append_op<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, Arg && arg, Args && ... rest);
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && detect_string_can_append_iter<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, const Arg& arg, Args && ... rest);
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && !detect_string_can_append_iter<OutStringType, Arg>::value
+                         && detect_string_can_append_data<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, const Arg& arg, Args && ... rest);
+
+template<typename OutStringType, typename Arg, typename... Args,
+         enable_if_t<detect_string_can_append<OutStringType, Arg>::value, int> = 0>
+inline void concat_into(OutStringType& out, Arg && arg, Args && ... rest)
+{
+    out.append(std::forward<Arg>(arg));
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && detect_string_can_append_op<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, Arg&& arg, Args&& ... rest)
+{
+    out += std::forward<Arg>(arg);
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && detect_string_can_append_iter<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, const Arg& arg, Args&& ... rest)
+{
+    out.append(arg.begin(), arg.end());
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && !detect_string_can_append_iter<OutStringType, Arg>::value
+                         && detect_string_can_append_data<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, const Arg& arg, Args&& ... rest)
+{
+    out.append(arg.data(), arg.size());
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template<typename OutStringType = std::string, typename... Args>
+inline OutStringType concat(Args && ... args)
+{
+    OutStringType str;
+    str.reserve(concat_length(std::forward<Args>(args)...));
+    concat_into(str, std::forward<Args>(args)...);
+    return str;
+}
+
+} // namespace detail
+} // namespace nlohmann

include/nlohmann/detail/string_escape.hpp

 #pragma once
 
-#include <string>
 #include <nlohmann/detail/macro_scope.hpp>
 
 namespace nlohmann
@@ -21,12 +20,13 @@ enforced with an assertion.**
 
 @since version 2.0.0
 */
-inline void replace_substring(std::string& s, const std::string& f,
-                              const std::string& t)
+template<typename StringType>
+inline void replace_substring(StringType& s, const StringType& f,
+                              const StringType& t)
 {
     JSON_ASSERT(!f.empty());
     for (auto pos = s.find(f);                // find first occurrence of f
-            pos != std::string::npos;         // make sure f was found
+            pos != StringType::npos;          // make sure f was found
             s.replace(pos, f.size(), t),      // replace with t, and
             pos = s.find(f, pos + t.size()))  // find next occurrence of f
     {}
@@ -39,10 +39,11 @@ inline void replace_substring(std::string& s, const std::string& f,
  *
  * Note the order of escaping "~" to "~0" and "/" to "~1" is important.
  */
-inline std::string escape(std::string s)
+template<typename StringType>
+inline StringType escape(StringType s)
 {
-    replace_substring(s, "~", "~0");
-    replace_substring(s, "/", "~1");
+    replace_substring(s, StringType{"~"}, StringType{"~0"});
+    replace_substring(s, StringType{"/"}, StringType{"~1"});
     return s;
 }
 
@@ -53,10 +54,11 @@ inline std::string escape(std::string s)
  *
  * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
  */
-static void unescape(std::string& s)
+template<typename StringType>
+static void unescape(StringType& s)
 {
-    replace_substring(s, "~1", "/");
-    replace_substring(s, "~0", "~");
+    replace_substring(s, StringType{"~1"}, StringType{"/"});
+    replace_substring(s, StringType{"~0"}, StringType{"~"});
 }
 
 } // namespace detail

test/src/unit-alt-string.cpp

@@ -49,6 +49,8 @@ class alt_string
   public:
     using value_type = std::string::value_type;
 
+    static constexpr auto npos = static_cast<std::size_t>(-1);
+
     alt_string(const char* str): str_impl(str) {}
     alt_string(const char* str, std::size_t count): str_impl(str, count) {}
     alt_string(size_t count, char chr): str_impl(count, chr) {}
@@ -144,11 +146,38 @@ class alt_string
         str_impl.clear();
     }
 
-    const value_type* data()
+    const value_type* data() const
     {
         return str_impl.data();
     }
 
