Reimplement value() access functions (#3663)

* Reimplement value() access functions

* Merges the 'const char *' with the 'ValueType &&' overloads.
* Fixes ambiguities when default value is 0.
* Fixes 'no matching function' error when specifying ValueType template
  parameter.
* Fixes incorrect template parameter order in previous overloads.

* Add additional value() tests

* Make JSON_MultipleHeaders visible to unit tests

Define the macro JSON_TEST_USING_MULTIPLE_HEADERS to 0/1 depending on
JSON_MultipleHeaders.

* Add type_traits unit test

* Update documentation

docs/mkdocs/docs/api/basic_json/value.md

@@ -7,7 +7,7 @@ ValueType value(const typename object_t::key_type& key,
                 ValueType&& default_value) const;
 
 // (2)
-template<class KeyType, class ValueType>
+template<class ValueType, class KeyType>
 ValueType value(KeyType&& key,
                 ValueType&& default_value) const;
 
@@ -155,5 +155,5 @@ changes to any JSON value.
 ## Version history
 
 1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version 3.11.0.
-2. Added in version 3.11.0.
+2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2.
 3. Added in version 2.0.2.

include/nlohmann/detail/meta/type_traits.hpp

@@ -684,5 +684,57 @@ inline constexpr bool value_in_range_of(T val)
     return value_in_range_of_impl1<OfType, T>::test(val);
 }
 
+template<bool Value>
+using bool_constant = std::integral_constant<bool, Value>;
+
+///////////////////////////////////////////////////////////////////////////////
+// is_c_string
+///////////////////////////////////////////////////////////////////////////////
+
+namespace impl
+{
+
+template<typename T>
+inline constexpr bool is_c_string()
+{
+    using TUnExt = typename std::remove_extent<T>::type;
+    using TUnCVExt = typename std::remove_cv<TUnExt>::type;
+    using TUnPtr = typename std::remove_pointer<T>::type;
+    using TUnCVPtr = typename std::remove_cv<TUnPtr>::type;
+    return
+        (std::is_array<T>::value && std::is_same<TUnCVExt, char>::value)
+        || (std::is_pointer<T>::value && std::is_same<TUnCVPtr, char>::value);
+}
+
+}  // namespace impl
+
+// checks whether T is a [cv] char */[cv] char[] C string
+template<typename T>
+struct is_c_string : bool_constant<impl::is_c_string<T>()> {};
+
+template<typename T>
+using is_c_string_uncvref = is_c_string<uncvref_t<T>>;
+
+///////////////////////////////////////////////////////////////////////////////
+// is_transparent
+///////////////////////////////////////////////////////////////////////////////
+
+namespace impl
+{
+
+template<typename T>
+inline constexpr bool is_transparent()
+{
+    return is_detected<detect_is_transparent, T>::value;
+}
+
+}  // namespace impl
+
+// checks whether T has a member named is_transparent
+template<typename T>
+struct is_transparent : bool_constant<impl::is_transparent<T>()> {};
+
+///////////////////////////////////////////////////////////////////////////////
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/json.hpp

@@ -2194,14 +2194,24 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         JSON_THROW(type_error::create(305, detail::concat("cannot use operator[] with a string argument with ", type_name()), this));
     }
 
+  private:
+    template<typename KeyType>
+    using is_comparable_with_object_key = detail::is_comparable <
+        object_comparator_t, const typename object_t::key_type&, KeyType >;
+
+    template<typename ValueType>
+    using value_return_type = std::conditional <
+        detail::is_c_string_uncvref<ValueType>::value,
+        string_t, typename std::decay<ValueType>::type >;
+
+  public:
     /// @brief access specified object element with default value
     /// @sa https://json.nlohmann.me/api/basic_json/value/
-    // this is the value(const typename object_t::key_type&) overload
+    template < class ValueType, detail::enable_if_t <
-    template < class KeyType, class ValueType, detail::enable_if_t <
+                   !detail::is_transparent<object_comparator_t>::value
-                   std::is_same<KeyType, typename object_t::key_type>::value
                    && detail::is_getable<basic_json_t, ValueType>::value
-                   && !std::is_same<value_t, ValueType>::value, int > = 0 >
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
-    typename std::decay<ValueType>::type value(const KeyType& key, ValueType && default_value) const
+    ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
     {
         // value only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
@@ -2210,7 +2220,32 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             const auto it = find(key);
             if (it != end())
             {
-                return it->template get<typename std::decay<ValueType>::type>();
+                return it->template get<ValueType>();
+            }
+
+            return default_value;
+        }
+
+        JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
+    }
+
+    /// @brief access specified object element with default value
+    /// @sa https://json.nlohmann.me/api/basic_json/value/
+    template < class ValueType, class ReturnType = typename value_return_type<ValueType>::type,
+               detail::enable_if_t <
+                   !detail::is_transparent<object_comparator_t>::value
+                   && detail::is_getable<basic_json_t, ReturnType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
+    ReturnType value(const typename object_t::key_type& key, ValueType && default_value) const
+    {
+        // value only works for objects
+        if (JSON_HEDLEY_LIKELY(is_object()))
+        {
+            // if key is found, return value and given default value otherwise
+            const auto it = find(key);
+            if (it != end())
+            {
+                return it->template get<ReturnType>();
             }
 
