diff --git a/.vscode/c_cpp_properties.json b/.vscode/c_cpp_properties.json
index eee6171..cc6468f 100644
--- a/.vscode/c_cpp_properties.json
+++ b/.vscode/c_cpp_properties.json
@@ -10,7 +10,7 @@
             "defines": [],
             "compilerPath": "/usr/bin/gcc",
             "cStandard": "c11",
-            "cppStandard": "c++17",
+            "cppStandard": "c++11",
             "intelliSenseMode": "clang-x64"
         }
     ],
diff --git a/include/dap/optional.h b/include/dap/optional.h
index 77b0587..deab3f7 100644
--- a/include/dap/optional.h
+++ b/include/dap/optional.h
@@ -17,6 +17,7 @@
 
 #include <assert.h>
 #include <type_traits>
+#include <utility>  // std::move
 
 namespace dap {
 
diff --git a/include/dap/serialization.h b/include/dap/serialization.h
index dbb4e68..95fcb56 100644
--- a/include/dap/serialization.h
+++ b/include/dap/serialization.h
@@ -18,6 +18,7 @@
 #include "typeof.h"
 #include "types.h"
 
+#include <cstddef>  // ptrdiff_t
 #include <type_traits>
 
 namespace dap {
diff --git a/src/any_test.cpp b/src/any_test.cpp
index 35b92d5..d52ff05 100644
--- a/src/any_test.cpp
+++ b/src/any_test.cpp
@@ -78,10 +78,10 @@ TEST(Any, Object) {
   ASSERT_TRUE(any.is<dap::object>());
   if (any.is<dap::object>()) {
     auto got = any.get<dap::object>();
-    ASSERT_EQ(got.size(), 3);
-    ASSERT_EQ(got.count("one"), 1);
-    ASSERT_EQ(got.count("two"), 1);
-    ASSERT_EQ(got.count("three"), 1);
+    ASSERT_EQ(got.size(), 3U);
+    ASSERT_EQ(got.count("one"), 1U);
+    ASSERT_EQ(got.count("two"), 1U);
+    ASSERT_EQ(got.count("three"), 1U);
     ASSERT_TRUE(got["one"].is<dap::integer>());
     ASSERT_TRUE(got["two"].is<dap::integer>());
     ASSERT_TRUE(got["three"].is<dap::integer>());
diff --git a/src/io.cpp b/src/io.cpp
index 00665dd..7e3a52f 100644
--- a/src/io.cpp
+++ b/src/io.cpp
@@ -18,6 +18,7 @@
 #include <condition_variable>
 #include <cstdarg>
 #include <cstdio>
+#include <cstring>  // strlen
 #include <deque>
 #include <mutex>
 
diff --git a/src/session_test.cpp b/src/session_test.cpp
index 247c8a4..04409e5 100644
--- a/src/session_test.cpp
+++ b/src/session_test.cpp
@@ -179,7 +179,7 @@ TEST_F(SessionTest, Request) {
   ASSERT_EQ(received.i, request.i);
   ASSERT_EQ(received.n, request.n);
   ASSERT_EQ(received.a, request.a);
-  ASSERT_EQ(received.o.size(), 3);
+  ASSERT_EQ(received.o.size(), 3U);
   ASSERT_EQ(received.o["a"].get<dap::integer>(),
             request.o["a"].get<dap::integer>());
   ASSERT_EQ(received.o["b"].get<dap::number>(),
@@ -208,7 +208,7 @@ TEST_F(SessionTest, RequestResponseSuccess) {
   ASSERT_EQ(got.response.i, dap::integer(99));
   ASSERT_EQ(got.response.n, dap::number(123.456));
   ASSERT_EQ(got.response.a, dap::array<dap::integer>({5, 4, 3, 2, 1}));
-  ASSERT_EQ(got.response.o.size(), 3);
+  ASSERT_EQ(got.response.o.size(), 3U);
   ASSERT_EQ(got.response.o["one"].get<dap::integer>(), dap::integer(1));
   ASSERT_EQ(got.response.o["two"].get<dap::number>(), dap::number(2));
   ASSERT_EQ(got.response.o["three"].get<dap::string>(), dap::string("3"));
@@ -267,7 +267,7 @@ TEST_F(SessionTest, ResponseSentHandlerSuccess) {
   ASSERT_EQ(got.response.i, dap::integer(99));
   ASSERT_EQ(got.response.n, dap::number(123.456));
   ASSERT_EQ(got.response.a, dap::array<dap::integer>({5, 4, 3, 2, 1}));
-  ASSERT_EQ(got.response.o.size(), 3);
+  ASSERT_EQ(got.response.o.size(), 3U);
   ASSERT_EQ(got.response.o["one"].get<dap::integer>(), dap::integer(1));
   ASSERT_EQ(got.response.o["two"].get<dap::number>(), dap::number(2));
   ASSERT_EQ(got.response.o["three"].get<dap::string>(), dap::string("3"));
@@ -308,7 +308,7 @@ TEST_F(SessionTest, Event) {
   ASSERT_EQ(got.i, event.i);
   ASSERT_EQ(got.n, event.n);
   ASSERT_EQ(got.a, event.a);
-  ASSERT_EQ(got.o.size(), 3);
+  ASSERT_EQ(got.o.size(), 3U);
   ASSERT_EQ(got.o["a"].get<dap::integer>(), event.o["a"].get<dap::integer>());
   ASSERT_EQ(got.o["b"].get<dap::number>(), event.o["b"].get<dap::number>());
   ASSERT_EQ(got.o["c"].get<dap::string>(), event.o["c"].get<dap::string>());