+    bool empty() const
+    {
+        return str_impl.empty();
+    }
+
+    std::size_t find(const alt_string& str, std::size_t pos = 0) const
+    {
+        return str_impl.find(str.str_impl, pos);
+    }
+
+    std::size_t find_first_of(char c, std::size_t pos = 0) const
+    {
+        return str_impl.find_first_of(c, pos);
+    }
+
+    alt_string substr(std::size_t pos = 0, std::size_t count = npos) const
+    {
+        std::string s = str_impl.substr(pos, count);
+        return {s.data(), s.size()};
+    }
+
+    alt_string& replace(std::size_t pos, std::size_t count, const alt_string& str)
+    {
+        str_impl.replace(pos, count, str.str_impl);
+        return *this;
+    }
+
   private:
     std::string str_impl {};
 
@@ -296,4 +325,20 @@ TEST_CASE("alternative string type")
             CHECK_FALSE(const_doc["Who are you?"] == "I'm Bruce Wayne");
         }
     }
+
+    SECTION("JSON pointer")
+    {
+        // conversion from json to alt_json fails to compile (see #3425);
+        // attempted fix(*) produces: [[['b','a','r'],['b','a','z']]] (with each char being an integer)
+        // (*) disable implicit conversion for json_refs of any basic_json type
+        // alt_json j = R"(
+        // {
+        //     "foo": ["bar", "baz"]
+        // }
+        // )"_json;
+        auto j = alt_json::parse(R"({"foo": ["bar", "baz"]})");
+
+        CHECK(j.at(alt_json::json_pointer("/foo/0")) == j["foo"][0]);
+        CHECK(j.at(alt_json::json_pointer("/foo/1")) == j["foo"][1]);
+    }
 }

test/src/unit-convenience.cpp

@@ -37,6 +37,84 @@ using nlohmann::json;
 
 namespace
 {
+struct alt_string_iter
+{
+    alt_string_iter() = default;
+    alt_string_iter(const char* cstr)
+        : impl(cstr)
+    {}
+
+    void reserve(std::size_t s)
+    {
+        impl.reserve(s);
+    }
+
+    template<typename Iter>
+    void append(Iter first, Iter last)
+    {
+        impl.append(first, last);
+    }
+
+    std::string::const_iterator begin() const
+    {
+        return impl.begin();
+    }
+
+    std::string::const_iterator end() const
+    {
+        return impl.end();
+    }
+
+    std::size_t size() const
+    {
+        return impl.size();
+    }
+
+    alt_string_iter& operator+=(const char c)
+    {
+        impl += c;
+        return *this;
+    }
+
+    std::string impl{};
+};
+
+struct alt_string_data
+{
+    alt_string_data() = default;
+    alt_string_data(const char* cstr)
+        : impl(cstr)
+    {}
+
+    void reserve(std::size_t s)
+    {
+        impl.reserve(s);
+    }
+
+    void append(const char* p, std::size_t s)
+    {
+        impl.append(p, s);
+    }
+
+    const char* data() const
+    {
+        return impl.data();
+    }
+
+    std::size_t size() const
+    {
+        return impl.size();
+    }
+
+    alt_string_data& operator+=(const char c)
+    {
+        impl += c;
+        return *this;
+    }
+
+    std::string impl{};
+};
+
 void check_escaped(const char* original, const char* escaped = "", bool ensure_ascii = false);
 void check_escaped(const char* original, const char* escaped, const bool ensure_ascii)
 {
@@ -110,4 +188,39 @@ TEST_CASE("convenience functions")
 
         CHECK_THROWS_WITH_AS(check_escaped("\xC2"), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
     }
+
+    SECTION("string concat")
+    {
+        using nlohmann::detail::concat;
+
+        const char* expected = "Hello, world!";
+        alt_string_iter hello_iter{"Hello, "};
+        alt_string_data hello_data{"Hello, "};
+        std::string world = "world";
+
+        SECTION("std::string")
+        {
+            std::string str1 = concat(hello_iter, world, '!');
+            std::string str2 = concat(hello_data, world, '!');
+            std::string str3 = concat("Hello, ", world, '!');
+
+            CHECK(str1 == expected);
+            CHECK(str2 == expected);
+            CHECK(str3 == expected);
+        }
+
+        SECTION("alt_string_iter")
+        {
+            alt_string_iter str = concat<alt_string_iter>(hello_iter, world, '!');
+
+            CHECK(str.impl == expected);
+        }
+
+        SECTION("alt_string_data")
+        {
+            alt_string_data str = concat<alt_string_data>(hello_data, world, '!');
+
+            CHECK(str.impl == expected);
+        }
+    }
 }