             return std::forward<ValueType>(default_value);
@@ -2221,36 +2256,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief access specified object element with default value
     /// @sa https://json.nlohmann.me/api/basic_json/value/
-    /// overload for a default value of type const char*
+    template < class ValueType, class KeyType, detail::enable_if_t <
-    string_t value(const typename object_t::key_type& key, const char* default_value) const
+                   detail::is_transparent<object_comparator_t>::value
-    {
+                   && !detail::is_json_pointer<KeyType>::value
-        return value(key, string_t(default_value));
+                   && is_comparable_with_object_key<KeyType>::value
-    }
+                   && detail::is_getable<basic_json_t, ValueType>::value
-
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
-    // these two functions, in conjunction with value(const KeyType &, ValueType &&),
+    ValueType value(KeyType && key, const ValueType& default_value) const
-    // resolve an ambiguity that would otherwise occur between the json_pointer and
-    // typename object_t::key_type & overloads
-    template < class ValueType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value
-                   && !std::is_same<value_t, ValueType>::value, int > = 0 >
-    typename std::decay<ValueType>::type value(const char* key, ValueType && default_value) const
-    {
-        return value(typename object_t::key_type(key), std::forward<ValueType>(default_value));
-    }
-
-    string_t value(const char* key, const char* default_value) const
-    {
-        return value(typename object_t::key_type(key), string_t(default_value));
-    }
-
-    /// @brief access specified object element with default value
-    /// @sa https://json.nlohmann.me/api/basic_json/value/
-    /// using std::is_convertible in a std::enable_if will fail when using explicit conversions
-    template < class KeyType, class ValueType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value
-                   && !std::is_same<value_t, ValueType>::value
-                   && detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int > = 0 >
-    typename std::decay<ValueType>::type value(KeyType && key, ValueType && default_value) const
     {
         // value only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
@@ -2259,7 +2271,34 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             const auto it = find(std::forward<KeyType>(key));
             if (it != end())
             {
-                return it->template get<typename std::decay<ValueType>::type>();
+                return it->template get<ValueType>();
+            }
+
+            return default_value;
+        }
+
+        JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
+    }
+
+    /// @brief access specified object element via JSON Pointer with default value
+    /// @sa https://json.nlohmann.me/api/basic_json/value/
+    template < class ValueType, class KeyType, class ReturnType = typename value_return_type<ValueType>::type,
+               detail::enable_if_t <
+                   detail::is_transparent<object_comparator_t>::value
+                   && !detail::is_json_pointer<KeyType>::value
+                   && is_comparable_with_object_key<KeyType>::value
+                   && detail::is_getable<basic_json_t, ReturnType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
+    ReturnType value(KeyType && key, ValueType && default_value) const
+    {
+        // value only works for objects
+        if (JSON_HEDLEY_LIKELY(is_object()))
+        {
+            // if key is found, return value and given default value otherwise
+            const auto it = find(std::forward<KeyType>(key));
+            if (it != end())
+            {
+                return it->template get<ReturnType>();
             }
 
             return std::forward<ValueType>(default_value);
@@ -2268,20 +2307,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
     }
 
-    /// @brief access specified object element with default value
-    /// @sa https://json.nlohmann.me/api/basic_json/value/
-    /// overload for a default value of type const char*
-    template < class KeyType, detail::enable_if_t <
-                   !detail::is_json_pointer<KeyType>::value, int > = 0 >
-    string_t value(KeyType && key, const char* default_value) const
-    {
-        return value(std::forward<KeyType>(key), string_t(default_value));
-    }
-
     /// @brief access specified object element via JSON Pointer with default value
     /// @sa https://json.nlohmann.me/api/basic_json/value/
     template < class ValueType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value, int> = 0 >
+                   detail::is_getable<basic_json_t, ValueType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
     ValueType value(const json_pointer& ptr, const ValueType& default_value) const
     {
         // value only works for objects
@@ -2301,29 +2331,50 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
     }
 
