diff --git a/src/ConsoleStatCalc.h b/src/ConsoleStatCalc.h
index 3940ef0e..76bfd5b7 100644
--- a/src/ConsoleStatCalc.h
+++ b/src/ConsoleStatCalc.h
@@ -45,7 +45,7 @@
 
 namespace aria2 {
 
-class SizeFormatter : public std::unary_function<int64_t, std::string> {
+class SizeFormatter {
 protected:
   virtual std::string format(int64_t size) const = 0;
 
diff --git a/src/CookieStorage.cc b/src/CookieStorage.cc
index 56db33ed..3dbafdec 100644
--- a/src/CookieStorage.cc
+++ b/src/CookieStorage.cc
@@ -352,7 +352,7 @@ public:
 } // namespace
 
 namespace {
-class OrderByPathDepthDesc : public std::binary_function<Cookie, Cookie, bool> {
+class OrderByPathDepthDesc {
 public:
   bool operator()(const CookiePathDivider& lhs,
                   const CookiePathDivider& rhs) const
diff --git a/src/MetalinkEntry.cc b/src/MetalinkEntry.cc
index 3567f06d..d18afd22 100644
--- a/src/MetalinkEntry.cc
+++ b/src/MetalinkEntry.cc
@@ -102,8 +102,7 @@ void MetalinkEntry::reorderMetaurlsByPriority()
 }
 
 namespace {
-class Supported
-    : public std::unary_function<std::shared_ptr<MetalinkResource>, bool> {
+class Supported {
 public:
   bool operator()(const std::shared_ptr<MetalinkResource>& res) const
   {
diff --git a/src/a2functional.h b/src/a2functional.h
index c1059fef..bacfff57 100644
--- a/src/a2functional.h
+++ b/src/a2functional.h
@@ -102,8 +102,7 @@ std::string strjoin(InputIterator first, InputIterator last,
   return result;
 }
 
-template <typename T>
-class LeastRecentAccess : public std::binary_function<T, T, bool> {
+template <typename T> class LeastRecentAccess {
 public:
   bool operator()(const std::shared_ptr<T>& lhs,
                   const std::shared_ptr<T>& rhs) const