diff --git a/stl/src/StlCompareStringA.cpp b/stl/src/StlCompareStringA.cpp
index 19e3dbe56b5..47ffe52bfea 100644
--- a/stl/src/StlCompareStringA.cpp
+++ b/stl/src/StlCompareStringA.cpp
@@ -2,8 +2,8 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <crtdbg.h>
+#include <cstring> // for __strncnt
 #include <internal_shared.h>
-#include <string.h>
 
 #include <Windows.h>
 
diff --git a/stl/src/StlCompareStringW.cpp b/stl/src/StlCompareStringW.cpp
index 6dede598e49..424593c03a0 100644
--- a/stl/src/StlCompareStringW.cpp
+++ b/stl/src/StlCompareStringW.cpp
@@ -1,9 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <crtdbg.h>
-#include <internal_shared.h>
-#include <string.h>
+#include <cstring> // for wcsnlen
 
 #include <Windows.h>
 
diff --git a/stl/src/StlLCMapStringA.cpp b/stl/src/StlLCMapStringA.cpp
index bce72ee1ee2..d176fcc4151 100644
--- a/stl/src/StlLCMapStringA.cpp
+++ b/stl/src/StlLCMapStringA.cpp
@@ -2,7 +2,6 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <internal_shared.h>
-#include <locale.h>
 
 #include "awint.hpp"
 
diff --git a/stl/src/StlLCMapStringW.cpp b/stl/src/StlLCMapStringW.cpp
index 1008910611a..625e8f8075c 100644
--- a/stl/src/StlLCMapStringW.cpp
+++ b/stl/src/StlLCMapStringW.cpp
@@ -1,8 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <internal_shared.h>
-#include <locale.h>
+#include <cstring> // for wcsnlen
 
 #include "awint.hpp"
 
diff --git a/stl/src/_tolower.cpp b/stl/src/_tolower.cpp
index 21d35f55a10..6d91a498b0f 100644
--- a/stl/src/_tolower.cpp
+++ b/stl/src/_tolower.cpp
@@ -5,11 +5,10 @@
 
 #include <yvals.h>
 
-#include <ctype.h>
+#include <cctype>
+#include <clocale>
+#include <cstdlib>
 #include <internal_shared.h>
-#include <locale.h>
-#include <stddef.h>
-#include <stdlib.h>
 #include <xlocinfo.h>
 
 #include "awint.hpp"
diff --git a/stl/src/_toupper.cpp b/stl/src/_toupper.cpp
index e0cc01cc5af..78953a811a6 100644
--- a/stl/src/_toupper.cpp
+++ b/stl/src/_toupper.cpp
@@ -5,9 +5,9 @@
 
 #include <yvals.h>
 
-#include <ctype.h>
-#include <locale.h>
-#include <stddef.h>
+#include <cctype>
+#include <clocale>
+#include <cstddef>
 #include <xlocinfo.h>
 
 #include "awint.hpp"
diff --git a/stl/src/cond.cpp b/stl/src/cond.cpp
index 49f79e4633e..c619d264dea 100644
--- a/stl/src/cond.cpp
+++ b/stl/src/cond.cpp
@@ -3,9 +3,8 @@
 
 // condition variable functions
 
+#include <cstdlib>
 #include <internal_shared.h>
-#include <limits.h>
-#include <stdlib.h>
 #include <type_traits>
 #include <xthreads.h>
 #include <xtimec.h>
diff --git a/stl/src/cthread.cpp b/stl/src/cthread.cpp
index 7b144c82151..5a66e009874 100644
--- a/stl/src/cthread.cpp
+++ b/stl/src/cthread.cpp
@@ -3,11 +3,11 @@
 
 // thread functions
 
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <process.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 #include <xthreads.h>
 
 #include <Windows.h>
diff --git a/stl/src/excptptr.cpp b/stl/src/excptptr.cpp
index 6b5879e328b..ed5460df8cd 100644
--- a/stl/src/excptptr.cpp
+++ b/stl/src/excptptr.cpp
@@ -11,6 +11,8 @@
 #endif // _VCRT_ALLOW_INTERNALS
 
