diff --git a/cpp/unpack.cpp b/cpp/unpack.cpp
index ea4d414c..8edf4072 100644
--- a/cpp/unpack.cpp
+++ b/cpp/unpack.cpp
@@ -178,13 +178,14 @@ private:
 };
 
 
-unpacker::unpacker() :
+unpacker::unpacker(size_t initial_buffer_size) :
 	m_buffer(NULL),
 	m_used(0),
 	m_free(0),
 	m_off(0),
 	m_zone(new zone()),
-	m_ctx(new context(&*m_zone))
+	m_ctx(new context(&*m_zone)),
+	m_initial_buffer_size(initial_buffer_size)
 { }
 
 
@@ -199,14 +200,14 @@ void unpacker::expand_buffer(size_t len)
 	if(m_off == 0) {
 		size_t next_size;
 		if(m_used != 0) { next_size = (m_used + m_free) * 2; }
-		else { next_size = UNPACKER_INITIAL_BUFFER_SIZE; }
+		else { next_size = m_initial_buffer_size; }
 		while(next_size < len + m_used) { next_size *= 2; }
 
 		m_buffer = m_zone->realloc(m_buffer, next_size);
 		m_free = next_size - m_used;
 
 	} else {
-		size_t next_size = UNPACKER_INITIAL_BUFFER_SIZE;
+		size_t next_size = m_initial_buffer_size;
 		while(next_size < len + m_used - m_off) { next_size *= 2; }
 
 		char* tmp = m_zone->malloc(next_size);
diff --git a/cpp/unpack.hpp b/cpp/unpack.hpp
index ffee60d6..ac20de3b 100644
--- a/cpp/unpack.hpp
+++ b/cpp/unpack.hpp
@@ -23,16 +23,13 @@
 #include <memory>
 #include <stdexcept>
 
-#ifndef MSGPACK_UNPACKER_INITIAL_BUFFER_SIZE
-#define MSGPACK_UNPACKER_INITIAL_BUFFER_SIZE 8*1024
+#ifndef MSGPACK_UNPACKER_DEFAULT_INITIAL_BUFFER_SIZE
+#define MSGPACK_UNPACKER_DEFAULT_INITIAL_BUFFER_SIZE 8*1024
 #endif
 
 namespace msgpack {
 
 
-static const size_t UNPACKER_INITIAL_BUFFER_SIZE = MSGPACK_UNPACKER_INITIAL_BUFFER_SIZE;
-
-
 struct unpack_error : public std::runtime_error {
 	unpack_error(const std::string& msg) :
 		std::runtime_error(msg) { }
@@ -41,7 +38,7 @@ struct unpack_error : public std::runtime_error {
 
 class unpacker {
 public:
-	unpacker();
+	unpacker(size_t initial_buffer_size = MSGPACK_UNPACKER_DEFAULT_INITIAL_BUFFER_SIZE);
 	~unpacker();
 
 public:
@@ -139,6 +136,8 @@ private:
 	struct context;
 	context* m_ctx;
 
+	const size_t m_initial_buffer_size;
+
 private:
 	void expand_buffer(size_t len);