diff --git a/.github/workflows/build-and-test-rh.yml b/.github/workflows/build-and-test-rh.yml
index c30fc19..65f88a9 100644
--- a/.github/workflows/build-and-test-rh.yml
+++ b/.github/workflows/build-and-test-rh.yml
@@ -87,6 +87,15 @@ jobs:
         run:  sudo yum -y install findutils
 
       - name: Checkout shell test framework
+        if: matrix.image != 'centos:7'
+        uses: actions/checkout@v4
+        with:
+          repository: kward/shunit2
+          path: ${{github.workspace}}/tests/shunit2
+          fetch-depth: 1
+
+      - name: Checkout shell test framework centos7
+        if: matrix.image == 'centos:7'
         uses: actions/checkout@v3
         with:
           repository: kward/shunit2
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9ac363c..d65b30e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,7 +51,6 @@ if (BUILD_SHARED_LIBS AND MSVC)
     message(FATAL_ERROR "Building sexp shared library with MSVC is not supported")
 endif(BUILD_SHARED_LIBS AND MSVC)
 
-
 message(STATUS "Building ${TYPE} library")
 
 if (WITH_SANITIZERS)
@@ -107,7 +106,6 @@ if(NOT MSVC)
     -Wno-unused-parameter
     -Wno-missing-field-initializers
   )
-
 endif(NOT MSVC)
 
 add_library(sexpp ${TYPE}
@@ -132,7 +130,10 @@ target_include_directories(sexpp PUBLIC
 
 if (BUILD_SHARED_LIBS)
     target_compile_definitions(sexpp PUBLIC BUILD_SHARED_LIBS)
-    set_target_properties(sexpp PROPERTIES CXX_VISIBILITY_PRESET "hidden")
+    set_target_properties(sexpp PROPERTIES
+        CXX_VISIBILITY_PRESET "hidden"
+        VISIBILITY_INLINES_HIDDEN true
+    )
 endif (BUILD_SHARED_LIBS)
 
 set_target_properties(sexpp PROPERTIES
diff --git a/include/sexpp/sexp.h b/include/sexpp/sexp.h
index bb6ae4e..7e67d93 100644
--- a/include/sexpp/sexp.h
+++ b/include/sexpp/sexp.h
@@ -101,25 +101,25 @@ class sexp_input_stream_t;
 
 typedef uint8_t octet_t;
 
-class SEXP_PUBLIC_SYMBOL sexp_simple_string_t : public std::basic_string<octet_t>,
+class SEXP_PUBLIC_SYMBOL sexp_simple_string_t : public std::vector<octet_t>,
                                                 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) : std::vector<octet_t>() { for (; *dt; ++dt) push_back(*dt); }
+    sexp_simple_string_t(const octet_t *bt, size_t ln) : std::vector<octet_t>(ln) { for (size_t s = 0; s < ln; ++bt, ++s ) push_back(*bt); }
     sexp_simple_string_t &append(int c)
     {
-        (*this) += (octet_t)(c & 0xFF);
+        push_back((octet_t)(c & 0xFF));
         return *this;
     }
     // Returns length for printing simple string as a token
-    size_t advanced_length_token(void) const { return length(); }
+    size_t advanced_length_token(void) const { return size(); }
     // Returns length for printing simple string as a base64 string
-    size_t advanced_length_base64(void) const { return (2 + 4 * ((length() + 2) / 3)); }
+    size_t advanced_length_base64(void) const { return (2 + 4 * ((size() + 2) / 3)); }
     // Returns length for printing simple string ss in quoted-string mode
-    size_t advanced_length_quoted(void) const { return (1 + length() + 1); }
+    size_t advanced_length_quoted(void) const { return (1 + size() + 1); }
     // Returns length for printing simple string ss in hexadecimal mode
-    size_t advanced_length_hexadecimal(void) const { return (1 + 2 * length() + 1); }
+    size_t advanced_length_hexadecimal(void) const { return (1 + 2 * size() + 1); }
     size_t advanced_length(sexp_output_stream_t *os) const;
 
     sexp_output_stream_t *print_canonical_verbatim(sexp_output_stream_t *os) const;
@@ -134,19 +134,15 @@ class SEXP_PUBLIC_SYMBOL sexp_simple_string_t : public std::basic_string<octet_t
 
     bool operator==(const char *right) const noexcept
     {
-        return length() == std::strlen(right) && std::memcmp(data(), right, length()) == 0;
+        return size() == std::strlen(right) && std::memcmp(data(), right, size()) == 0;
     }
 
     bool operator!=(const char *right) const noexcept
     {
-        return length() != std::strlen(right) || std::memcmp(data(), right, length()) != 0;
+        return size() != std::strlen(right) || std::memcmp(data(), right, size()) != 0;
     }
 