 #include <Unknwn.h>
+#include <cstdlib> // for abort
+#include <cstring> // for memcpy
 #include <eh.h>
 #include <ehdata.h>
 #include <exception>
@@ -18,9 +20,7 @@
 #include <malloc.h>
 #include <memory>
 #include <new>
-#include <stddef.h>
 #include <stdexcept>
-#include <string.h>
 #include <trnsctrl.h>
 #include <xcall_once.h>
 
diff --git a/stl/src/filesys.cpp b/stl/src/filesys.cpp
index e6588539b1c..02d1fe85a8d 100644
--- a/stl/src/filesys.cpp
+++ b/stl/src/filesys.cpp
@@ -8,10 +8,10 @@
 
 #include <yvals.h>
 
+#include <cstring>
 #include <direct.h>
 #include <experimental/filesystem>
 #include <io.h>
-#include <string.h>
 
 #include <Windows.h>
 
diff --git a/stl/src/filesystem.cpp b/stl/src/filesystem.cpp
index 29079e5fca5..a8493ec49ae 100644
--- a/stl/src/filesystem.cpp
+++ b/stl/src/filesystem.cpp
@@ -9,16 +9,15 @@
 // Do not include or define anything else here.
 // In particular, basic_string must not be included here.
 
+#include <clocale>
 #include <corecrt_terminate.h>
+#include <cstdlib>
+#include <cstring>
 #include <internal_shared.h>
-#include <limits.h>
-#include <locale.h>
-#include <stdlib.h>
-#include <string.h>
 #include <xfilesystem_abi.h>
 
 #include <Windows.h>
-#include <WinIoCtl.h>
+#include <winioctl.h>
 
 // We have several switches that do not have case statements for every possible enum value.
 // Hence, disabling this warning.
diff --git a/stl/src/future.cpp b/stl/src/future.cpp
index 77264b2de59..5ff6f82f924 100644
--- a/stl/src/future.cpp
+++ b/stl/src/future.cpp
@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <cstdlib>
 #include <exception>
 #include <future>
 
diff --git a/stl/src/ios.cpp b/stl/src/ios.cpp
index 9f15c44dc4a..8640a63169b 100644
--- a/stl/src/ios.cpp
+++ b/stl/src/ios.cpp
@@ -3,7 +3,6 @@
 
 // ios_base basic members
 
-#include <new>
 #include <xiosbase>
 _STD_BEGIN
 
@@ -26,20 +25,6 @@ __PURE_APPDOMAIN_GLOBAL static ios_base* stdstr[_Nstdstr + 2] = {
     nullptr}; // [1, _Nstdstr] hold pointers to standard streams
 __PURE_APPDOMAIN_GLOBAL static char stdopens[_Nstdstr + 2] = {0}; // [1, _Nstdstr] hold open counts for standard streams
 
-// void __CLR_OR_THIS_CALL ios_base::clear(iostate state, bool reraise) { // set state, possibly reraise exception
-//     _Mystate = (iostate)(state & _Statmask);
-//     if ((_Mystate & _Except) == 0)
-//         ;
-//     else if (reraise)
-//         _RERAISE;
-//     else if (_Mystate & _Except & badbit)
-//         _THROW(failure("ios_base::badbit set"));
-//     else if (_Mystate & _Except & failbit)
-//         _THROW(failure("ios_base::failbit set"));
-//     else
-//         _THROW(failure("ios_base::eofbit set"));
-// }
-
 void __CLRCALL_PURE_OR_CDECL ios_base::_Ios_base_dtor(ios_base* _This) { // destroy the object
     if (0 < _This->_Stdstr && 0 < --stdopens[_This->_Stdstr]) {
         return;
@@ -49,30 +34,6 @@ void __CLRCALL_PURE_OR_CDECL ios_base::_Ios_base_dtor(ios_base* _This) { // dest
     delete _This->_Ploc;
 }
 