+    /// @brief access specified object element via JSON Pointer with default value
+    /// @sa https://json.nlohmann.me/api/basic_json/value/
+    template < class ValueType, class ReturnType = typename value_return_type<ValueType>::type,
+               detail::enable_if_t <
+                   detail::is_getable<basic_json_t, ReturnType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
+    ReturnType value(const json_pointer& ptr, ValueType && default_value) const
+    {
+        // value only works for objects
+        if (JSON_HEDLEY_LIKELY(is_object()))
+        {
+            // if pointer resolves a value, return it or use default value
+            JSON_TRY
+            {
+                return ptr.get_checked(this).template get<ReturnType>();
+            }
+            JSON_INTERNAL_CATCH (out_of_range&)
+            {
+                return std::forward<ValueType>(default_value);
+            }
+        }
+
+        JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
+    }
+
     template < class ValueType, class BasicJsonType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value, int> = 0 >
+                   detail::is_basic_json<BasicJsonType>::value
+                   && detail::is_getable<basic_json_t, ValueType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
     ValueType value(const ::nlohmann::json_pointer<BasicJsonType>& ptr, const ValueType& default_value) const
     {
         return value(ptr.convert(), default_value);
     }
 
-    /// @brief access specified object element via JSON Pointer with default value
+    template < class ValueType, class BasicJsonType, class ReturnType = typename value_return_type<ValueType>::type,
-    /// @sa https://json.nlohmann.me/api/basic_json/value/
+               detail::enable_if_t <
-    /// overload for a default value of type const char*
+                   detail::is_basic_json<BasicJsonType>::value
-    JSON_HEDLEY_NON_NULL(3)
+                   && detail::is_getable<basic_json_t, ReturnType>::value
-    string_t value(const json_pointer& ptr, const char* default_value) const
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
-    {
-        return value(ptr, string_t(default_value));
-    }
-
-    template<typename BasicJsonType>
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
-    JSON_HEDLEY_NON_NULL(3)
+    ReturnType value(const ::nlohmann::json_pointer<BasicJsonType>& ptr, ValueType && default_value) const
-    string_t value(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr, const char* default_value) const
     {
-        return value(ptr.convert(), default_value);
+        return value(ptr.convert(), std::forward<ValueType>(default_value));
     }
 
     /// @brief access the first element

single_include/nlohmann/json.hpp

@@ -4059,6 +4059,58 @@ inline constexpr bool value_in_range_of(T val)
     return value_in_range_of_impl1<OfType, T>::test(val);
 }
 
+template<bool Value>
+using bool_constant = std::integral_constant<bool, Value>;
+
+///////////////////////////////////////////////////////////////////////////////
+// is_c_string
+///////////////////////////////////////////////////////////////////////////////
+
+namespace impl
+{
+
+template<typename T>
+inline constexpr bool is_c_string()
+{
+    using TUnExt = typename std::remove_extent<T>::type;
+    using TUnCVExt = typename std::remove_cv<TUnExt>::type;
+    using TUnPtr = typename std::remove_pointer<T>::type;
+    using TUnCVPtr = typename std::remove_cv<TUnPtr>::type;
+    return
+        (std::is_array<T>::value && std::is_same<TUnCVExt, char>::value)
+        || (std::is_pointer<T>::value && std::is_same<TUnCVPtr, char>::value);
+}
+
+}  // namespace impl
+
+// checks whether T is a [cv] char */[cv] char[] C string
+template<typename T>
+struct is_c_string : bool_constant<impl::is_c_string<T>()> {};
+
+template<typename T>
+using is_c_string_uncvref = is_c_string<uncvref_t<T>>;
+
+///////////////////////////////////////////////////////////////////////////////
+// is_transparent
+///////////////////////////////////////////////////////////////////////////////
+
+namespace impl
+{
+
+template<typename T>
+inline constexpr bool is_transparent()
+{
+    return is_detected<detect_is_transparent, T>::value;
+}
+
+}  // namespace impl
+
+// checks whether T has a member named is_transparent
+template<typename T>
+struct is_transparent : bool_constant<impl::is_transparent<T>()> {};
+
+///////////////////////////////////////////////////////////////////////////////
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
 
@@ -21274,14 +21326,24 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         JSON_THROW(type_error::create(305, detail::concat("cannot use operator[] with a string argument with ", type_name()), this));
     }
 