-    unsigned as_unsigned() const noexcept
-    {
-        return empty() ? std::numeric_limits<uint32_t>::max() :
-                         (unsigned) atoi(reinterpret_cast<const char *>(c_str()));
-    }
+    uint32_t as_unsigned() const noexcept;
 };
 
 inline bool operator==(const sexp_simple_string_t *left, const std::string &right) noexcept
@@ -193,7 +189,7 @@ class SEXP_PUBLIC_SYMBOL sexp_object_t {
     }
     virtual bool     operator==(const char *right) const noexcept { return false; }
     virtual bool     operator!=(const char *right) const noexcept { return true; }
-    virtual unsigned as_unsigned() const noexcept
+    virtual uint32_t as_unsigned() const noexcept
     {
         return std::numeric_limits<uint32_t>::max();
     }
@@ -250,17 +246,19 @@ class SEXP_PUBLIC_SYMBOL sexp_string_t : public sexp_object_t {
     virtual bool operator!=(const char *right) const noexcept { return data_string != right; }
 
     void             parse(sexp_input_stream_t *sis);
-    virtual unsigned as_unsigned() const noexcept { return data_string.as_unsigned(); }
+    virtual uint32_t as_unsigned() const noexcept { return data_string.as_unsigned(); }
 };
 
 inline bool operator==(const sexp_string_t *left, const std::string &right) noexcept
 {
-    return *left == right.c_str();
+    return left->get_string().size() == right.length() &&
+           memcmp(left->get_string().data(), right.c_str(), left->get_string().size()) == 0;
 }
 
 inline bool operator!=(const sexp_string_t *left, const std::string &right) noexcept
 {
-    return *left != right.c_str();
+    return left->get_string().size() != right.length() ||
+           memcmp(left->get_string().data(), right.c_str(), left->get_string().size()) != 0;
 }
 
 /*
diff --git a/src/sexp-input.cpp b/src/sexp-input.cpp
index cb21686..3538497 100644
--- a/src/sexp-input.cpp
+++ b/src/sexp-input.cpp
@@ -234,9 +234,9 @@ void sexp_input_stream_t::scan_verbatim_string(sexp_simple_string_t &ss, uint32_
 void sexp_input_stream_t::scan_quoted_string(sexp_simple_string_t &ss, uint32_t length)
 {
     skip_char('"');
-    while (ss.length() <= length) {
+    while (ss.size() <= length) {
         if (next_char == '\"') {
-            if (length == std::numeric_limits<uint32_t>::max() || (ss.length() == length)) {
+            if (length == std::numeric_limits<uint32_t>::max() || (ss.size() == length)) {
                 skip_char('\"');
                 return;
             } else
@@ -367,10 +367,10 @@ void sexp_input_stream_t::scan_hexadecimal_string(sexp_simple_string_t &ss, uint
         get_char();
     }
     skip_char('#');
-    if (ss.length() != length && length != std::numeric_limits<uint32_t>::max())
+    if (ss.size() != length && length != std::numeric_limits<uint32_t>::max())
         sexp_error(sexp_exception_t::warning,
                    "Hex string has length %d different than declared length %d",
-                   ss.length(),
+                   ss.size(),
                    length,
                    count);
 }
@@ -389,10 +389,10 @@ void sexp_input_stream_t::scan_base64_string(sexp_simple_string_t &ss, uint32_t
         get_char();
     }
     skip_char('|');
-    if (ss.length() != length && length != std::numeric_limits<uint32_t>::max())
+    if (ss.size() != length && length != std::numeric_limits<uint32_t>::max())
         sexp_error(sexp_exception_t::warning,
                    "Base64 string has length %d different than declared length %d",
-                   ss.length(),
+                   ss.size(),
                    length,
                    count);
 }
@@ -441,7 +441,7 @@ sexp_simple_string_t sexp_input_stream_t::scan_simple_string(void)
         }
     }
 
-    if (ss.length() == 0)
+    if (ss.size() == 0)
         sexp_error(sexp_exception_t::warning, "Simple string has zero length", 0, 0, count);
     return ss;
 }
diff --git a/src/sexp-simple-string.cpp b/src/sexp-simple-string.cpp
index 0e4c869..3029850 100644
--- a/src/sexp-simple-string.cpp
+++ b/src/sexp-simple-string.cpp
@@ -29,6 +29,27 @@
 #include "sexpp/sexp.h"
 