-// ios_base::_Iosarray& __CLR_OR_THIS_CALL ios_base::_Findarr(int idx) { // locate or make a variable array element
-//     static _Iosarray stub(0, 0);
-//     _Iosarray *p, *q;
-//
-//     if (idx < 0) { // handle bad index
-//         setstate(badbit);
-//         return stub;
-//     }
-//
-//     for (p = _Arr, q = 0; p != 0; p = p->_Next)
-//         if (p->_Index == idx)
-//             return *p; // found element, return it
-//         else if (q == 0 && p->_Lo == 0 && p->_Vp == 0)
-//             q = p; // found recycling candidate
-//
-//     if (q != 0) { // recycle existing element
-//         q->_Index = idx;
-//         return *q;
-//     }
-//
-//     _Arr = new _Iosarray(idx, _Arr); // make a new element
-//     return *_Arr;
-// }
-
 void __CLRCALL_PURE_OR_CDECL ios_base::_Addstd(ios_base* _This) { // add standard stream to destructor list
     _BEGIN_LOCK(_LOCK_STREAM)
     for (_This->_Stdstr = 1; _This->_Stdstr < _Nstdstr; ++_This->_Stdstr) {
diff --git a/stl/src/locale0.cpp b/stl/src/locale0.cpp
index 46a6f679fe3..acd2fc944a2 100644
--- a/stl/src/locale0.cpp
+++ b/stl/src/locale0.cpp
@@ -73,8 +73,8 @@ _STD_END
 
 #if !STDCPP_IMPLIB || defined(_M_CEE_PURE)
 
+#include <cstdlib>
 #include <locale>
-#include <stdlib.h>
 
 _EXTERN_C
 
diff --git a/stl/src/mpiostream.cpp b/stl/src/mpiostream.cpp
index cbc2b38c93e..a0d1818eafd 100644
--- a/stl/src/mpiostream.cpp
+++ b/stl/src/mpiostream.cpp
@@ -6,7 +6,6 @@
 #error This file must be built with /clr:pure.
 #endif
 
-#include <fstream>
 #include <iostream>
 
 #pragma warning(disable : 4074)
diff --git a/stl/src/mutex.cpp b/stl/src/mutex.cpp
index 79ca2067e08..8749f4544b9 100644
--- a/stl/src/mutex.cpp
+++ b/stl/src/mutex.cpp
@@ -3,9 +3,9 @@
 
 // mutex functions
 
+#include <cstdio>
+#include <cstdlib>
 #include <internal_shared.h>
-#include <stdio.h>
-#include <stdlib.h>
 #include <type_traits>
 #include <xthreads.h>
 #include <xtimec.h>
diff --git a/stl/src/nothrow.cpp b/stl/src/nothrow.cpp
index 6e01ef81976..d12dcdcf336 100644
--- a/stl/src/nothrow.cpp
+++ b/stl/src/nothrow.cpp
@@ -11,8 +11,6 @@
 #undef MRTDLL
 #endif
 
-#include <yvals.h>
-
 #include <new>
 _STD_BEGIN
 
diff --git a/stl/src/ppltasks.cpp b/stl/src/ppltasks.cpp
index 95a5a011dbe..b603576f41e 100644
--- a/stl/src/ppltasks.cpp
+++ b/stl/src/ppltasks.cpp
@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <crtdbg.h>
 #include <crtdefs.h>
 #include <pplinterface.h>
 #include <ppltasks.h>
diff --git a/stl/src/primitives.hpp b/stl/src/primitives.hpp
index ca877252967..fa82288c1cf 100644
--- a/stl/src/primitives.hpp
+++ b/stl/src/primitives.hpp
@@ -7,9 +7,9 @@
 #pragma warning(disable : 4324) // structure was padded due to alignment specifier
 
 #include <crtdefs.h>
+#include <cstdlib> // for __max
 #include <exception>
 #include <new>
-#include <stdlib.h>
 
 #include <Windows.h>
 
diff --git a/stl/src/taskscheduler.cpp b/stl/src/taskscheduler.cpp
index 385c9d8fbf5..a4908fe95ad 100644
--- a/stl/src/taskscheduler.cpp
+++ b/stl/src/taskscheduler.cpp
@@ -2,9 +2,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <condition_variable>
+#include <cstddef> // for size_t
 #include <mutex>
 #include <ppltaskscheduler.h>
-#include <stddef.h>
 
 #include <Windows.h>
 
diff --git a/stl/src/thread0.cpp b/stl/src/thread0.cpp
index a4067600697..bffe7faf63b 100644
--- a/stl/src/thread0.cpp
+++ b/stl/src/thread0.cpp
@@ -3,9 +3,7 @@
 
