diff --git a/Makefile.in b/Makefile.in
index 0835d87..e8117a0 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -54,6 +54,7 @@ SOURCE=\
 	persistent-data/lock_tracker.cc \
 	persistent-data/transaction_manager.cc \
 	\
+	persistent-data/data-structures/bitset.cc \
 	persistent-data/data-structures/btree.cc \
 	\
 	persistent-data/space_map.cc \
diff --git a/persistent-data/data-structures/bitset.cc b/persistent-data/data-structures/bitset.cc
new file mode 100644
index 0000000..9400222
--- /dev/null
+++ b/persistent-data/data-structures/bitset.cc
@@ -0,0 +1,202 @@
+#include "persistent-data/data-structures/array.h"
+#include "persistent-data/data-structures/bitset.h"
+#include "persistent-data/math_utils.h"
+
+using namespace persistent_data;
+using namespace persistent_data::bitset_detail;
+using namespace std;
+
+//----------------------------------------------------------------
+
+namespace {
+	struct bitset_traits {
+		typedef base::le64 disk_type;
+		typedef uint64_t value_type;
+		typedef no_op_ref_counter<uint64_t> ref_counter;
+
+		static void unpack(disk_type const &disk, value_type &value) {
+			value = base::to_cpu<uint64_t>(disk);
+		}
+
+		static void pack(value_type const &value, disk_type &disk) {
+			disk = base::to_disk<base::le64>(value);
+		}
+	};
+}
+
+namespace persistent_data {
+	namespace bitset_detail {
+		class bitset_impl {
+		public:
+			typedef boost::shared_ptr<bitset_impl> ptr;
+			typedef typename persistent_data::transaction_manager::ptr tm_ptr;
+
+			bitset_impl(tm_ptr tm)
+			: nr_bits_(0),
+			  array_(tm, rc_) {
+			}
+
+			bitset_impl(tm_ptr tm, block_address root, unsigned nr_bits)
+				: nr_bits_(nr_bits),
+				  array_(tm, rc_, root, nr_bits) {
+			}
+
+			block_address get_root() const {
+				return array_.get_root();
+			}
+
+			void grow(unsigned new_nr_bits, bool default_value) {
+				pad_last_block(default_value);
+				resize_array(new_nr_bits, default_value);
+				nr_bits_ = new_nr_bits;
+			}
+
+			void destroy() {
+				throw runtime_error("bitset.destroy() not implemented");
+			}
+
+			// May trigger a flush, so cannot be const
+			bool get(unsigned n) {
+				check_bounds(n);
+				return get_bit(array_.get(word(n)), bit(n));
+			}
+
+			void set(unsigned n, bool value) {
+				check_bounds(n);
+				unsigned w_index = word(n);
+				uint64_t w = array_.get(w_index);
+				if (value)
+					w = set_bit(w, bit(n));
+				else
+					w = clear_bit(w, bit(n));
+				array_.set(w_index, w);
+			}
+
+			void flush() {
+			}
+
+		private:
+			void pad_last_block(bool default_value) {
+				// Set defaults in the final word
+				if (bit(nr_bits_)) {
+					unsigned w_index = word(nr_bits_);
+					uint64_t w = array_.get(w_index);
+
+					for (unsigned b = bit(nr_bits_); b < 64; b++)
+						if (default_value)
+							w = set_bit(w, b);
+						else
+							w = clear_bit(w, b);
+
+					array_.set(w_index, w);
+				}
+			}
+
+			void resize_array(unsigned new_nr_bits, bool default_value) {
+				unsigned old_nr_words = words_needed(nr_bits_);
+				unsigned new_nr_words = words_needed(new_nr_bits);
+
+				if (new_nr_words < old_nr_words)
+					throw runtime_error("bitset grow actually asked to shrink");
+
+				if (new_nr_words > old_nr_words)
+					array_.grow(new_nr_words, default_value ? ~0 : 0);
+			}
+
+			unsigned words_needed(unsigned nr_bits) const {
+				return base::div_up<unsigned>(nr_bits, 64u);
+			}
+
+			unsigned word(unsigned bit) const {
+				return bit / 64;
+			}
+
+			uint64_t mask(unsigned bit) const {
+				return 1ull << bit;
+			}
+
+			bool get_bit(uint64_t w, unsigned bit) const {
+				return w & mask(bit);
+			}
+
+			uint64_t set_bit(uint64_t w, unsigned bit) const {
+				return w | mask(bit);
+			}
+
+			uint64_t clear_bit(uint64_t w, unsigned bit) const {
+				return w & (~mask(bit));
+			}
+
+			unsigned bit(unsigned bit) const {
+				return bit % 64;
+			}
+
+			// The last word may be only partially full, so we have to
+			// do our own bounds checking rather than relying on array
+			// to do it.
+			void check_bounds(unsigned n) const {
+				if (n >= nr_bits_) {
+					std::ostringstream str;
+					str << "bitset index out of bounds ("
+					    << n << " >= " << nr_bits_ << endl;
+					throw runtime_error(str.str());
+				}
+			}
+
+			unsigned nr_bits_;
+			no_op_ref_counter<uint64_t> rc_;
+			array<bitset_traits> array_;
+		};
+	}
+}
+
+//----------------------------------------------------------------
+
+bitset::bitset(tm_ptr tm)
+	: impl_(new bitset_impl(tm))
+{
+}
+
+bitset::bitset(tm_ptr tm, block_address root, unsigned nr_bits)
+	: impl_(new bitset_impl(tm, root, nr_bits))
+{
+}
+
+block_address
+bitset::get_root() const
+{
+	return impl_->get_root();
+}
+
+void
+bitset::grow(unsigned new_nr_bits, bool default_value)
+{
+	impl_->grow(new_nr_bits, default_value);
+}
+
+void
+bitset::destroy()
+{
+	impl_->destroy();
+}
+
+bool
+bitset::get(unsigned n)
+{
+	return impl_->get(n);
+}
+
+void
+bitset::set(unsigned n, bool value)
+{
+	impl_->set(n, value);
+}
+
+void
+bitset::flush()
+{
+	impl_->flush();
+}
+
+//----------------------------------------------------------------
+
diff --git a/persistent-data/data-structures/bitset.h b/persistent-data/data-structures/bitset.h
index d0fa572..89fa5e1 100644
--- a/persistent-data/data-structures/bitset.h
+++ b/persistent-data/data-structures/bitset.h
@@ -19,26 +19,11 @@
 #ifndef BITSET_H
 #define BITSET_H
 
