diff --git a/simplecpp.cpp b/simplecpp.cpp
index a7ced05a..d3511792 100644
--- a/simplecpp.cpp
+++ b/simplecpp.cpp
@@ -3,20 +3,6 @@
  * Copyright (C) 2016-2023 simplecpp team
  */
 
-#if defined(_WIN32)
-#  ifndef _WIN32_WINNT
-#    define _WIN32_WINNT 0x0602
-#  endif
-#  ifndef NOMINMAX
-#    define NOMINMAX
-#  endif
-#  ifndef WIN32_LEAN_AND_MEAN
-#    define WIN32_LEAN_AND_MEAN
-#  endif
-#  include <windows.h>
-#  undef ERROR
-#endif
-
 #include "simplecpp.h"
 
 #if defined(_WIN32) || defined(__CYGWIN__) || defined(__MINGW32__)
@@ -51,10 +37,22 @@
 #include <utility>
 #include <vector>
 
-#ifdef _WIN32
+#if defined(_WIN32)
+#  ifndef _WIN32_WINNT
+#    define _WIN32_WINNT 0x0602
+#  endif
+#  ifndef NOMINMAX
+#    define NOMINMAX
+#  endif
+#  ifndef WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN
+#  endif
+#  include <windows.h>
+#  undef ERROR
 #  include <direct.h>
 #else
 #  include <sys/stat.h>
+#  include <sys/types.h>
 #endif
 
 static bool isHex(const std::string &s)
@@ -3075,6 +3073,65 @@ static std::string openHeader(std::ifstream &f, const simplecpp::DUI &dui, const
     return "";
 }
 
+namespace {
+    struct FileID {
+#ifdef _WIN32
+        struct {
+            std::uint64_t VolumeSerialNumber;
+            struct {
+                std::uint64_t IdentifierHi;
+                std::uint64_t IdentifierLo;
+            } FileId;
+        } fileIdInfo;
+
+        bool operator==(const FileID &that) const noexcept {
+            return fileIdInfo.VolumeSerialNumber == that.fileIdInfo.VolumeSerialNumber &&
+                   fileIdInfo.FileId.IdentifierHi == that.fileIdInfo.FileId.IdentifierHi &&
+                   fileIdInfo.FileId.IdentifierLo == that.fileIdInfo.FileId.IdentifierLo;
+        }
+#else
+        dev_t dev;
+        ino_t ino;
+
+        bool operator==(const FileID& that) const noexcept {
+            return dev == that.dev && ino == that.ino;
+        }
+#endif
+        struct Hasher {
+            std::size_t operator()(const FileID &id) const {
+#ifdef _WIN32
+                return static_cast<std::size_t>(id.fileIdInfo.FileId.IdentifierHi ^ id.fileIdInfo.FileId.IdentifierLo ^
+                                                id.fileIdInfo.VolumeSerialNumber);
+#else
+                return static_cast<std::size_t>(id.dev) ^ static_cast<std::size_t>(id.ino);
+#endif
+            }
+        };
+    };
+}
+
+struct simplecpp::FileDataCache::Impl
+{
+    void clear()
+    {
+        mIdMap.clear();
+    }
+
+    using id_map_type = std::unordered_map<FileID, FileData *, FileID::Hasher>;
+
+    id_map_type mIdMap;
+};
+
+simplecpp::FileDataCache::FileDataCache()
+    : mImpl(new Impl)
+{}
+
+simplecpp::FileDataCache::~FileDataCache() = default;
+simplecpp::FileDataCache::FileDataCache(FileDataCache &&) noexcept = default;
+simplecpp::FileDataCache &simplecpp::FileDataCache::operator=(simplecpp::FileDataCache &&) noexcept = default;
+
+static bool getFileId(const std::string &path, FileID &id);
+
 std::pair<simplecpp::FileData *, bool> simplecpp::FileDataCache::tryload(FileDataCache::name_map_type::iterator &name_it, const simplecpp::DUI &dui, std::vector<std::string> &filenames, simplecpp::OutputList *outputList)
 {
     const std::string &path = name_it->first;
@@ -3083,8 +3140,8 @@ std::pair<simplecpp::FileData *, bool> simplecpp::FileDataCache::tryload(FileDat
     if (!getFileId(path, fileId))
         return {nullptr, false};
 