 // thread support functions
 
-#include <yvals.h>
-
-#include <stdlib.h>
+#include <cstdlib>
 #include <system_error>
 #include <xthreads.h>
 
diff --git a/stl/src/vector_algorithms.cpp b/stl/src/vector_algorithms.cpp
index 4823e4fdd9d..9c7fc413e99 100644
--- a/stl/src/vector_algorithms.cpp
+++ b/stl/src/vector_algorithms.cpp
@@ -11,7 +11,6 @@
 #include <immintrin.h>
 #include <intrin0.h>
 #include <isa_availability.h>
-#include <xmmintrin.h>
 
 extern "C" long __isa_enabled;
 
diff --git a/stl/src/winapinls.cpp b/stl/src/winapinls.cpp
index 503072ab96f..44f792e55a4 100644
--- a/stl/src/winapinls.cpp
+++ b/stl/src/winapinls.cpp
@@ -6,8 +6,8 @@
 #if _STL_WIN32_WINNT < _WIN32_WINNT_VISTA
 
 #include <algorithm>
-#include <ctype.h>
-#include <stdlib.h>
+#include <cctype>
+#include <cstdlib> // for _countof
 #include <string_view>
 
 namespace {
diff --git a/stl/src/winapisupp.cpp b/stl/src/winapisupp.cpp
index 846e20376da..aa583d138a7 100644
--- a/stl/src/winapisupp.cpp
+++ b/stl/src/winapisupp.cpp
@@ -4,17 +4,17 @@
 // clang-format off
 // Prevent clang-format from reordering <AppModel.h> before <Windows.h>
 #include <Windows.h>
-#include <AppModel.h>
+#include <AppModel.h> // for APPMODEL_ERROR_NO_PACKAGE
 #include "awint.hpp"
 #include <internal_shared.h>
-#include <stdlib.h>
+#include <cstdlib>
 // clang-format on
 
 #pragma warning(push)
 #pragma warning(disable : 4265) // non-virtual destructor in base class
 #include <wrl/wrappers/corewrappers.h>
 #pragma warning(pop)
-#include <stdint.h>
+#include <cstdint>
 
 #if !defined(_CRT_WINDOWS) && !defined(UNDOCKED_WINDOWS_UCRT)
 // GetCurrentPackageId retrieves the current package id, if the app is deployed via a package.
diff --git a/stl/src/xferaise.cpp b/stl/src/xferaise.cpp
index b1683d6e194..68d8bbbf82a 100644
--- a/stl/src/xferaise.cpp
+++ b/stl/src/xferaise.cpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h>
 
-#include <errno.h>
+#include <cerrno>
 #include <ymath.h>
 
 _EXTERN_C_UNLESS_PURE
diff --git a/stl/src/xfprec.cpp b/stl/src/xfprec.cpp
index 31ca4b8aad1..4c4e438e406 100644
--- a/stl/src/xfprec.cpp
+++ b/stl/src/xfprec.cpp
@@ -3,6 +3,5 @@
 
 // _FXp* functions
 
-#include "xmath.hpp"
 #include "xxfftype.hpp"
 #include "xxxprec.hpp"
diff --git a/stl/src/xgetwctype.cpp b/stl/src/xgetwctype.cpp
index b3e444a2376..2063e0cfded 100644
--- a/stl/src/xgetwctype.cpp
+++ b/stl/src/xgetwctype.cpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h>
 
-#include <wchar.h>
+#include <cwchar>
 #include <xlocinfo.h>
 
 #include "awint.hpp"
diff --git a/stl/src/xlock.cpp b/stl/src/xlock.cpp
index 1ffa80e9362..e6274ddd3e5 100644
--- a/stl/src/xlock.cpp
+++ b/stl/src/xlock.cpp
@@ -5,8 +5,8 @@
 
 #include <yvals.h>
 
-#include <locale.h>
-#include <stdlib.h>
+#include <clocale>
+#include <cstdlib>
 
 #include "xmtx.hpp"
 
diff --git a/stl/src/xlprec.cpp b/stl/src/xlprec.cpp
index 3061c43c846..34963b7bee6 100644
--- a/stl/src/xlprec.cpp
+++ b/stl/src/xlprec.cpp
@@ -3,6 +3,5 @@
 