test/src/unit-json_pointer.cpp

@@ -660,4 +660,54 @@ TEST_CASE("JSON pointers")
         CHECK(j[ptr] == j["object"]["/"]);
         CHECK(ptr.to_string() == "/object/~1");
     }
+
+    SECTION("equality comparison")
+    {
+        auto ptr1 = json::json_pointer("/foo/bar");
+        auto ptr2 = json::json_pointer("/foo/bar");
+
+        CHECK(ptr1 == ptr2);
+        CHECK_FALSE(ptr1 != ptr2);
+    }
+
+    SECTION("backwards compatibility and mixing")
+    {
+        json j = R"(
+        {
+            "foo": ["bar", "baz"]
+        }
+        )"_json;
+
+        using nlohmann::ordered_json;
+        using json_ptr_str = nlohmann::json_pointer<std::string>;
+        using json_ptr_j = nlohmann::json_pointer<json>;
+        using json_ptr_oj = nlohmann::json_pointer<ordered_json>;
+
+        CHECK(std::is_same<json_ptr_str::string_t, json::json_pointer::string_t>::value);
+        CHECK(std::is_same<json_ptr_str::string_t, ordered_json::json_pointer::string_t>::value);
+        CHECK(std::is_same<json_ptr_str::string_t, json_ptr_j::string_t>::value);
+        CHECK(std::is_same<json_ptr_str::string_t, json_ptr_oj::string_t>::value);
+
+        json_ptr_str ptr{"/foo/0"};
+        json_ptr_j ptr_j{"/foo/0"};
+        json_ptr_oj ptr_oj{"/foo/0"};
+
+        CHECK(j.contains(ptr));
+        CHECK(j.contains(ptr_j));
+        CHECK(j.contains(ptr_oj));
+
+        CHECK(j.at(ptr) == j.at(ptr_j));
+        CHECK(j.at(ptr) == j.at(ptr_oj));
+
+        CHECK(j[ptr] == j[ptr_j]);
+        CHECK(j[ptr] == j[ptr_oj]);
+
+        CHECK(j.value(ptr, "x") == j.value(ptr_j, "x"));
+        CHECK(j.value(ptr, "x") == j.value(ptr_oj, "x"));
+
+        CHECK(ptr == ptr_j);
+        CHECK(ptr == ptr_oj);
+        CHECK_FALSE(ptr != ptr_j);
+        CHECK_FALSE(ptr != ptr_oj);
+    }
 }

test/src/unit-regression2.cpp

@@ -760,7 +760,6 @@ TEST_CASE("regression tests 2")
     {
         std::string p = "/root";
 
-        // matching types
         json test1;
         test1[json::json_pointer(p)] = json::object();
         CHECK(test1.dump() == "{\"root\":{}}");
@@ -769,10 +768,11 @@ TEST_CASE("regression tests 2")
         test2[ordered_json::json_pointer(p)] = json::object();
         CHECK(test2.dump() == "{\"root\":{}}");
 
-        // mixed type - the JSON Pointer is implicitly converted into a string "/root"
+        // json::json_pointer and ordered_json::json_pointer are the same type; behave as above
         ordered_json test3;
         test3[json::json_pointer(p)] = json::object();
-        CHECK(test3.dump() == "{\"/root\":{}}");
+        CHECK(std::is_same<json::json_pointer::string_t, ordered_json::json_pointer::string_t>::value);
+        CHECK(test3.dump() == "{\"root\":{}}");
     }
 
     SECTION("issue #2982 - to_{binary format} does not provide a mechanism for specifying a custom allocator for the returned type")