+  private:
+    template<typename KeyType>
+    using is_comparable_with_object_key = detail::is_comparable <
+        object_comparator_t, const typename object_t::key_type&, KeyType >;
+
+    template<typename ValueType>
+    using value_return_type = std::conditional <
+        detail::is_c_string_uncvref<ValueType>::value,
+        string_t, typename std::decay<ValueType>::type >;
+
+  public:
     /// @brief access specified object element with default value
     /// @sa https://json.nlohmann.me/api/basic_json/value/
-    // this is the value(const typename object_t::key_type&) overload
+    template < class ValueType, detail::enable_if_t <
-    template < class KeyType, class ValueType, detail::enable_if_t <
+                   !detail::is_transparent<object_comparator_t>::value
-                   std::is_same<KeyType, typename object_t::key_type>::value
                    && detail::is_getable<basic_json_t, ValueType>::value
-                   && !std::is_same<value_t, ValueType>::value, int > = 0 >
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
-    typename std::decay<ValueType>::type value(const KeyType& key, ValueType && default_value) const
+    ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
     {
         // value only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
@@ -21290,7 +21352,32 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             const auto it = find(key);
             if (it != end())
             {
-                return it->template get<typename std::decay<ValueType>::type>();
+                return it->template get<ValueType>();
+            }
+
+            return default_value;
+        }
+
+        JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
+    }
+
+    /// @brief access specified object element with default value
+    /// @sa https://json.nlohmann.me/api/basic_json/value/
+    template < class ValueType, class ReturnType = typename value_return_type<ValueType>::type,
+               detail::enable_if_t <
+                   !detail::is_transparent<object_comparator_t>::value
+                   && detail::is_getable<basic_json_t, ReturnType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
+    ReturnType value(const typename object_t::key_type& key, ValueType && default_value) const
+    {
+        // value only works for objects
+        if (JSON_HEDLEY_LIKELY(is_object()))
+        {
+            // if key is found, return value and given default value otherwise
+            const auto it = find(key);
+            if (it != end())
+            {
+                return it->template get<ReturnType>();
             }
 
             return std::forward<ValueType>(default_value);
@@ -21301,36 +21388,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief access specified object element with default value
     /// @sa https://json.nlohmann.me/api/basic_json/value/
-    /// overload for a default value of type const char*
+    template < class ValueType, class KeyType, detail::enable_if_t <
-    string_t value(const typename object_t::key_type& key, const char* default_value) const
+                   detail::is_transparent<object_comparator_t>::value
-    {
+                   && !detail::is_json_pointer<KeyType>::value
-        return value(key, string_t(default_value));
+                   && is_comparable_with_object_key<KeyType>::value
-    }
+                   && detail::is_getable<basic_json_t, ValueType>::value
-
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
-    // these two functions, in conjunction with value(const KeyType &, ValueType &&),
+    ValueType value(KeyType && key, const ValueType& default_value) const
-    // resolve an ambiguity that would otherwise occur between the json_pointer and
-    // typename object_t::key_type & overloads
-    template < class ValueType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value
-                   && !std::is_same<value_t, ValueType>::value, int > = 0 >
-    typename std::decay<ValueType>::type value(const char* key, ValueType && default_value) const
-    {
-        return value(typename object_t::key_type(key), std::forward<ValueType>(default_value));
-    }
-
-    string_t value(const char* key, const char* default_value) const
-    {
-        return value(typename object_t::key_type(key), string_t(default_value));
-    }
-
-    /// @brief access specified object element with default value
-    /// @sa https://json.nlohmann.me/api/basic_json/value/
-    /// using std::is_convertible in a std::enable_if will fail when using explicit conversions
-    template < class KeyType, class ValueType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value
-                   && !std::is_same<value_t, ValueType>::value
-                   && detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int > = 0 >
-    typename std::decay<ValueType>::type value(KeyType && key, ValueType && default_value) const
     {
         // value only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
@@ -21339,7 +21403,34 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             const auto it = find(std::forward<KeyType>(key));
             if (it != end())
             {
-                return it->template get<typename std::decay<ValueType>::type>();
+                return it->template get<ValueType>();
+            }
+
+            return default_value;
+        }
+
+        JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
+    }
+
+    /// @brief access specified object element via JSON Pointer with default value
+    /// @sa https://json.nlohmann.me/api/basic_json/value/
+    template < class ValueType, class KeyType, class ReturnType = typename value_return_type<ValueType>::type,
+               detail::enable_if_t <
+                   detail::is_transparent<object_comparator_t>::value
+                   && !detail::is_json_pointer<KeyType>::value
+                   && is_comparable_with_object_key<KeyType>::value
+                   && detail::is_getable<basic_json_t, ReturnType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
+    ReturnType value(KeyType && key, ValueType && default_value) const
+    {
+        // value only works for objects
+        if (JSON_HEDLEY_LIKELY(is_object()))
+        {
+            // if key is found, return value and given default value otherwise
+            const auto it = find(std::forward<KeyType>(key));
+            if (it != end())
+            {
+                return it->template get<ReturnType>();
             }
 
             return std::forward<ValueType>(default_value);
@@ -21348,20 +21439,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
     }
 