 namespace sexp {
+
+/*
+ * sexp_simple_string_t::sexp_simple_string_t(const octet_t *dt)
+ */
+sexp_simple_string_t::sexp_simple_string_t(const octet_t *dt) : std::vector<octet_t>()
+{
+    for (; *dt; ++dt)
+        push_back(*dt);
+}
+
+/*
+ * sexp_simple_string_t::sexp_simple_string_t(const octet_t *bt, size_t ln)
+ */
+sexp_simple_string_t::sexp_simple_string_t(const octet_t *bt, size_t ln)
+    : std::vector<octet_t>()
+{
+    reserve(ln);
+    for (size_t s = 0; s < ln; ++bt, ++s)
+        push_back(*bt);
+}
+
 /*
  * sexp_simple_string_t::print_canonical_verbatim(os)
  * Print out simple string on output stream os as verbatim string.
@@ -36,11 +57,11 @@ namespace sexp {
 sexp_output_stream_t *sexp_simple_string_t::print_canonical_verbatim(
   sexp_output_stream_t *os) const
 {
-    const octet_t *c = c_str();
+    const octet_t *c = data();
     /* print out len: */
-    os->print_decimal(length())->var_put_char(':');
+    os->print_decimal(size())->var_put_char(':');
     /* print characters in fragment */
-    for (uint32_t i = 0; i < length(); i++)
+    for (uint32_t i = 0; i < size(); i++)
         os->var_put_char((int) *c++);
     return os;
 }
@@ -55,7 +76,7 @@ size_t sexp_simple_string_t::advanced_length(sexp_output_stream_t *os) const
         return advanced_length_token();
     else if (can_print_as_quoted_string())
         return advanced_length_quoted();
-    else if (length() <= 4 && os->get_byte_size() == 8)
+    else if (size() <= 4 && os->get_byte_size() == 8)
         return advanced_length_hexadecimal();
     else if (os->get_byte_size() == 8)
         return advanced_length_base64();
@@ -63,6 +84,26 @@ size_t sexp_simple_string_t::advanced_length(sexp_output_stream_t *os) const
         return 0; /* an error condition */
 }
 
+/*
+ * sexp_simple_string_t::as_unsigned
+ * Converts simple string to unsigned integer.
+ * Returns std::numeric_limits<uint32_t>::max() if simple string is empty
+ * Returns 0 if simple string contains non-digit characters
+ */
+uint32_t sexp_simple_string_t::as_unsigned() const noexcept
+{
+    if (empty())
+        return std::numeric_limits<uint32_t>::max();
+
+    uint32_t result = 0;
+    for (octet_t c : *this) {
+        if (!is_dec_digit(c))
+            return 0;
+        result = result * 10 + (c - '0');
+    }
+    return result;
+}
+
 /*
  * sexp_simple_string_t::print_token(os)
  * Prints out simple string ss as a token (assumes that this is OK).
@@ -70,10 +111,10 @@ size_t sexp_simple_string_t::advanced_length(sexp_output_stream_t *os) const
  */
 sexp_output_stream_t *sexp_simple_string_t::print_token(sexp_output_stream_t *os) const
 {
-    const octet_t *c = c_str();
-    if (os->get_max_column() > 0 && os->get_column() > (os->get_max_column() - length()))
+    const octet_t *c = data();
+    if (os->get_max_column() > 0 && os->get_column() > (os->get_max_column() - size()))
         os->new_line(sexp_output_stream_t::advanced);
-    for (uint32_t i = 0; i < length(); i++)
+    for (uint32_t i = 0; i < size(); i++)
         os->put_char((int) (*c++));
     return os;
 }
@@ -84,9 +125,9 @@ sexp_output_stream_t *sexp_simple_string_t::print_token(sexp_output_stream_t *os
  */
 sexp_output_stream_t *sexp_simple_string_t::print_base64(sexp_output_stream_t *os) const
 {
-    const octet_t *c = c_str();
+    const octet_t *c = data();
     os->var_put_char('|')->change_output_byte_size(6, sexp_output_stream_t::advanced);
-    for (uint32_t i = 0; i < length(); i++)
+    for (uint32_t i = 0; i < size(); i++)
         os->var_put_char((int) (*c++));
     return os->flush()
       ->change_output_byte_size(8, sexp_output_stream_t::advanced)
@@ -99,9 +140,9 @@ sexp_output_stream_t *sexp_simple_string_t::print_base64(sexp_output_stream_t *o
  */
 sexp_output_stream_t *sexp_simple_string_t::print_hexadecimal(sexp_output_stream_t *os) const
 {
-    const octet_t *c = c_str();
+    const octet_t *c = data();
     os->put_char('#')->change_output_byte_size(4, sexp_output_stream_t::advanced);
-    for (uint32_t i = 0; i < length(); i++)
+    for (uint32_t i = 0; i < size(); i++)
         os->var_put_char((int) (*c++));
     return os->flush()
       ->change_output_byte_size(8, sexp_output_stream_t::advanced)
@@ -117,9 +158,9 @@ sexp_output_stream_t *sexp_simple_string_t::print_hexadecimal(sexp_output_stream
  */
 sexp_output_stream_t *sexp_simple_string_t::print_quoted(sexp_output_stream_t *os) const
 {
-    const octet_t *c = c_str();
+    const octet_t *c = data();
     os->put_char('\"');
-    for (uint32_t i = 0; i < length(); i++) {
+    for (uint32_t i = 0; i < size(); i++) {
         if (os->get_max_column() > 0 && os->get_column() >= os->get_max_column() - 2) {
             os->put_char('\\')->put_char('\n');
             os->reset_column();
@@ -139,7 +180,7 @@ sexp_output_stream_t *sexp_simple_string_t::print_advanced(sexp_output_stream_t
         print_token(os);
     else if (can_print_as_quoted_string())
         print_quoted(os);
-    else if (length() <= 4 && os->get_byte_size() == 8)
+    else if (size() <= 4 && os->get_byte_size() == 8)
         print_hexadecimal(os);
     else if (os->get_byte_size() == 8)
         print_base64(os);
@@ -159,8 +200,8 @@ sexp_output_stream_t *sexp_simple_string_t::print_advanced(sexp_output_stream_t
  */
 bool sexp_simple_string_t::can_print_as_quoted_string(void) const
 {
-    const octet_t *c = c_str();
-    for (uint32_t i = 0; i < length(); i++, c++) {
+    const octet_t *c = data();
+    for (uint32_t i = 0; i < size(); i++, c++) {
         if (!is_token_char((int) (*c)) && *c != ' ')
             return false;
     }
@@ -174,14 +215,14 @@ bool sexp_simple_string_t::can_print_as_quoted_string(void) const
  */
 bool sexp_simple_string_t::can_print_as_token(const sexp_output_stream_t *os) const
 {
-    const octet_t *c = c_str();
-    if (length() <= 0)
+    const octet_t *c = data();
+    if (size() <= 0)
         return false;
     if (is_dec_digit((int) *c))
         return false;
-    if (os->get_max_column() > 0 && os->get_column() + length() >= os->get_max_column())
+    if (os->get_max_column() > 0 && os->get_column() + size() >= os->get_max_column())
         return false;
-    for (uint32_t i = 0; i < length(); i++) {
+    for (uint32_t i = 0; i < size(); i++) {
         if (!is_token_char((int) (*c++)))
             return false;
     }
diff --git a/tests/src/primitives-tests.cpp b/tests/src/primitives-tests.cpp
index 0689d27..64b4b4f 100644
--- a/tests/src/primitives-tests.cpp
+++ b/tests/src/primitives-tests.cpp
@@ -201,7 +201,8 @@ TEST_F(PrimitivesTests, at4rnp)
     }
 
