fix page level -- operator

2 files changed, 11 insertions, 6 deletions

diff --git a/arch/x86_64/include/arch/memory/paging/page_table.hpp b/arch/x86_64/include/arch/memory/paging/page_table.hpp
index feb327a..e6542a2 100644
--- a/arch/x86_64/include/arch/memory/paging/page_table.hpp
+++ b/arch/x86_64/include/arch/memory/paging/page_table.hpp
@@ -58,7 +58,7 @@ namespace teachos::arch::memory::paging
      *
      * @param table_index Index of this page table in the page table one level lower.
      */
-    auto next_table(std::size_t table_index) const -> std::optional<page_table_handle>;
+    auto next_table(std::size_t table_index) -> std::optional<page_table_handle>;
 
     /**
      * @brief Index operator overload to access specific immutable entry directy.
@@ -83,7 +83,7 @@ namespace teachos::arch::memory::paging
      * @param value Value we want to decrement on
      * @return level New level value decrement by one, meaning the level is also decrement by one Level4 --> Level3, ...
      */
-    friend auto operator--(level value, int) -> level;
+    friend auto operator--(level & value, int) -> level;
 
   private:
     page_table * handle;  ///< Handle to underlying page table, can never be null (invariant ensured by constructor)
diff --git a/arch/x86_64/src/memory/paging/page_table.cpp b/arch/x86_64/src/memory/paging/page_table.cpp
index 555d38a..b229ff7 100644
--- a/arch/x86_64/src/memory/paging/page_table.cpp
+++ b/arch/x86_64/src/memory/paging/page_table.cpp
@@ -99,25 +99,30 @@ namespace teachos::arch::memory::paging
 
   auto page_table_handle::zero_entries() -> void { handle->zero_entries(); }
 
-  auto page_table_handle::next_table(std::size_t table_index) const -> std::optional<page_table_handle>
+  auto page_table_handle::next_table(std::size_t table_index) -> std::optional<page_table_handle>
   {
     exception_handling::assert(handle_level != page_table_handle::LEVEL1,
                                "[Page Table] Attempted to call next_table on level 1 page table");
     auto next_table = handle->next_table(table_index);
     if (next_table)
     {
-      return page_table_handle{next_table.value(), handle_level--};
+      handle_level--;
+      return page_table_handle{next_table.value(), handle_level};
     }
     return std::nullopt;
   }
 
   auto page_table_handle::operator[](std::size_t index) const -> entry { return handle->operator[](index); }
 
-  auto operator--(page_table_handle::level value, int) -> page_table_handle::level
+  auto operator--(page_table_handle::level & value, int) -> page_table_handle::level
   {
     exception_handling::assert(value != page_table_handle::LEVEL1,
                                "[Page table] Attempted to decrement enum to value outside of range");
+
+    page_table_handle::level original_value = value;
     auto new_value = static_cast<std::underlying_type<page_table_handle::level>::type>(value);
-    return static_cast<page_table_handle::level>(--new_value);
+    value = static_cast<page_table_handle::level>(--new_value);
+
+    return original_value;
   }
 }  // namespace teachos::arch::memory::paging