-    /// @brief access specified object element with default value
-    /// @sa https://json.nlohmann.me/api/basic_json/value/
-    /// overload for a default value of type const char*
-    template < class KeyType, detail::enable_if_t <
-                   !detail::is_json_pointer<KeyType>::value, int > = 0 >
-    string_t value(KeyType && key, const char* default_value) const
-    {
-        return value(std::forward<KeyType>(key), string_t(default_value));
-    }
-
     /// @brief access specified object element via JSON Pointer with default value
     /// @sa https://json.nlohmann.me/api/basic_json/value/
     template < class ValueType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value, int> = 0 >
+                   detail::is_getable<basic_json_t, ValueType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
     ValueType value(const json_pointer& ptr, const ValueType& default_value) const
     {
         // value only works for objects
@@ -21381,29 +21463,50 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
     }
 
+    /// @brief access specified object element via JSON Pointer with default value
+    /// @sa https://json.nlohmann.me/api/basic_json/value/
+    template < class ValueType, class ReturnType = typename value_return_type<ValueType>::type,
+               detail::enable_if_t <
+                   detail::is_getable<basic_json_t, ReturnType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
+    ReturnType value(const json_pointer& ptr, ValueType && default_value) const
+    {
+        // value only works for objects
+        if (JSON_HEDLEY_LIKELY(is_object()))
+        {
+            // if pointer resolves a value, return it or use default value
+            JSON_TRY
+            {
+                return ptr.get_checked(this).template get<ReturnType>();
+            }
+            JSON_INTERNAL_CATCH (out_of_range&)
+            {
+                return std::forward<ValueType>(default_value);
+            }
+        }
+
+        JSON_THROW(type_error::create(306, detail::concat("cannot use value() with ", type_name()), this));
+    }
+
     template < class ValueType, class BasicJsonType, detail::enable_if_t <
-                   detail::is_getable<basic_json_t, ValueType>::value, int> = 0 >
+                   detail::is_basic_json<BasicJsonType>::value
+                   && detail::is_getable<basic_json_t, ValueType>::value
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
     ValueType value(const ::nlohmann::json_pointer<BasicJsonType>& ptr, const ValueType& default_value) const
     {
         return value(ptr.convert(), default_value);
     }
 
-    /// @brief access specified object element via JSON Pointer with default value
+    template < class ValueType, class BasicJsonType, class ReturnType = typename value_return_type<ValueType>::type,
-    /// @sa https://json.nlohmann.me/api/basic_json/value/
+               detail::enable_if_t <
-    /// overload for a default value of type const char*
+                   detail::is_basic_json<BasicJsonType>::value
-    JSON_HEDLEY_NON_NULL(3)
+                   && detail::is_getable<basic_json_t, ReturnType>::value
-    string_t value(const json_pointer& ptr, const char* default_value) const
+                   && !std::is_same<value_t, detail::uncvref_t<ValueType>>::value, int > = 0 >
-    {
-        return value(ptr, string_t(default_value));
-    }
-
-    template<typename BasicJsonType>
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
-    JSON_HEDLEY_NON_NULL(3)
+    ReturnType value(const ::nlohmann::json_pointer<BasicJsonType>& ptr, ValueType && default_value) const
-    string_t value(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr, const char* default_value) const
     {
-        return value(ptr.convert(), default_value);
+        return value(ptr.convert(), std::forward<ValueType>(default_value));
     }
 
     /// @brief access the first element

tests/CMakeLists.txt

@@ -37,7 +37,8 @@ endif()
 add_library(test_main OBJECT src/unit.cpp)
 target_compile_definitions(test_main PUBLIC
     DOCTEST_CONFIG_SUPER_FAST_ASSERTS
-    JSON_TEST_KEEP_MACROS)
+    JSON_TEST_KEEP_MACROS
+    JSON_TEST_USING_MULTIPLE_HEADERS=$<BOOL:${JSON_MultipleHeaders}>)
 target_compile_features(test_main PRIVATE cxx_std_11)
 target_compile_options(test_main PUBLIC
     $<$<CXX_COMPILER_ID:MSVC>:/EHsc;$<$<CONFIG:Release>:/Od>>