 // _LXp* functions
 
-#include "xmath.hpp"
 #include "xxlftype.hpp"
 #include "xxxprec.hpp"
diff --git a/stl/src/xmath.hpp b/stl/src/xmath.hpp
index 200d7a9d981..a2ebd22d505 100644
--- a/stl/src/xmath.hpp
+++ b/stl/src/xmath.hpp
@@ -3,9 +3,8 @@
 
 #ifndef _XMATH
 #define _XMATH
-#include <errno.h>
-#include <math.h>
-#include <stddef.h>
+#include <cerrno>
+#include <cmath>
 #include <ymath.h>
 
 // FLOAT PROPERTIES
diff --git a/stl/src/xmbtowc.cpp b/stl/src/xmbtowc.cpp
index 5bc58a872cf..d415eeb8c5a 100644
--- a/stl/src/xmbtowc.cpp
+++ b/stl/src/xmbtowc.cpp
@@ -3,14 +3,11 @@
 
 // Convert multibyte char to wide char.
 
+#include <cctype>
+#include <cerrno>
+#include <climits> // for INT_MAX
 #include <crtdbg.h>
-#include <ctype.h>
-#include <errno.h>
 #include <internal_shared.h>
-#include <limits.h> // for INT_MAX
-#include <locale.h>
-#include <stdio.h> // for EOF
-#include <stdlib.h>
 #include <xlocinfo.h> // for _Cvtvec, _Mbrtowc
 
 _EXTERN_C_UNLESS_PURE
diff --git a/stl/src/xmtx.hpp b/stl/src/xmtx.hpp
index f891af3175f..9380716a3c0 100644
--- a/stl/src/xmtx.hpp
+++ b/stl/src/xmtx.hpp
@@ -6,7 +6,7 @@
 #define _XMTX
 #include <yvals.h>
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include <Windows.h>
 
diff --git a/stl/src/xnotify.cpp b/stl/src/xnotify.cpp
index bf362c717ea..64a513855d9 100644
--- a/stl/src/xnotify.cpp
+++ b/stl/src/xnotify.cpp
@@ -3,8 +3,7 @@
 
 // thread exit notification functions
 
-#include <stdlib.h>
-#include <string.h>
+#include <cstdlib>
 #include <xthreads.h>
 
 #include <Windows.h>
diff --git a/stl/src/xprec.cpp b/stl/src/xprec.cpp
index 4097ba6a9fa..cb7eaa21f20 100644
--- a/stl/src/xprec.cpp
+++ b/stl/src/xprec.cpp
@@ -3,6 +3,5 @@
 
 // _Xp* functions
 
-#include "xmath.hpp"
 #include "xxdftype.hpp"
 #include "xxxprec.hpp"
diff --git a/stl/src/xrngabort.cpp b/stl/src/xrngabort.cpp
index b221d651871..87642321b95 100644
--- a/stl/src/xrngabort.cpp
+++ b/stl/src/xrngabort.cpp
@@ -5,8 +5,8 @@
 
 #include <yvals.h>
 
-#include <stdio.h>
-#include <stdlib.h>
+#include <cstdio>
+#include <cstdlib>
 
 // clang-format off
 _STD_BEGIN
diff --git a/stl/src/xstod.cpp b/stl/src/xstod.cpp
index 821cd15fb8a..34425729a3c 100644
--- a/stl/src/xstod.cpp
+++ b/stl/src/xstod.cpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h> // include first, for native shared
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "xmath.hpp"
 #include "xxcctype.hpp"
diff --git a/stl/src/xstof.cpp b/stl/src/xstof.cpp
index 0c98e1a855f..1f0107573d3 100644
--- a/stl/src/xstof.cpp
+++ b/stl/src/xstof.cpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h> // include first, for native shared
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "xmath.hpp"
 #include "xxcctype.hpp"
diff --git a/stl/src/xstoflt.cpp b/stl/src/xstoflt.cpp
index 57dfdaf5707..3a9b6adf214 100644
--- a/stl/src/xstoflt.cpp
+++ b/stl/src/xstoflt.cpp
@@ -3,9 +3,9 @@
 
 // _Stoflt function
 
-#include <ctype.h>
-#include <locale.h>
-#include <stdlib.h>
+#include <cctype>
+#include <clocale>
+#include <cstdlib>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xstol.cpp b/stl/src/xstol.cpp
index 35a9e052fee..3cd73becd64 100644
--- a/stl/src/xstol.cpp
+++ b/stl/src/xstol.cpp
@@ -5,10 +5,10 @@
 
