diff --git a/include/upa/str_arg.h b/include/upa/str_arg.h
index 9e6e643..829b6fb 100644
--- a/include/upa/str_arg.h
+++ b/include/upa/str_arg.h
@@ -6,9 +6,9 @@
 /**************************************************************
 // Usage example:
 
-template <class ...Args, enable_if_str_arg_t<Args...> = 0>
-inline void procfn(Args&&... args) {
-    const auto inp = make_str_arg(std::forward<Args>(args)...);
+template <class StrT, enable_if_str_arg_t<StrT> = 0>
+inline void procfn(StrT&& str) {
+    const auto inp = make_str_arg(std::forward<StrT>(str));
     const auto* first = inp.begin();
     const auto* last = inp.end();
     // do something with first ... last
@@ -76,18 +76,17 @@ class str_arg {
         : first_(s)
         , last_(s + traits_type::length(s))
     {}
-    str_arg(const CharT* s, std::size_t length)
-        : first_(s)
-        , last_(s + length)
-    {}
-    str_arg(const CharT* s, std::ptrdiff_t length)
+
+    template <typename SizeT, std::enable_if_t<is_size_type_v<SizeT>, int> = 0>
+    str_arg(const CharT* s, SizeT length)
         : first_(s)
         , last_(s + length)
-    {}
+    { assert(length >= 0); }
+
     str_arg(const CharT* first, const CharT* last)
         : first_(first)
         , last_(last)
-    {}
+    { assert(first <= last); }
 
     // destructor
     ~str_arg() noexcept = default;
@@ -150,33 +149,10 @@ namespace detail {
 
 // Requirements for string arguments
 
-template<class ...Args>
+template<class StrT, typename = void>
 struct str_arg_char {};
 
-// two pointers
-template<class CharT>
-struct str_arg_char<CharT*, CharT*> : std::remove_cv<CharT> {
-
-    template <typename T>
-    static str_arg<T> to_str_arg(const T* first, const T* last) {
-        assert(first <= last);
-        return { first, last };
-    }
-};
-
-// pointer and size
-template<class CharT, class SizeT>
-struct str_arg_char<CharT*, SizeT> : std::enable_if<
-    is_size_type_v<SizeT>,
-    std::remove_cv_t<CharT>> {
-
-    template <typename T>
-    static str_arg<T> to_str_arg(const T* s, std::size_t length) {
-        return { s, length };
-    }
-};
-
-// one pointer (null terminated string)
+// Null terminated string
 template<class CharT>
 struct str_arg_char<CharT*> : std::remove_cv<CharT> {
 
@@ -186,7 +162,7 @@ struct str_arg_char<CharT*> : std::remove_cv<CharT> {
     }
 };
 
-// one string class argument
+// String that has data() and length() members
 template<class StrT>
 struct str_arg_char<StrT> : std::enable_if<
     std::is_pointer_v<detail::data_member_t<StrT>> &&
@@ -202,24 +178,24 @@ struct str_arg_char<StrT> : std::enable_if<
 
 // String arguments helper types
 
-template<class ...Args>
-using str_arg_char_s = str_arg_char<std::decay_t<Args>...>;
+template<class StrT>
+using str_arg_char_s = str_arg_char<std::decay_t<StrT>>;
 
-template<class ...Args>
-using str_arg_char_t = typename str_arg_char_s<Args...>::type;
+template<class StrT>
+using str_arg_char_t = typename str_arg_char_s<StrT>::type;
 
 
-template<class ...Args>
+template<class StrT>
 using enable_if_str_arg_t = std::enable_if_t<
-    is_char_type_v<str_arg_char_t<Args...>>,
+    is_char_type_v<str_arg_char_t<StrT>>,
     int>;
 
 
 // String arguments helper function
 
-template <class ...Args>
-inline auto make_str_arg(Args&&... args) -> str_arg<str_arg_char_t<Args...>> {
-    return str_arg_char_s<Args...>::to_str_arg(std::forward<Args>(args)...);
+template <class StrT>
+inline auto make_str_arg(StrT&& str) -> str_arg<str_arg_char_t<StrT>> {
+    return str_arg_char_s<StrT>::to_str_arg(std::forward<StrT>(str));
 }
 
 
@@ -233,9 +209,9 @@ constexpr bool is_char8_type_v =
 #endif
 ;
 
-template<class ...Args>
+template<class StrT>
 using enable_if_str_arg_to_char8_t = std::enable_if_t<
-    is_char8_type_v<str_arg_char_t<Args...>>,
+    is_char8_type_v<str_arg_char_t<StrT>>,
     int>;
 
 template<class CharT>
@@ -244,9 +220,9 @@ constexpr bool is_charW_type_v =
     std::is_same_v<CharT, char32_t> ||
     std::is_same_v<CharT, wchar_t>;
 
-template<class ...Args>
+template<class StrT>
 using enable_if_str_arg_to_charW_t = std::enable_if_t<
-    is_charW_type_v<str_arg_char_t<Args...>>,
+    is_charW_type_v<str_arg_char_t<StrT>>,
     int>;
 
 
@@ -254,15 +230,15 @@ inline std::string&& make_string(std::string&& str) {
     return std::move(str);
 }
 