-    const auto id_it = mIdMap.find(fileId);
-    if (id_it != mIdMap.end()) {
+    const auto id_it = mImpl->mIdMap.find(fileId);
+    if (id_it != mImpl->mIdMap.end()) {
         name_it->second = id_it->second;
         return {id_it->second, false};
     }
@@ -3095,7 +3152,7 @@ std::pair<simplecpp::FileData *, bool> simplecpp::FileDataCache::tryload(FileDat
         data->tokens.removeComments();
 
     name_it->second = data;
-    mIdMap.emplace(fileId, data);
+    mImpl->mIdMap.emplace(fileId, data);
     mData.emplace_back(data);
 
     return {data, true};
@@ -3147,7 +3204,14 @@ std::pair<simplecpp::FileData *, bool> simplecpp::FileDataCache::get(const std::
     return {nullptr, false};
 }
 
-bool simplecpp::FileDataCache::getFileId(const std::string &path, FileID &id)
+void simplecpp::FileDataCache::clear()
+{
+    mImpl->clear();
+    mNameMap.clear();
+    mData.clear();
+}
+
+bool getFileId(const std::string &path, FileID &id)
 {
 #ifdef _WIN32
     HANDLE hFile = CreateFileA(path.c_str(), 0, FILE_SHARE_READ | FILE_SHARE_WRITE, nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
diff --git a/simplecpp.h b/simplecpp.h
index f29166ff..de20ae2e 100644
--- a/simplecpp.h
+++ b/simplecpp.h
@@ -41,10 +41,6 @@
 #  define SIMPLECPP_LIB
 #endif
 
-#ifndef _WIN32
-#  include <sys/types.h>
-#endif
-
 #if defined(_MSC_VER)
 #  pragma warning(push)
 // suppress warnings about "conversion from 'type1' to 'type2', possible loss of data"
@@ -452,13 +448,14 @@ namespace simplecpp {
 
     class SIMPLECPP_LIB FileDataCache {
     public:
-        FileDataCache() = default;
+        FileDataCache();
+        ~FileDataCache();
 
         FileDataCache(const FileDataCache &) = delete;
-        FileDataCache(FileDataCache &&) = default;
+        FileDataCache(FileDataCache &&) noexcept;
 
         FileDataCache &operator=(const FileDataCache &) = delete;
-        FileDataCache &operator=(FileDataCache &&) = default;
+        FileDataCache &operator=(FileDataCache &&) noexcept;
 
         /** Get the cached data for a file, or load and then return it if it isn't cached.
          *  returns the file data and true if the file was loaded, false if it was cached. */
@@ -472,11 +469,7 @@ namespace simplecpp {
             mNameMap.emplace(newdata->filename, newdata);
         }
 
-        void clear() {
-            mNameMap.clear();
-            mIdMap.clear();
-            mData.clear();
-        }
+        void clear();
 
         using container_type = std::vector<std::unique_ptr<FileData>>;
         using iterator = container_type::iterator;
@@ -506,51 +499,15 @@ namespace simplecpp {
         }
 
     private:
-        struct FileID {
-#ifdef _WIN32
-            struct {
-                std::uint64_t VolumeSerialNumber;
-                struct {
-                    std::uint64_t IdentifierHi;
-                    std::uint64_t IdentifierLo;
-                } FileId;
-            } fileIdInfo;
-
-            bool operator==(const FileID &that) const noexcept {
-                return fileIdInfo.VolumeSerialNumber == that.fileIdInfo.VolumeSerialNumber &&
-                       fileIdInfo.FileId.IdentifierHi == that.fileIdInfo.FileId.IdentifierHi &&
-                       fileIdInfo.FileId.IdentifierLo == that.fileIdInfo.FileId.IdentifierLo;
-            }
-#else
-            dev_t dev;
-            ino_t ino;
-
-            bool operator==(const FileID& that) const noexcept {
-                return dev == that.dev && ino == that.ino;
-            }
-#endif
-            struct Hasher {
-                std::size_t operator()(const FileID &id) const {
-#ifdef _WIN32
-                    return static_cast<std::size_t>(id.fileIdInfo.FileId.IdentifierHi ^ id.fileIdInfo.FileId.IdentifierLo ^
-                                                    id.fileIdInfo.VolumeSerialNumber);
-#else
-                    return static_cast<std::size_t>(id.dev) ^ static_cast<std::size_t>(id.ino);
-#endif
-                }
-            };
-        };
+        struct Impl;
+        std::unique_ptr<Impl> mImpl;
 
         using name_map_type = std::unordered_map<std::string, FileData *>;
-        using id_map_type = std::unordered_map<FileID, FileData *, FileID::Hasher>;
-
-        static bool getFileId(const std::string &path, FileID &id);
 
         std::pair<FileData *, bool> tryload(name_map_type::iterator &name_it, const DUI &dui, std::vector<std::string> &filenames, OutputList *outputList);
 
         container_type mData;
         name_map_type mNameMap;
-        id_map_type mIdMap;
     };
 
     /** Converts character literal (including prefix, but not ud-suffix) to long long value.