 #include <yvals.h>
 
-#include <ctype.h>
-#include <errno.h>
-#include <limits.h>
-#include <stdlib.h>
+#include <cctype>
+#include <cerrno>
+#include <climits>
+#include <cstdlib>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xstold.cpp b/stl/src/xstold.cpp
index f8a66cb8827..cea4090ffc4 100644
--- a/stl/src/xstold.cpp
+++ b/stl/src/xstold.cpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h> // include first, for native shared
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "xmath.hpp"
 #include "xxcctype.hpp"
diff --git a/stl/src/xstoll.cpp b/stl/src/xstoll.cpp
index af01eeb597c..63ad80e1493 100644
--- a/stl/src/xstoll.cpp
+++ b/stl/src/xstoll.cpp
@@ -3,10 +3,10 @@
 
 // _Stoll function
 
-#include <ctype.h>
-#include <errno.h>
-#include <limits.h>
-#include <stdlib.h>
+#include <cctype>
+#include <cerrno>
+#include <climits>
+#include <cstdlib>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xstopfx.cpp b/stl/src/xstopfx.cpp
index b9d44fe14b6..207b4cd7c61 100644
--- a/stl/src/xstopfx.cpp
+++ b/stl/src/xstopfx.cpp
@@ -3,7 +3,7 @@
 
 // _Stopfx function
 
-#include <ctype.h>
+#include <cctype>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xstoul.cpp b/stl/src/xstoul.cpp
index c9a3214ed5b..48f48489b22 100644
--- a/stl/src/xstoul.cpp
+++ b/stl/src/xstoul.cpp
@@ -3,14 +3,13 @@
 
 // _Stoul function
 
-#include <ctype.h>
-#include <errno.h>
-#include <limits.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-
-#include "xmath.hpp"
+#include <cctype>
+#include <cerrno>
+#include <climits>
+#include <cstddef>
+#include <cstdlib>
+#include <cstring>
+
 
 _EXTERN_C_UNLESS_PURE
 
diff --git a/stl/src/xstoull.cpp b/stl/src/xstoull.cpp
index 2d04d149ee8..8a29d7ab260 100644
--- a/stl/src/xstoull.cpp
+++ b/stl/src/xstoull.cpp
@@ -3,14 +3,12 @@
 
 // _Stoull function
 
-#include <ctype.h>
-#include <errno.h>
-#include <limits.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-
-#include "xmath.hpp"
+#include <cctype>
+#include <cerrno>
+#include <climits>
+#include <cstddef>
+#include <cstdlib>
+#include <cstring>
 
 _EXTERN_C_UNLESS_PURE
 
diff --git a/stl/src/xstoxflt.cpp b/stl/src/xstoxflt.cpp
index a4e33a6a6e7..d8b2c590574 100644
--- a/stl/src/xstoxflt.cpp
+++ b/stl/src/xstoxflt.cpp
@@ -3,10 +3,10 @@
 
 // _Stoxflt function
 
-#include <ctype.h>
-#include <locale.h>
-#include <stdlib.h>
-#include <string.h>
+#include <cctype>
+#include <clocale>
+#include <cstdlib>
+#include <cstring>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xstrcoll.cpp b/stl/src/xstrcoll.cpp
index 2def872bf86..752999db66e 100644
--- a/stl/src/xstrcoll.cpp
+++ b/stl/src/xstrcoll.cpp
@@ -3,12 +3,12 @@
 
 // Compare two strings using the locale LC_COLLATE information.
 
+#include <cerrno>
+#include <clocale>
 #include <crtdbg.h>
-#include <errno.h>
-#include <locale.h>
+#include <cstdlib>
+#include <cstring>
 #include <malloc.h>
-#include <stdlib.h>
-#include <string.h>
 #include <xlocinfo.h> // for _Collvec, _Strcoll
 
 #include "awint.hpp"
diff --git a/stl/src/xstrxfrm.cpp b/stl/src/xstrxfrm.cpp
index e7988a747ab..e814ecfe618 100644
--- a/stl/src/xstrxfrm.cpp
+++ b/stl/src/xstrxfrm.cpp
@@ -3,11 +3,10 @@
 
 // Transform a string using the locale information as set by LC_COLLATE.
 