tests/src/unit-element_access2.cpp

@@ -14,6 +14,9 @@
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
+// build test with C++14
+// JSON_HAS_CPP_14
+
 TEST_CASE_TEMPLATE("element access 2", Json, nlohmann::json, nlohmann::ordered_json)
 {
     SECTION("object")
@@ -1488,3 +1491,304 @@ TEST_CASE_TEMPLATE("element access 2 (throwing tests)", Json, nlohmann::json, nl
     }
 }
 #endif
+
+// TODO(falbrechtskirchinger) merge with the other test case; clean up
+TEST_CASE_TEMPLATE("element access 2 (additional value() tests)", Json, nlohmann::json, nlohmann::ordered_json)
+{
+    using string_t = typename Json::string_t;
+    using number_integer_t = typename Json::number_integer_t;
+
+    // test assumes string_t and object_t::key_type are the same
+    REQUIRE(std::is_same<string_t, typename Json::object_t::key_type>::value);
+
+    Json j
+    {
+        {"foo", "bar"},
+        {"baz", 42}
+    };
+
+    const char* cpstr = "default";
+    const char castr[] = "default"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+    string_t str = "default";
+
+    number_integer_t integer = 69;
+    std::size_t size = 69;
+
+    SECTION("deduced ValueType")
+    {
+        SECTION("literal key")
+        {
+            CHECK(j.value("foo", "default") == "bar");
+            CHECK(j.value("foo", cpstr) == "bar");
+            CHECK(j.value("foo", castr) == "bar");
+            CHECK(j.value("foo", str) == "bar");
+            // this test is in fact different than the one below,
+            // because of 0 considering const char * overloads
+            // where as any other number does not
+            CHECK(j.value("baz", 0) == 42);
+            CHECK(j.value("baz", 47) == 42);
+            CHECK(j.value("baz", integer) == 42);
+            CHECK(j.value("baz", size) == 42);
+
+            CHECK(j.value("bar", "default") == "default");
+            CHECK(j.value("bar", 0) == 0);
+            CHECK(j.value("bar", 47) == 47);
+            CHECK(j.value("bar", integer) == integer);
+            CHECK(j.value("bar", size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().value("foo", "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().value("foo", str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+        SECTION("const char * key")
+        {
+            const char* key = "foo";
+            const char* key2 = "baz";
+            const char* key_notfound = "bar";
+
+            CHECK(j.value(key, "default") == "bar");
+            CHECK(j.value(key, cpstr) == "bar");
+            CHECK(j.value(key, castr) == "bar");
+            CHECK(j.value(key, str) == "bar");
+            CHECK(j.value(key2, 0) == 42);
+            CHECK(j.value(key2, 47) == 42);
+            CHECK(j.value(key2, integer) == 42);
+            CHECK(j.value(key2, size) == 42);
+
+            CHECK(j.value(key_notfound, "default") == "default");
+            CHECK(j.value(key_notfound, 0) == 0);
+            CHECK(j.value(key_notfound, 47) == 47);
+            CHECK(j.value(key_notfound, integer) == integer);
+            CHECK(j.value(key_notfound, size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().value(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().value(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+        SECTION("const char(&)[] key")
+        {
+            const char key[] = "foo"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+            const char key2[] = "baz"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+            const char key_notfound[] = "bar"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+
+            CHECK(j.value(key, "default") == "bar");
+            CHECK(j.value(key, cpstr) == "bar");
+            CHECK(j.value(key, castr) == "bar");
+            CHECK(j.value(key, str) == "bar");
+            CHECK(j.value(key2, 0) == 42);
+            CHECK(j.value(key2, 47) == 42);
+            CHECK(j.value(key2, integer) == 42);
+            CHECK(j.value(key2, size) == 42);
+
+            CHECK(j.value(key_notfound, "default") == "default");
+            CHECK(j.value(key_notfound, 0) == 0);
+            CHECK(j.value(key_notfound, 47) == 47);
+            CHECK(j.value(key_notfound, integer) == integer);
+            CHECK(j.value(key_notfound, size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().value(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().value(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+        SECTION("string_t/object_t::key_type key")
+        {
+            string_t key = "foo";
+            string_t key2 = "baz";
+            string_t key_notfound = "bar";
+
+            CHECK(j.value(key, "default") == "bar");
+            CHECK(j.value(key, cpstr) == "bar");
+            CHECK(j.value(key, castr) == "bar");
+            CHECK(j.value(key, str) == "bar");
+            CHECK(j.value(key2, 0) == 42);
+            CHECK(j.value(key2, 47) == 42);
+            CHECK(j.value(key2, integer) == 42);
+            CHECK(j.value(key2, size) == 42);
+
+            CHECK(j.value(key_notfound, "default") == "default");
+            CHECK(j.value(key_notfound, 0) == 0);
+            CHECK(j.value(key_notfound, 47) == 47);
+            CHECK(j.value(key_notfound, integer) == integer);
+            CHECK(j.value(key_notfound, size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().value(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().value(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+#ifdef JSON_HAS_CPP_17
+        SECTION("std::string_view key")
+        {
+            std::string_view key = "foo";
+            std::string_view key2 = "baz";
+            std::string_view key_notfound = "bar";
+
+            CHECK(j.value(key, "default") == "bar");
+            CHECK(j.value(key, cpstr) == "bar");
+            CHECK(j.value(key, castr) == "bar");
+            CHECK(j.value(key, str) == "bar");
+            CHECK(j.value(key2, 0) == 42);
+            CHECK(j.value(key2, 47) == 42);
+            CHECK(j.value(key2, integer) == 42);
+            CHECK(j.value(key2, size) == 42);
+
+            CHECK(j.value(key_notfound, "default") == "default");
+            CHECK(j.value(key_notfound, 0) == 0);
+            CHECK(j.value(key_notfound, 47) == 47);
+            CHECK(j.value(key_notfound, integer) == integer);
+            CHECK(j.value(key_notfound, size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().value(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().value(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+#endif
+    }
+
+    SECTION("explicit ValueType")
+    {
+        SECTION("literal key")
+        {
+            CHECK(j.template value<string_t>("foo", "default") == "bar");
+            CHECK(j.template value<string_t>("foo", cpstr) == "bar");
+            CHECK(j.template value<string_t>("foo", castr) == "bar");
+            CHECK(j.template value<string_t>("foo", str) == "bar");
+            CHECK(j.template value<number_integer_t>("baz", 0) == 42);
+            CHECK(j.template value<number_integer_t>("baz", 47) == 42);
+            CHECK(j.template value<number_integer_t>("baz", integer) == 42);
+            CHECK(j.template value<std::size_t>("baz", 0) == 42);
