diff --git a/src/util/AllocatedString.hxx b/src/util/AllocatedString.hxx
index ceb307b3a..74e98333a 100644
--- a/src/util/AllocatedString.hxx
+++ b/src/util/AllocatedString.hxx
@@ -33,6 +33,7 @@
 #include "StringPointer.hxx"
 
 #include <algorithm>
+#include <cstddef>
 
 /**
  * A string pointer whose memory is managed by this class.
@@ -47,7 +48,7 @@ public:
 	typedef typename StringPointer<T>::const_reference_type const_reference_type;
 	typedef typename StringPointer<T>::pointer_type pointer_type;
 	typedef typename StringPointer<T>::const_pointer_type const_pointer_type;
-	typedef size_t size_type;
+	typedef std::size_t size_type;
 
 	static constexpr value_type SENTINEL = '\0';
 
diff --git a/src/util/ConstBuffer.hxx b/src/util/ConstBuffer.hxx
index eddee80e5..f9cf48cc6 100644
--- a/src/util/ConstBuffer.hxx
+++ b/src/util/ConstBuffer.hxx
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2013-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2013-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -43,7 +43,7 @@ struct ConstBuffer;
 
 template<>
 struct ConstBuffer<void> {
-	typedef size_t size_type;
+	typedef std::size_t size_type;
 	typedef void value_type;
 	typedef const void *pointer_type;
 	typedef pointer_type const_pointer_type;
@@ -91,7 +91,7 @@ struct ConstBuffer<void> {
  */
 template<typename T>
 struct ConstBuffer {
-	typedef size_t size_type;
+	typedef std::size_t size_type;
 	typedef T value_type;
 	typedef const T &reference_type;
 	typedef reference_type const_reference_type;
diff --git a/src/util/ForeignFifoBuffer.hxx b/src/util/ForeignFifoBuffer.hxx
index bea0c1e5f..9d4013323 100644
--- a/src/util/ForeignFifoBuffer.hxx
+++ b/src/util/ForeignFifoBuffer.hxx
@@ -51,7 +51,7 @@
 template<typename T>
 class ForeignFifoBuffer {
 public:
-	using size_type = size_t;
+	using size_type = std::size_t;
 	using Range = WritableBuffer<T>;
 	using pointer_type = typename Range::pointer_type;
 	using const_pointer_type = typename Range::const_pointer_type;
diff --git a/src/util/StaticFifoBuffer.hxx b/src/util/StaticFifoBuffer.hxx
index f1a9d14ee..968b6b330 100644
--- a/src/util/StaticFifoBuffer.hxx
+++ b/src/util/StaticFifoBuffer.hxx
@@ -34,9 +34,9 @@
 
 #include <utility>
 #include <algorithm>
+#include <cstddef>
 
 #include <assert.h>
-#include <stddef.h>
 
 /**
  * A first-in-first-out buffer: you can append data at the end, and
@@ -46,7 +46,7 @@
 template<class T, size_t size>
 class StaticFifoBuffer {
 public:
-	using size_type = size_t;
+	using size_type = std::size_t;
 	using Range = WritableBuffer<T>;
 
 protected:
diff --git a/src/util/WritableBuffer.hxx b/src/util/WritableBuffer.hxx
index ae6e14af5..73b26abbb 100644
--- a/src/util/WritableBuffer.hxx
+++ b/src/util/WritableBuffer.hxx
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2013-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2013-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -44,7 +44,7 @@ struct WritableBuffer;
 
 template<>
 struct WritableBuffer<void> {
-	typedef size_t size_type;
+	typedef std::size_t size_type;
 	typedef void value_type;
 	typedef void *pointer_type;
 	typedef const void *const_pointer_type;
@@ -90,7 +90,7 @@ struct WritableBuffer<void> {
  */
 template<typename T>
 struct WritableBuffer {
-	typedef size_t size_type;
+	typedef std::size_t size_type;
 	typedef T value_type;
 	typedef T &reference_type;
 	typedef const T &const_reference_type;