-#include <limits.h>
-#include <locale.h>
+#include <clocale>
+#include <cstdlib>
+#include <cstring>
 #include <malloc.h>
-#include <stdlib.h>
-#include <string.h>
 #include <xlocinfo.h> // for _Collvec
 
 #include <Windows.h>
diff --git a/stl/src/xtime.cpp b/stl/src/xtime.cpp
index 0de318718d8..6d4bb94dc6c 100644
--- a/stl/src/xtime.cpp
+++ b/stl/src/xtime.cpp
@@ -4,8 +4,6 @@
 // xtime functions
 
 #include <atomic>
-#include <stdlib.h>
-#include <time.h>
 #include <xtimec.h>
 
 #include "awint.hpp"
diff --git a/stl/src/xtowlower.cpp b/stl/src/xtowlower.cpp
index 630f148bc69..b7417de4a96 100644
--- a/stl/src/xtowlower.cpp
+++ b/stl/src/xtowlower.cpp
@@ -3,7 +3,7 @@
 
 // _Towlower -- convert wchar_t to lower case
 
-#include <wchar.h>
+#include <cwchar>
 #include <xlocinfo.h>
 
 #include "awint.hpp"
diff --git a/stl/src/xtowupper.cpp b/stl/src/xtowupper.cpp
index 84901eae6ef..943b160ffcb 100644
--- a/stl/src/xtowupper.cpp
+++ b/stl/src/xtowupper.cpp
@@ -3,7 +3,7 @@
 
 // _Towupper -- convert wchar_t to upper case
 
-#include <wchar.h>
+#include <cwchar>
 #include <xlocinfo.h>
 
 #include "awint.hpp"
diff --git a/stl/src/xwcscoll.cpp b/stl/src/xwcscoll.cpp
index 9bf62598030..c8a105d94cf 100644
--- a/stl/src/xwcscoll.cpp
+++ b/stl/src/xwcscoll.cpp
@@ -3,10 +3,10 @@
 
 // Compare two wchar_t strings using the locale LC_COLLATE information.
 
-#include <errno.h>
-#include <locale.h>
-#include <stdlib.h>
-#include <string.h>
+#include <cerrno>
+#include <clocale>
+#include <cstdlib>
+#include <cstring> // for wmemcmp
 #include <xlocinfo.h> // for _Collvec, _Wcscoll
 
 #include "awint.hpp"
diff --git a/stl/src/xwcsxfrm.cpp b/stl/src/xwcsxfrm.cpp
index ab3692be410..9c283e289b7 100644
--- a/stl/src/xwcsxfrm.cpp
+++ b/stl/src/xwcsxfrm.cpp
@@ -3,11 +3,11 @@
 
 // Transform a wide-character string using the locale information as set by LC_COLLATE.
 
+#include <climits>
+#include <clocale>
+#include <cstdlib>
+#include <cstring>
 #include <internal_shared.h>
-#include <limits.h>
-#include <locale.h>
-#include <stdlib.h>
-#include <string.h>
 #include <xlocinfo.h> // for _Collvec, _Wcsxfrm
 
 #include "awint.hpp"
diff --git a/stl/src/xwctomb.cpp b/stl/src/xwctomb.cpp
index d82f8a5bdaa..80ce47e3abe 100644
--- a/stl/src/xwctomb.cpp
+++ b/stl/src/xwctomb.cpp
@@ -3,13 +3,11 @@
 
 // Convert wide character to multibyte character, with locale.
 