+            CHECK(j.template value<std::size_t>("baz", 47) == 42);
+            CHECK(j.template value<std::size_t>("baz", size) == 42);
+
+            CHECK(j.template value<string_t>("bar", "default") == "default");
+            CHECK(j.template value<number_integer_t>("bar", 0) == 0);
+            CHECK(j.template value<number_integer_t>("bar", 47) == 47);
+            CHECK(j.template value<number_integer_t>("bar", integer) == integer);
+            CHECK(j.template value<std::size_t>("bar", 0) == 0);
+            CHECK(j.template value<std::size_t>("bar", 47) == 47);
+            CHECK(j.template value<std::size_t>("bar", size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>("foo", "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>("foo", str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+        SECTION("const char * key")
+        {
+            const char* key = "foo";
+            const char* key2 = "baz";
+            const char* key_notfound = "bar";
+
+            CHECK(j.template value<string_t>(key, "default") == "bar");
+            CHECK(j.template value<string_t>(key, cpstr) == "bar");
+            CHECK(j.template value<string_t>(key, castr) == "bar");
+            CHECK(j.template value<string_t>(key, str) == "bar");
+            CHECK(j.template value<number_integer_t>(key2, 0) == 42);
+            CHECK(j.template value<number_integer_t>(key2, 47) == 42);
+            CHECK(j.template value<number_integer_t>(key2, integer) == 42);
+            CHECK(j.template value<std::size_t>(key2, 0) == 42);
+            CHECK(j.template value<std::size_t>(key2, 47) == 42);
+            CHECK(j.template value<std::size_t>(key2, size) == 42);
+
+            CHECK(j.template value<string_t>(key_notfound, "default") == "default");
+            CHECK(j.template value<number_integer_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<number_integer_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<number_integer_t>(key_notfound, integer) == integer);
+            CHECK(j.template value<std::size_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<std::size_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<std::size_t>(key_notfound, size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+        SECTION("const char(&)[] key")
+        {
+            const char key[] = "foo"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+            const char key2[] = "baz"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+            const char key_notfound[] = "bar"; // NOLINT(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+
+            CHECK(j.template value<string_t>(key, "default") == "bar");
+            CHECK(j.template value<string_t>(key, cpstr) == "bar");
+            CHECK(j.template value<string_t>(key, castr) == "bar");
+            CHECK(j.template value<string_t>(key, str) == "bar");
+            CHECK(j.template value<number_integer_t>(key2, 0) == 42);
+            CHECK(j.template value<number_integer_t>(key2, 47) == 42);
+            CHECK(j.template value<number_integer_t>(key2, integer) == 42);
+            CHECK(j.template value<std::size_t>(key2, 0) == 42);
+            CHECK(j.template value<std::size_t>(key2, 47) == 42);
+            CHECK(j.template value<std::size_t>(key2, size) == 42);
+
+            CHECK(j.template value<string_t>(key_notfound, "default") == "default");
+            CHECK(j.template value<number_integer_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<number_integer_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<number_integer_t>(key_notfound, integer) == integer);
+            CHECK(j.template value<std::size_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<std::size_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<std::size_t>(key_notfound, size) == size);
+
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+        SECTION("string_t/object_t::key_type key")
+        {
+            string_t key = "foo";
+            string_t key2 = "baz";
+            string_t key_notfound = "bar";
+
+            CHECK(j.template value<string_t>(key, "default") == "bar");
+            CHECK(j.template value<string_t>(key, cpstr) == "bar");
+            CHECK(j.template value<string_t>(key, castr) == "bar");
+            CHECK(j.template value<string_t>(key, str) == "bar");
+            CHECK(j.template value<number_integer_t>(key2, 0) == 42);
+            CHECK(j.template value<number_integer_t>(key2, 47) == 42);
+            CHECK(j.template value<std::size_t>(key2, 0) == 42);
+            CHECK(j.template value<std::size_t>(key2, 47) == 42);
+
+            CHECK(j.template value<string_t>(key_notfound, "default") == "default");
+            CHECK(j.template value<number_integer_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<number_integer_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<std::size_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<std::size_t>(key_notfound, 47) == 47);
+
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+
+#ifdef JSON_HAS_CPP_17
+        SECTION("std::string_view key")
+        {
+            std::string_view key = "foo";
+            std::string_view key2 = "baz";
+            std::string_view key_notfound = "bar";
+
+            CHECK(j.template value<string_t>(key, "default") == "bar");
+            CHECK(j.template value<string_t>(key, cpstr) == "bar");
+            CHECK(j.template value<string_t>(key, castr) == "bar");
+            CHECK(j.template value<string_t>(key, str) == "bar");
+            CHECK(j.template value<number_integer_t>(key2, 0) == 42);
+            CHECK(j.template value<number_integer_t>(key2, 47) == 42);
+            CHECK(j.template value<number_integer_t>(key2, integer) == 42);
+            CHECK(j.template value<std::size_t>(key2, 0) == 42);
+            CHECK(j.template value<std::size_t>(key2, 47) == 42);
+            CHECK(j.template value<std::size_t>(key2, size) == 42);
+
+            CHECK(j.template value<string_t>(key_notfound, "default") == "default");
+            CHECK(j.template value<number_integer_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<number_integer_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<number_integer_t>(key_notfound, integer) == integer);
+            CHECK(j.template value<std::size_t>(key_notfound, 0) == 0);
+            CHECK(j.template value<std::size_t>(key_notfound, 47) == 47);
+            CHECK(j.template value<std::size_t>(key_notfound, size) == size);
+
+            CHECK(j.template value<std::string_view>(key, "default") == "bar");
+            CHECK(j.template value<std::string_view>(key, cpstr) == "bar");
+            CHECK(j.template value<std::string_view>(key, castr) == "bar");
+            CHECK(j.template value<std::string_view>(key, str) == "bar");
+
+            CHECK(j.template value<std::string_view>(key_notfound, "default") == "default");
+
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, "default"), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+            CHECK_THROWS_WITH_AS(Json().template value<string_t>(key, str), "[json.exception.type_error.306] cannot use value() with null", typename Json::type_error&);
+        }
+#endif
+    }
+}

