diff --git a/membuffer/membuffer.h b/membuffer/membuffer.h
index 21ef016..dea0493 100644
--- a/membuffer/membuffer.h
+++ b/membuffer/membuffer.h
@@ -5,7 +5,7 @@
  */
 
 /* 
- * Copyright (c) 2012-2013, Tomasz Sowa
+ * Copyright (c) 2012-2021, Tomasz Sowa
  * All rights reserved.
  * 
  * Redistribution and use in source and binary forms, with or without
@@ -58,8 +58,8 @@ public:
 	{
 	public:
 
-		bool operator==(const iterator & i);
-		bool operator!=(const iterator & i);
+		bool operator==(const iterator & i) const;
+		bool operator!=(const iterator & i) const;
 
 		iterator & operator++(); 		// prefix  ++
 		iterator   operator++(int); 	// postfix ++
@@ -89,8 +89,8 @@ public:
 		const_iterator & operator=(const const_iterator & i);
 		const_iterator & operator=(const iterator & i);
 
-		bool operator==(const const_iterator & i);
-		bool operator!=(const const_iterator & i);
+		bool operator==(const const_iterator & i) const;
+		bool operator!=(const const_iterator & i) const;
 
 		const_iterator & operator++(); 		// prefix  ++
 		const_iterator   operator++(int); 	// postfix ++
@@ -264,7 +264,7 @@ item_type & MemBuffer<item_type, stack_size, heap_block_size>::iterator::operato
 
 
 template<typename item_type, size_t stack_size, size_t heap_block_size>
-bool MemBuffer<item_type, stack_size, heap_block_size>::iterator::operator==(const iterator & i)
+bool MemBuffer<item_type, stack_size, heap_block_size>::iterator::operator==(const iterator & i) const
 {
 	return	mem_buffer == i.mem_buffer && 
 			dynamic_array_index == i.dynamic_array_index &&
@@ -273,7 +273,7 @@ bool MemBuffer<item_type, stack_size, heap_block_size>::iterator::operator==(con
 
 
 template<typename item_type, size_t stack_size, size_t heap_block_size>
-bool MemBuffer<item_type, stack_size, heap_block_size>::iterator::operator!=(const iterator & i)
+bool MemBuffer<item_type, stack_size, heap_block_size>::iterator::operator!=(const iterator & i) const
 {
 	return	mem_buffer != i.mem_buffer ||
 			dynamic_array_index != i.dynamic_array_index ||
@@ -422,7 +422,7 @@ item_type MemBuffer<item_type, stack_size, heap_block_size>::const_iterator::ope
 
 
 template<typename item_type, size_t stack_size, size_t heap_block_size>
-bool MemBuffer<item_type, stack_size, heap_block_size>::const_iterator::operator==(const const_iterator & i)
+bool MemBuffer<item_type, stack_size, heap_block_size>::const_iterator::operator==(const const_iterator & i) const
 {
 	return	mem_buffer == i.mem_buffer && 
 			dynamic_array_index == i.dynamic_array_index &&
@@ -431,7 +431,7 @@ bool MemBuffer<item_type, stack_size, heap_block_size>::const_iterator::operator
 
 
 template<typename item_type, size_t stack_size, size_t heap_block_size>
-bool MemBuffer<item_type, stack_size, heap_block_size>::const_iterator::operator!=(const const_iterator & i)
+bool MemBuffer<item_type, stack_size, heap_block_size>::const_iterator::operator!=(const const_iterator & i) const
 {
 	return	mem_buffer != i.mem_buffer ||
 			dynamic_array_index != i.dynamic_array_index ||