diff --git a/include/sexpp/sexp.h b/include/sexpp/sexp.h
index bb6ae4e..3bba921 100644
--- a/include/sexpp/sexp.h
+++ b/include/sexpp/sexp.h
@@ -99,14 +99,69 @@ class sexp_input_stream_t;
  * SEXP simple string
  */
 
-typedef uint8_t octet_t;
+using octet_t = uint8_t;
 
-class SEXP_PUBLIC_SYMBOL sexp_simple_string_t : public std::basic_string<octet_t>,
-                                                private sexp_char_defs_t {
+struct octet_traits : std::char_traits<octet_t> {
+    static void assign(char_type &r, const char_type &a) { r = a; }
+
+    static bool eq(const char_type &c1, const char_type &c2) { return c1 == c2; }
+
+    static bool lt(const char_type &c1, const char_type &c2) { return c1 < c2; }
+
+    static int compare(const char_type *s1, const char_type *s2, std::size_t n)
+    {
+        while (n-- != 0) {
+            if (*s1 < *s2)
+                return -1;
+            if (*s2++ < *s1++)
+                return 1;
+        }
+        return 0;
+    }
+
+    static const char_type *find(const char_type *s, std::size_t n, const char_type &a)
+    {
+        while (n-- != 0) {
+            if (*s == a)
+                return s;
+            s++;
+        }
+        return nullptr;
+    }
+
+    static char_type *move(char_type *s1, const char_type *s2, std::size_t n)
+    {
+        return (char_type *) std::memmove(s1, s2, n);
+    }
+
+    static char_type *copy(char_type *s1, const char_type *s2, std::size_t n)
+    {
+        return (char_type *) std::memcpy(s1, s2, n);
+    }
+
+    static char_type *assign(char_type *s, std::size_t n, char_type a)
+    {
+        return (char_type *) std::memset(s, a, n);
+    }
+
+    static char_type to_char_type(const int_type &c) { return char_type(c); }
+
+    static int_type to_int_type(const char_type &c) { return int_type(c); }
+
+    static bool eq_int_type(const int_type &c1, const int_type &c2) { return c1 == c2; }
+
+    static int_type eof() { return static_cast<int_type>(EOF); }
+
+    static int_type not_eof(const int_type &c) { return eq_int_type(c, eof()) ? 0 : c; }
+};
+
+using octet_string = std::basic_string<octet_t, octet_traits>;
+
+class SEXP_PUBLIC_SYMBOL sexp_simple_string_t : public octet_string, private sexp_char_defs_t {
   public:
     sexp_simple_string_t(void) = default;
-    sexp_simple_string_t(const octet_t *dt) : std::basic_string<octet_t>{dt} {}
-    sexp_simple_string_t(const octet_t *bt, size_t ln) : std::basic_string<octet_t>{bt, ln} {}
+    sexp_simple_string_t(const octet_t *dt) : octet_string{dt} {}
+    sexp_simple_string_t(const octet_t *bt, size_t ln) : octet_string{bt, ln} {}
     sexp_simple_string_t &append(int c)
     {
         (*this) += (octet_t)(c & 0xFF);
diff --git a/tests/src/traits-tests.cpp b/tests/src/traits-tests.cpp
new file mode 100644
index 0000000..52e1019
--- /dev/null
+++ b/tests/src/traits-tests.cpp
@@ -0,0 +1,116 @@
+/**
+ *
+ * Copyright 2024 Ribose Inc. (https://www.ribose.com)
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#include "sexp-tests.h"
+
+using namespace sexp;
+
+namespace {
+
+TEST(OctetTraitsTest, Assign)
+{
+    octet_t a = 0x12;
+    octet_t b = 0x34;
+    octet_traits::assign(a, b);
+    EXPECT_EQ(a, b);
+}
+
+TEST(OctetTraitsTest, Eq)
+{
+    octet_t a = 0x12;
+    octet_t b = 0x12;
+    EXPECT_TRUE(octet_traits::eq(a, b));
+}
+
+TEST(OctetTraitsTest, Lt)
+{
+    octet_t a = 0x12;
+    octet_t b = 0x34;
+    EXPECT_TRUE(octet_traits::lt(a, b));
+}
+
+TEST(OctetTraitsTest, Compare)
+{
+    octet_t s1[] = {0x12, 0x34, 0x56};
+    octet_t s2[] = {0x12, 0x34, 0x57};
+    EXPECT_LT(octet_traits::compare(s1, s2, 3), 0);
+}
+
+TEST(OctetTraitsTest, Find)
+{
+    octet_t s[] = {0x12, 0x34, 0x56};
+    octet_t a = 0x34;
+    EXPECT_EQ(octet_traits::find(s, 3, a), s + 1);
+}
+
+TEST(OctetTraitsTest, Move)
+{
+    octet_t s1[] = {0x12, 0x34, 0x56};
+    octet_t s2[3];
+    octet_traits::move(s2, s1, 3);
+    EXPECT_EQ(memcmp(s1, s2, 3), 0);
+}
+
+TEST(OctetTraitsTest, Copy)
+{
+    octet_t s1[] = {0x12, 0x34, 0x56};
+    octet_t s2[3];
+    octet_traits::copy(s2, s1, 3);
+    EXPECT_EQ(memcmp(s1, s2, 3), 0);
+}
+
+TEST(OctetTraitsTest, AssignMultiple)
+{
+    octet_t s[3];
+    octet_t a = 0x12;
+    octet_traits::assign(s, 3, a);
+    for (int i = 0; i < 3; i++) {
+        EXPECT_EQ(s[i], a);
+    }
+}
+
+TEST(OctetTraitsTest, ToCharType)
+{
+    octet_traits::int_type a = 0x12;
+    EXPECT_EQ(octet_traits::to_char_type(a), 0x12);
+}
+
+TEST(OctetTraitsTest, ToIntType)
+{
+    octet_t a = 0x12;
+    EXPECT_EQ(octet_traits::to_int_type(a), 0x12);
+}
+
+TEST(OctetTraitsTest, EqIntType)
+{
+    octet_traits::int_type a = 0x12;
+    octet_traits::int_type b = 0x12;
+    EXPECT_TRUE(octet_traits::eq_int_type(a, b));
+}
+
+TEST(OctetTraitsTest, NotEof)
+{
+    octet_traits::int_type a = 0x12;
+    EXPECT_EQ(octet_traits::not_eof(a), 0x12);
+}
+} // namespace
diff --git a/version.txt b/version.txt
index 1e9b46b..6201b5f 100644
--- a/version.txt
+++ b/version.txt
@@ -1 +1 @@
-0.8.7
+0.8.8