diff --git a/include/plist/Array.h b/include/plist/Array.h
index 745b750..1b7728a 100644
--- a/include/plist/Array.h
+++ b/include/plist/Array.h
@@ -34,7 +34,7 @@
     Array(Node* parent = NULL);
     Array(plist_t node, Node* parent = NULL);
     Array(const Array& a);
-    Array& operator=(Array& a);
+    Array& operator=(const Array& a);
     virtual ~Array();
 
     Node* Clone() const;
diff --git a/include/plist/Boolean.h b/include/plist/Boolean.h
index 307a1ff..9ecbc48 100644
--- a/include/plist/Boolean.h
+++ b/include/plist/Boolean.h
@@ -33,7 +33,7 @@
     Boolean(Node* parent = NULL);
     Boolean(plist_t node, Node* parent = NULL);
     Boolean(const Boolean& b);
-    Boolean& operator=(Boolean& b);
+    Boolean& operator=(const Boolean& b);
     Boolean(bool b);
     virtual ~Boolean();
 
diff --git a/include/plist/Data.h b/include/plist/Data.h
index a100d28..b566a6c 100644
--- a/include/plist/Data.h
+++ b/include/plist/Data.h
@@ -34,7 +34,7 @@
     Data(Node* parent = NULL);
     Data(plist_t node, Node* parent = NULL);
     Data(const Data& d);
-    Data& operator=(Data& b);
+    Data& operator=(const Data& b);
     Data(const std::vector<char>& buff);
     virtual ~Data();
 
diff --git a/include/plist/Date.h b/include/plist/Date.h
index 510a349..3cfb7d1 100644
--- a/include/plist/Date.h
+++ b/include/plist/Date.h
@@ -35,7 +35,7 @@
     Date(Node* parent = NULL);
     Date(plist_t node, Node* parent = NULL);
     Date(const Date& d);
-    Date& operator=(Date& d);
+    Date& operator=(const Date& d);
     Date(timeval t);
     virtual ~Date();
 
diff --git a/include/plist/Dictionary.h b/include/plist/Dictionary.h
index 931a569..a3ac3a7 100644
--- a/include/plist/Dictionary.h
+++ b/include/plist/Dictionary.h
@@ -35,7 +35,7 @@
     Dictionary(Node* parent = NULL);
     Dictionary(plist_t node, Node* parent = NULL);
     Dictionary(const Dictionary& d);
-    Dictionary& operator=(Dictionary& d);
+    Dictionary& operator=(const Dictionary& d);
     virtual ~Dictionary();
 
     Node* Clone() const;
diff --git a/include/plist/Integer.h b/include/plist/Integer.h
index adbc39a..bdabc6f 100644
--- a/include/plist/Integer.h
+++ b/include/plist/Integer.h
@@ -33,7 +33,7 @@
     Integer(Node* parent = NULL);
     Integer(plist_t node, Node* parent = NULL);
     Integer(const Integer& i);
-    Integer& operator=(Integer& i);
+    Integer& operator=(const Integer& i);
     Integer(uint64_t i);
     virtual ~Integer();
 
diff --git a/include/plist/Key.h b/include/plist/Key.h
index e75aabc..bd5f7bc 100644
--- a/include/plist/Key.h
+++ b/include/plist/Key.h
@@ -34,7 +34,7 @@
     Key(Node* parent = NULL);
     Key(plist_t node, Node* parent = NULL);
     Key(const Key& k);
-    Key& operator=(Key& k);
+    Key& operator=(const Key& k);
     Key(const std::string& s);
     virtual ~Key();
 
diff --git a/include/plist/Real.h b/include/plist/Real.h
index c2d55f8..5afb0c0 100644
--- a/include/plist/Real.h
+++ b/include/plist/Real.h
@@ -33,7 +33,7 @@
     Real(Node* parent = NULL);
     Real(plist_t node, Node* parent = NULL);
     Real(const Real& d);
-    Real& operator=(Real& d);
+    Real& operator=(const Real& d);
     Real(double d);
     virtual ~Real();
 
diff --git a/include/plist/String.h b/include/plist/String.h
index 80290b3..9aba16b 100644
--- a/include/plist/String.h
+++ b/include/plist/String.h
@@ -34,7 +34,7 @@
     String(Node* parent = NULL);
     String(plist_t node, Node* parent = NULL);
     String(const String& s);
-    String& operator=(String& s);
+    String& operator=(const String& s);
     String(const std::string& s);
     virtual ~String();
 
diff --git a/include/plist/Uid.h b/include/plist/Uid.h
index 2d8375b..af6e51d 100644
--- a/include/plist/Uid.h
+++ b/include/plist/Uid.h
@@ -33,7 +33,7 @@
     Uid(Node* parent = NULL);
     Uid(plist_t node, Node* parent = NULL);
     Uid(const Uid& i);
-    Uid& operator=(Uid& i);
+    Uid& operator=(const Uid& i);
     Uid(uint64_t i);
     virtual ~Uid();
 
diff --git a/src/Array.cpp b/src/Array.cpp
index 22c254b..8838e5b 100644
--- a/src/Array.cpp
+++ b/src/Array.cpp
@@ -58,7 +58,7 @@
     array_fill(this, _array, _node);
 }
 