-#include <errno.h>
-#include <limits.h> // for MB_LEN_MAX
-#include <locale.h>
+#include <cerrno>
+#include <climits> // for MB_LEN_MAX
+#include <clocale>
+#include <cstdlib>
 #include <mbctype.h>
-#include <stdio.h> // for EOF
-#include <stdlib.h>
-#include <string.h> // for memcpy
 #include <xlocinfo.h> // for _Cvtvec, _Wcrtomb
 
 #include <Windows.h>
diff --git a/stl/src/xwstod.cpp b/stl/src/xwstod.cpp
index 499d19e2852..a2a4c927158 100644
--- a/stl/src/xwstod.cpp
+++ b/stl/src/xwstod.cpp
@@ -3,8 +3,8 @@
 
 // _WStod function
 
-#include <stdlib.h>
-#include <wchar.h>
+#include <cstdlib>
+#include <cwchar>
 
 #include "xmath.hpp"
 #include "xxdftype.hpp"
diff --git a/stl/src/xwstof.cpp b/stl/src/xwstof.cpp
index cbe81b4f3f0..8419e7f4974 100644
--- a/stl/src/xwstof.cpp
+++ b/stl/src/xwstof.cpp
@@ -3,7 +3,7 @@
 
 // _WStof function
 
-#include <wchar.h>
+#include <cwchar>
 
 #include "xmath.hpp"
 #include "xxfftype.hpp"
diff --git a/stl/src/xwstoflt.cpp b/stl/src/xwstoflt.cpp
index 0631679e5d2..0da23a067de 100644
--- a/stl/src/xwstoflt.cpp
+++ b/stl/src/xwstoflt.cpp
@@ -3,9 +3,9 @@
 
 // _WStoflt function
 
-#include <locale.h>
-#include <wchar.h>
-#include <wctype.h>
+#include <clocale>
+#include <cwchar>
+#include <cwctype>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xwstold.cpp b/stl/src/xwstold.cpp
index e00bb8194aa..ff8bbe800fa 100644
--- a/stl/src/xwstold.cpp
+++ b/stl/src/xwstold.cpp
@@ -3,7 +3,7 @@
 
 // _WStold function
 
-#include <wchar.h>
+#include <cwchar>
 
 #include "xmath.hpp"
 #include "xxlftype.hpp"
diff --git a/stl/src/xwstopfx.cpp b/stl/src/xwstopfx.cpp
index b3d74711b4d..a9447bca098 100644
--- a/stl/src/xwstopfx.cpp
+++ b/stl/src/xwstopfx.cpp
@@ -3,7 +3,7 @@
 
 // _WStopfx function
 
-#include <wctype.h>
+#include <cwctype>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xwstoxfl.cpp b/stl/src/xwstoxfl.cpp
index 871501031e3..174ab1cf351 100644
--- a/stl/src/xwstoxfl.cpp
+++ b/stl/src/xwstoxfl.cpp
@@ -3,10 +3,10 @@
 
 // _WStoxflt function
 
-#include <ctype.h>
-#include <locale.h>
-#include <wchar.h>
-#include <wctype.h>
+#include <cctype>
+#include <clocale>
+#include <cwchar>
+#include <cwctype>
 
 #include "xmath.hpp"
 
diff --git a/stl/src/xxdftype.hpp b/stl/src/xxdftype.hpp
index ecf8baff491..db33883c974 100644
--- a/stl/src/xxdftype.hpp
+++ b/stl/src/xxdftype.hpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h>
 
-#include <float.h>
+#include <cfloat>
 
 #define FTYPE   double
 #define FBITS   DBL_MANT_DIG
diff --git a/stl/src/xxfftype.hpp b/stl/src/xxfftype.hpp
index 387570dbb66..cd2e1ad4d80 100644
--- a/stl/src/xxfftype.hpp
+++ b/stl/src/xxfftype.hpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h>
 
-#include <float.h>
+#include <cfloat>
 
 #define FTYPE   float
 #define FBITS   FLT_MANT_DIG
diff --git a/stl/src/xxlftype.hpp b/stl/src/xxlftype.hpp
index 09b46c0c42c..8657da52ed6 100644
--- a/stl/src/xxlftype.hpp
+++ b/stl/src/xxlftype.hpp
@@ -5,7 +5,7 @@
 
 #include <yvals.h>
 
-#include <float.h>
+#include <cfloat>
 
 #define FTYPE   long double
 #define FBITS   LDBL_MANT_DIG
diff --git a/stl/src/xxxprec.hpp b/stl/src/xxxprec.hpp
index 7f63882fa38..7eea90d532a 100644
--- a/stl/src/xxxprec.hpp
+++ b/stl/src/xxxprec.hpp
@@ -3,7 +3,7 @@
 
 // common extended precision functionality
 
-#include <string.h>
+#include <cstring>
 
 #include "xmath.hpp"