Remove need for object_t to be fully instanced, allowing unordered_map

the unordered_map template cannot be instanced with an incomplete type
so this patch replaces all places where this is made necessary with
alternative though equivilent types.

src/json.hpp

@@ -1003,7 +1003,7 @@ class basic_json
     */
     template <class CompatibleObjectType, typename
               std::enable_if<
-                  std::is_constructible<typename object_t::key_type, typename CompatibleObjectType::key_type>::value and
+                  std::is_constructible<StringType, typename CompatibleObjectType::key_type>::value and
                   std::is_constructible<basic_json, typename CompatibleObjectType::mapped_type>::value, int>::type
               = 0>
     basic_json(const CompatibleObjectType& val)
@@ -2203,7 +2203,7 @@ class basic_json
     /// get an object (explicit)
     template <class T, typename
               std::enable_if<
-                  std::is_convertible<typename object_t::key_type, typename T::key_type>::value and
+                  std::is_convertible<StringType, typename T::key_type>::value and
                   std::is_convertible<basic_json_t, typename T::mapped_type>::value
                   , int>::type = 0>
     T get_impl(T*) const
@@ -2751,7 +2751,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    reference at(const typename object_t::key_type& key)
+    reference at(const StringType& key)
     {
         // at only works for objects
         if (is_object())
@@ -2798,7 +2798,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    const_reference at(const typename object_t::key_type& key) const
+    const_reference at(const StringType& key) const
     {
         // at only works for objects
         if (is_object())
@@ -2928,7 +2928,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    reference operator[](const typename object_t::key_type& key)
+    reference operator[](const StringType& key)
     {
         // implicitly convert null to object
         if (is_null())
@@ -2975,7 +2975,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    const_reference operator[](const typename object_t::key_type& key) const
+    const_reference operator[](const StringType& key) const
     {
         // [] only works for objects
         if (is_object())
@@ -3133,7 +3133,7 @@ class basic_json
               std::enable_if<
                   std::is_convertible<basic_json_t, ValueType>::value
                   , int>::type = 0>
-    ValueType value(const typename object_t::key_type& key, ValueType default_value) const
+    ValueType value(const StringType& key, ValueType default_value) const
     {
         // at only works for objects
         if (is_object())
@@ -3159,7 +3159,7 @@ class basic_json
     @brief overload for a default value of type const char*
     @copydoc basic_json::value()
     */
-    string_t value(const typename object_t::key_type& key, const char* default_value) const
+    string_t value(const StringType& key, const char* default_value) const
     {
         return value(key, string_t(default_value));
     }
@@ -3469,7 +3469,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    size_type erase(const typename object_t::key_type& key)
+    size_type erase(const StringType& key)
     {
         // this erase only works for objects
         if (is_object())
@@ -3541,7 +3541,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    iterator find(typename object_t::key_type key)
+    iterator find(StringType key)
     {
         auto result = end();
 
@@ -3557,7 +3557,7 @@ class basic_json
     @brief find an element in a JSON object
     @copydoc find(typename object_t::key_type)
     */
-    const_iterator find(typename object_t::key_type key) const
+    const_iterator find(StringType key) const
     {
         auto result = cend();
 
@@ -3587,7 +3587,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    size_type count(typename object_t::key_type key) const
+    size_type count(StringType key) const
     {
         // return 0 for all nonobject types
         return is_object() ? m_value.object->count(key) : 0;
@@ -4220,7 +4220,7 @@ class basic_json
 
     @since version 1.0.0
     */
-    void push_back(const typename object_t::value_type& val)
+    void push_back(const std::pair<StringType, basic_json>& val)
     {
         // push_back only works for null objects or objects
         if (not(is_null() or is_object()))
@@ -4241,9 +4241,9 @@ class basic_json
 
     /*!
     @brief add an object to an object
-    @copydoc push_back(const typename object_t::value_type&)
+    @copydoc push_back(const std::pair<StringType, basic_json>&)
     */
-    reference operator+=(const typename object_t::value_type& val)
+    reference operator+=(const std::pair<StringType, basic_json>& val)
     {
         push_back(val);
         return operator[](val.first);
@@ -6083,7 +6083,7 @@ class basic_json
         }
 
         /// return the key of an object iterator
-        typename object_t::key_type key() const
+        StringType key() const
         {
             if (m_object->is_object())
             {
@@ -6340,7 +6340,7 @@ class basic_json
         }
 
         /// return the key of an object iterator
-        typename object_t::key_type key() const
+        StringType key() const
         {
             auto it = --this->base();
             return it.key();