tests/src/unit-type_traits.cpp

@@ -0,0 +1,56 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.11.1
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2022 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#if JSON_TEST_USING_MULTIPLE_HEADERS
+    #include <nlohmann/detail/meta/type_traits.hpp>
+#else
+    #include <nlohmann/json.hpp>
+#endif
+
+TEST_CASE("type traits")
+{
+    SECTION("is_c_string")
+    {
+        using nlohmann::detail::is_c_string;
+        using nlohmann::detail::is_c_string_uncvref;
+
+        SECTION("char *")
+        {
+            CHECK(is_c_string<char*>::value);
+            CHECK(is_c_string<const char*>::value);
+            CHECK(is_c_string<char* const>::value);
+            CHECK(is_c_string<const char* const>::value);
+
+            CHECK_FALSE(is_c_string<char*&>::value);
+            CHECK_FALSE(is_c_string<const char*&>::value);
+            CHECK_FALSE(is_c_string<char* const&>::value);
+            CHECK_FALSE(is_c_string<const char* const&>::value);
+
+            CHECK(is_c_string_uncvref<char*&>::value);
+            CHECK(is_c_string_uncvref<const char*&>::value);
+            CHECK(is_c_string_uncvref<char* const&>::value);
+            CHECK(is_c_string_uncvref<const char* const&>::value);
+        }
+
+        SECTION("char[]")
+        {
+            // NOLINTBEGIN(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+            CHECK(is_c_string<char[]>::value);
+            CHECK(is_c_string<const char[]>::value);
+
+            CHECK_FALSE(is_c_string<char(&)[]>::value);
+            CHECK_FALSE(is_c_string<const char(&)[]>::value);
+
+            CHECK(is_c_string_uncvref<char(&)[]>::value);
+            CHECK(is_c_string_uncvref<const char(&)[]>::value);
+            // NOLINTEND(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+        }
+    }
+}