-template <class ...Args, enable_if_str_arg_to_char8_t<Args...> = 0>
-inline string_view make_string(Args&&... args) {
-    const auto inp = make_str_arg(std::forward<Args>(args)...);
+template <class StrT, enable_if_str_arg_to_char8_t<StrT> = 0>
+inline string_view make_string(StrT&& str) {
+    const auto inp = make_str_arg(std::forward<StrT>(str));
     return { inp.data(), inp.length() };
 }
 
-template <class ...Args, enable_if_str_arg_to_charW_t<Args...> = 0>
-inline std::string make_string(Args&&... args) {
-    const auto inp = make_str_arg(std::forward<Args>(args)...);
+template <class StrT, enable_if_str_arg_to_charW_t<StrT> = 0>
+inline std::string make_string(StrT&& str) {
+    const auto inp = make_str_arg(std::forward<StrT>(str));
     return url_utf::to_utf8_string(inp.begin(), inp.end());
 }
 
diff --git a/include/upa/url_for_atl.h b/include/upa/url_for_atl.h
index bd19e4e..9b08dbf 100644
--- a/include/upa/url_for_atl.h
+++ b/include/upa/url_for_atl.h
@@ -24,7 +24,7 @@ struct str_arg_char_for_atl {
     using type = typename StrT::XCHAR;
 
     static str_arg<type> to_str_arg(const StrT& str) {
-        return { str.GetString(), static_cast<std::ptrdiff_t>(str.GetLength()) };
+        return { str.GetString(), str.GetLength() };
     }
 };
 
diff --git a/include/upa/url_for_qt.h b/include/upa/url_for_qt.h
index 28ea476..f461252 100644
--- a/include/upa/url_for_qt.h
+++ b/include/upa/url_for_qt.h
@@ -22,7 +22,6 @@
 #if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
 # include <QUtf8StringView>
 #endif
-#include <cstddef> // std::ptrdiff_t
 
 namespace upa {
 
@@ -33,10 +32,7 @@ struct str_arg_char_for_qt {
     using type = CharT;
 
     static str_arg<type> to_str_arg(const StrT& str) {
-        return {
-            reinterpret_cast<const type*>(str.data()),
-            static_cast<std::ptrdiff_t>(str.length())
-        };
+        return { reinterpret_cast<const type*>(str.data()), str.length() };
     }
 };
 
diff --git a/test/test-str_arg.cpp b/test/test-str_arg.cpp
index 623a3a1..76fbd8e 100644
--- a/test/test-str_arg.cpp
+++ b/test/test-str_arg.cpp
@@ -13,11 +13,9 @@
 
 // Function to test
 
-using namespace upa;
-
-template <class ...Args, enable_if_str_arg_t<Args...> = 0>
-inline std::size_t procfn(Args&&... args) {
-    const auto inp = make_str_arg(std::forward<Args>(args)...);
+template <class StrT, upa::enable_if_str_arg_t<StrT> = 0>
+inline std::size_t procfn(StrT&& str) {
+    const auto inp = upa::make_str_arg(std::forward<StrT>(str));
     return std::distance(inp.begin(), inp.end());
 }
 
@@ -54,7 +52,7 @@ struct str_arg_char<CustomString<CharT>> {
 
 template <class CharT>
 inline void test_char() {
-    const size_t N = 3;
+    const std::size_t N = 3;
     CharT arr[] = { '1', '2', '3', 0 };
     const CharT carr[] = { '1', '2', '3', 0 };
     CharT* ptr = arr;
@@ -67,30 +65,33 @@ inline void test_char() {
     procfn(cptr);
     procfn(vptr);
 
-    procfn(arr, N);
-    procfn(carr, N);
-    procfn(ptr, N);
-    procfn(cptr, N);
-    procfn(vptr, N);
-
+    // upa::str_arg
+    upa::str_arg<CharT> arg(arr);
+    procfn(arg);
+    const upa::str_arg<CharT> carg(arr);
+    procfn(carg);
+
+    procfn(upa::str_arg<CharT>{arr, N});
+    procfn(upa::str_arg<CharT>{carr, N});
+    procfn(upa::str_arg<CharT>{ptr, N});
+    procfn(upa::str_arg<CharT>{cptr, N});
+    procfn(upa::str_arg<CharT>{vptr, N});
     // int size
-    procfn(arr, int(N));
-    procfn(cptr, int(N));
+    procfn(upa::str_arg<CharT>(arr, static_cast<int>(N)));
+    procfn(upa::str_arg<CharT>(cptr, static_cast<int>(N)));
 
-    procfn(arr, arr + N);
-    procfn(carr, carr + N);
-    procfn(ptr, ptr + N);
-    procfn(cptr, cptr + N);
-    procfn(vptr, vptr + N);
+    procfn(upa::str_arg<CharT>{arr, arr + N});
+    procfn(upa::str_arg<CharT>{carr, carr + N});
+    procfn(upa::str_arg<CharT>{ptr, ptr + N});
+    procfn(upa::str_arg<CharT>{cptr, cptr + N});
+    procfn(upa::str_arg<CharT>{vptr, vptr + N});
 
+    // std::basic_string
     const std::basic_string<CharT> str(arr);
     procfn(str);
     procfn(std::basic_string<CharT>(arr));
 
-    upa::str_arg<CharT> arg(arr);
-    procfn(arg);
-
-    // test custom string
+    // custom string
     procfn(CustomString<CharT>{cptr, N});
 }