diff --git a/algorithms_cpp/test/count_test.cc b/algorithms_cpp/test/count_test.cc
index 4efd422..e04d15d 100644
--- a/algorithms_cpp/test/count_test.cc
+++ b/algorithms_cpp/test/count_test.cc
@@ -32,7 +32,7 @@
 #include <functional>
 
 struct IsEven{
-  bool operator()(int val) {
+  bool operator()(int val) const {
     if (val % 2 == 0)
       return true;
     else
diff --git a/algorithms_cpp/test/for_each_test.cc b/algorithms_cpp/test/for_each_test.cc
index 95e8ae8..a8c58ae 100644
--- a/algorithms_cpp/test/for_each_test.cc
+++ b/algorithms_cpp/test/for_each_test.cc
@@ -38,7 +38,7 @@
  */
 struct Square {
   template<typename Type>
-  void operator()(Type& l) {
+  void operator()(Type& l) const {
     l = l * l;
   }
 };
diff --git a/algorithms_cpp/test/reduce_test.cc b/algorithms_cpp/test/reduce_test.cc
index d4c48a8..6922594 100644
--- a/algorithms_cpp/test/reduce_test.cc
+++ b/algorithms_cpp/test/reduce_test.cc
@@ -36,7 +36,7 @@
  */
 struct Square {
   template<typename Type>
-  Type operator()(Type& l) {
+  Type operator()(Type& l) const {
     return l * l;
   }
 };
diff --git a/algorithms_cpp/test/scan_test.cc b/algorithms_cpp/test/scan_test.cc
index 47988df..3597dda 100644
--- a/algorithms_cpp/test/scan_test.cc
+++ b/algorithms_cpp/test/scan_test.cc
@@ -35,7 +35,7 @@
  */
 struct Square {
   template<typename Type>
-  Type operator()(Type& l) {
+  Type operator()(Type& l) const {
     return l * l;
   }
 };