refactor detail::stack::reserve_impl

include/boost/json/detail/impl/stack.hpp

@@ -29,6 +29,13 @@ struct stack::non_trivial<void>
     virtual non_trivial* relocate(void*) noexcept = 0;
 };
 
+template<>
+struct stack::non_trivial<void const>
+    : stack::non_trivial<void>
+{
+    non_trivial* relocate(void*) noexcept { return nullptr; }
+};
+
 template< class T >
 struct stack::non_trivial
     : stack::non_trivial<void>
@@ -63,9 +70,11 @@ reserve_impl(std::size_t n)
         std::memcpy(base, base_, size_);
 
         // copy non-trivials
+        non_trivial<void const> start;
+        start.next = nullptr;
+
         non_trivial<>* src = head_;
-        non_trivial<>* prev = nullptr;
-        non_trivial<>* head = nullptr;
+        non_trivial<>* prev = &start;
         while(src)
         {
             auto const next = src->next;
@@ -77,15 +86,11 @@ reserve_impl(std::size_t n)
             dest->offset = offset;
             dest->next = nullptr;
 
-            if( prev )
-                prev->next = dest;
-            else
-                head = dest;
-
+            prev->next = dest;
             prev = dest;
             src = next;
         }
-        head_ = head;
+        head_ = start.next;
 
         if(base_ != buf_)
             sp_->deallocate(base_, cap_);