-#include "persistent-data/math_utils.h"
-#include "persistent-data/data-structures/array.h"
-
 //----------------------------------------------------------------
 
 namespace persistent_data {
 	namespace bitset_detail {
-		struct bitset_traits {
-			typedef base::le64 disk_type;
-			typedef uint64_t value_type;
-			typedef no_op_ref_counter<uint64_t> ref_counter;
-
-			static void unpack(disk_type const &disk, value_type &value) {
-				value = base::to_cpu<uint64_t>(disk);
-			}
-
-			static void pack(value_type const &value, disk_type &disk) {
-				disk = base::to_disk<base::le64>(value);
-			}
-		};
+		class bitset_impl;
 	}
 
 	class bitset {
@@ -46,119 +31,19 @@ namespace persistent_data {
 		typedef boost::shared_ptr<bitset> ptr;
 		typedef typename persistent_data::transaction_manager::ptr tm_ptr;
 
-		bitset(tm_ptr tm)
-		: nr_bits_(0),
-		  array_(tm, rc_) {
-		}
-
-		bitset(tm_ptr tm, block_address root, unsigned nr_bits)
-			: nr_bits_(nr_bits),
-			  array_(tm, rc_, root, nr_bits) {
-		}
-
-		block_address get_root() const {
-			return array_.get_root();
-		}
-
-		void grow(unsigned new_nr_bits, bool default_value) {
-			pad_last_block(default_value);
-			resize_array(new_nr_bits, default_value);
-			nr_bits_ = new_nr_bits;
-		}
-
+		bitset(tm_ptr tm);
+		bitset(tm_ptr tm, block_address root, unsigned nr_bits);
+		block_address get_root() const;
+		void grow(unsigned new_nr_bits, bool default_value);
 		void destroy();
 
 		// May trigger a flush, so cannot be const
-		bool get(unsigned n) {
-			check_bounds(n);
-			return get_bit(array_.get(word(n)), bit(n));
-		}
-
-		void set(unsigned n, bool value) {
-			check_bounds(n);
-			unsigned w_index = word(n);
-			uint64_t w = array_.get(w_index);
-			if (value)
-				w = set_bit(w, bit(n));
-			else
-				w = clear_bit(w, bit(n));
-			array_.set(w_index, w);
-		}
-
-		void flush() {
-		}
+		bool get(unsigned n);
+		void set(unsigned n, bool value);
+		void flush();
 
 	private:
-		void pad_last_block(bool default_value) {
-			// Set defaults in the final word
-			if (bit(nr_bits_)) {
-				unsigned w_index = word(nr_bits_);
-				uint64_t w = array_.get(w_index);
-
-				for (unsigned b = bit(nr_bits_); b < 64; b++)
-					if (default_value)
-						w = set_bit(w, b);
-					else
-						w = clear_bit(w, b);
-
-				array_.set(w_index, w);
-			}
-		}
-
-		void resize_array(unsigned new_nr_bits, bool default_value) {
-			unsigned old_nr_words = words_needed(nr_bits_);
-			unsigned new_nr_words = words_needed(new_nr_bits);
-
-			if (new_nr_words < old_nr_words)
-				throw runtime_error("bitset grow actually asked to shrink");
-
-			if (new_nr_words > old_nr_words)
-				array_.grow(new_nr_words, default_value ? ~0 : 0);
-		}
-
-		unsigned words_needed(unsigned nr_bits) const {
-			return base::div_up<unsigned>(nr_bits, 64u);
-		}
-
-		unsigned word(unsigned bit) const {
-			return bit / 64;
-		}
-
-		uint64_t mask(unsigned bit) const {
-			return 1ull << bit;
-		}
-
-		bool get_bit(uint64_t w, unsigned bit) const {
-			return w & mask(bit);
-		}
-
-		uint64_t set_bit(uint64_t w, unsigned bit) const {
-			return w | mask(bit);
-		}
-
-		uint64_t clear_bit(uint64_t w, unsigned bit) const {
-			return w & (~mask(bit));
-		}
-
-		unsigned bit(unsigned bit) const {
-			return bit % 64;
-		}
-
-		// The last word may be only partially full, so we have to
-		// do our own bounds checking rather than relying on array
-		// to do it.
-		void check_bounds(unsigned n) const {
-			if (n >= nr_bits_) {
-				std::ostringstream str;
-				str << "bitset index out of bounds ("
-				    << n << " >= " << nr_bits_ << endl;
-				throw runtime_error(str.str());
-			}
-		}
-
-		unsigned nr_bits_;
-		no_op_ref_counter<uint64_t> rc_;
-		array<bitset_detail::bitset_traits> array_;
+		boost::shared_ptr<bitset_detail::bitset_impl> impl_;
 	};
 }