-Array& Array::operator=(PList::Array& a)
+Array& Array::operator=(const PList::Array& a)
 {
     plist_free(_node);
     for (unsigned int it = 0; it < _array.size(); it++)
diff --git a/src/Boolean.cpp b/src/Boolean.cpp
index 9f14904..2c871c8 100644
--- a/src/Boolean.cpp
+++ b/src/Boolean.cpp
@@ -37,7 +37,7 @@
     plist_set_bool_val(_node, b.GetValue());
 }
 
-Boolean& Boolean::operator=(PList::Boolean& b)
+Boolean& Boolean::operator=(const PList::Boolean& b)
 {
     plist_free(_node);
     _node = plist_copy(b.GetPlist());
diff --git a/src/Data.cpp b/src/Data.cpp
index 89b4f4e..4515388 100644
--- a/src/Data.cpp
+++ b/src/Data.cpp
@@ -38,7 +38,7 @@
     plist_set_data_val(_node, &b[0], b.size());
 }
 
-Data& Data::operator=(PList::Data& b)
+Data& Data::operator=(const PList::Data& b)
 {
     plist_free(_node);
     _node = plist_copy(b.GetPlist());
diff --git a/src/Date.cpp b/src/Date.cpp
index a3c8592..8b8e650 100644
--- a/src/Date.cpp
+++ b/src/Date.cpp
@@ -38,7 +38,7 @@
     plist_set_date_val(_node, t.tv_sec, t.tv_usec);
 }
 
-Date& Date::operator=(PList::Date& d)
+Date& Date::operator=(const PList::Date& d)
 {
     plist_free(_node);
     _node = plist_copy(d.GetPlist());
diff --git a/src/Dictionary.cpp b/src/Dictionary.cpp
index 6da0eee..20e9710 100644
--- a/src/Dictionary.cpp
+++ b/src/Dictionary.cpp
@@ -62,7 +62,7 @@
     dictionary_fill(this, _map, _node);
 }
 
-Dictionary& Dictionary::operator=(PList::Dictionary& d)
+Dictionary& Dictionary::operator=(const PList::Dictionary& d)
 {
     for (Dictionary::iterator it = _map.begin(); it != _map.end(); it++)
     {
diff --git a/src/Integer.cpp b/src/Integer.cpp
index d394ee1..a40d026 100644
--- a/src/Integer.cpp
+++ b/src/Integer.cpp
@@ -37,7 +37,7 @@
     plist_set_uint_val(_node, i.GetValue());
 }
 
-Integer& Integer::operator=(PList::Integer& i)
+Integer& Integer::operator=(const PList::Integer& i)
 {
     plist_free(_node);
     _node = plist_copy(i.GetPlist());
diff --git a/src/Key.cpp b/src/Key.cpp
index 8c96e59..8ba497a 100644
--- a/src/Key.cpp
+++ b/src/Key.cpp
@@ -37,7 +37,7 @@
     plist_set_key_val(_node, k.GetValue().c_str());
 }
 
-Key& Key::operator=(PList::Key& k)
+Key& Key::operator=(const PList::Key& k)
 {
     plist_free(_node);
     _node = plist_copy(k.GetPlist());
diff --git a/src/Real.cpp b/src/Real.cpp
index 3ac67f2..6bdb920 100644
--- a/src/Real.cpp
+++ b/src/Real.cpp
@@ -37,7 +37,7 @@
     plist_set_real_val(_node, d.GetValue());
 }
 
-Real& Real::operator=(PList::Real& d)
+Real& Real::operator=(const PList::Real& d)
 {
     plist_free(_node);
     _node = plist_copy(d.GetPlist());
diff --git a/src/String.cpp b/src/String.cpp
index 39717c2..cd4f98f 100644
--- a/src/String.cpp
+++ b/src/String.cpp
@@ -37,7 +37,7 @@
     plist_set_string_val(_node, s.GetValue().c_str());
 }
 
-String& String::operator=(PList::String& s)
+String& String::operator=(const PList::String& s)
 {
     plist_free(_node);
     _node = plist_copy(s.GetPlist());
diff --git a/src/Uid.cpp b/src/Uid.cpp
index 204cd68..e83ed27 100644
--- a/src/Uid.cpp
+++ b/src/Uid.cpp
@@ -37,7 +37,7 @@
     plist_set_uid_val(_node, i.GetValue());
 }
 
-Uid& Uid::operator=(PList::Uid& i)
+Uid& Uid::operator=(const PList::Uid& i)
 {
     plist_free(_node);
     _node = plist_copy(i.GetPlist());