     const sexp_string_t *sstr = lst.sexp_string_at(0);
-    EXPECT_STREQ(reinterpret_cast<const char *>(sstr->get_string().c_str()), "rnp_block");
+    int result = memcmp(sstr->get_string().data(), "rnp_block", sstr->get_string().size());
+    EXPECT_EQ(result, 0);
 }
 
 TEST_F(PrimitivesTests, eq4rnp)
@@ -280,6 +281,15 @@ TEST_F(PrimitivesTests, u4rnp)
     EXPECT_EQ(lst.sexp_string_at(1)->as_unsigned(), 54321);
 }
 
+TEST_F(PrimitivesTests, asUnsigned)
+{
+    sexp_simple_string_t sss1(reinterpret_cast<const octet_t *>(""));
+    EXPECT_EQ(sss1.as_unsigned(), std::numeric_limits<uint32_t>::max());
+
+    sexp_simple_string_t sss2(reinterpret_cast<const octet_t *>("123A456"));
+    EXPECT_EQ(sss2.as_unsigned(), 0);
+}
+
 TEST_F(PrimitivesTests, proInheritance)
 {
     sexp_list_t lst;
@@ -391,4 +401,15 @@ TEST_F(PrimitivesTests, EnsureHexTest)
     EXPECT_EQ(oss.str(), "(#610963#)");
 }
 
+TEST_F(PrimitivesTests, SimpleStringConstructors)
+{
+    const char *tss = "test simple string";
+
+    sexp_simple_string_t sss1(reinterpret_cast<const octet_t *>(tss));
+    EXPECT_TRUE(sss1 == tss);
+
+    sexp_simple_string_t sss2(reinterpret_cast<const octet_t *>(tss), 4);
+    EXPECT_TRUE(sss2 == "test");
+}
+
 } // namespace
diff --git a/version.txt b/version.txt
index 1e9b46b..ac39a10 100644
--- a/version.txt
+++ b/version.txt
@@ -1 +1 @@
-0.8.7
+0.9.0