Modernise: Use variable templates

include/upa/str_arg.h

@@ -35,21 +35,19 @@ using string_view = std::string_view;
 // Supported char and size types
 
 template<class CharT>
-struct is_char_type : std::integral_constant<bool,
+constexpr bool is_char_type_v =
     std::is_same_v<CharT, char> ||
 #ifdef __cpp_char8_t
     std::is_same_v<CharT, char8_t> ||
 #endif
     std::is_same_v<CharT, char16_t> ||
     std::is_same_v<CharT, char32_t> ||
-    std::is_same_v<CharT, wchar_t>
-> {};
+    std::is_same_v<CharT, wchar_t>;
 
 template<class SizeT>
-struct is_size_type : std::integral_constant<bool,
+constexpr bool is_size_type_v =
     std::is_convertible_v<SizeT, std::size_t> ||
-    std::is_convertible_v<SizeT, std::ptrdiff_t>
-> {};
+    std::is_convertible_v<SizeT, std::ptrdiff_t>;
 
 
 // string args helper class
@@ -169,7 +167,7 @@ struct str_arg_char<CharT*, CharT*> : std::remove_cv<CharT> {
 // pointer and size
 template<class CharT, class SizeT>
 struct str_arg_char<CharT*, SizeT> : std::enable_if<
-    is_size_type<SizeT>::value,
+    is_size_type_v<SizeT>,
     std::remove_cv_t<CharT>> {
 
     template <typename T>
@@ -192,7 +190,7 @@ struct str_arg_char<CharT*> : std::remove_cv<CharT> {
 template<class StrT>
 struct str_arg_char<StrT> : std::enable_if<
     std::is_pointer_v<detail::data_member_t<StrT>> &&
-    is_size_type<detail::length_member_t<StrT>>::value,
+    is_size_type_v<detail::length_member_t<StrT>>,
     remove_cvptr_t<detail::data_member_t<StrT>>> {
 
     template <class STR, typename T = typename STR::value_type>
@@ -213,7 +211,7 @@ using str_arg_char_t = typename str_arg_char_s<Args...>::type;
 
 template<class ...Args>
 using enable_if_str_arg_t = std::enable_if_t<
-    is_char_type<str_arg_char_t<Args...>>::value,
+    is_char_type_v<str_arg_char_t<Args...>>,
     int>;
 
 
@@ -228,28 +226,27 @@ inline auto make_str_arg(Args&&... args) -> str_arg<str_arg_char_t<Args...>> {
 // Convert to std::string or string_view
 
 template<class CharT>
-struct is_char8_type : std::integral_constant<bool,
+constexpr bool is_char8_type_v =
     std::is_same_v<CharT, char>
 #ifdef __cpp_char8_t
     || std::is_same_v<CharT, char8_t>
 #endif
-> {};
+;
 
 template<class ...Args>
 using enable_if_str_arg_to_char8_t = std::enable_if_t<
-    is_char8_type<str_arg_char_t<Args...>>::value,
+    is_char8_type_v<str_arg_char_t<Args...>>,
     int>;
 
 template<class CharT>
-struct is_charW_type : std::integral_constant<bool,
+constexpr bool is_charW_type_v =
     std::is_same_v<CharT, char16_t> ||
     std::is_same_v<CharT, char32_t> ||
-    std::is_same_v<CharT, wchar_t>
-> {};
+    std::is_same_v<CharT, wchar_t>;
 
 template<class ...Args>
 using enable_if_str_arg_to_charW_t = std::enable_if_t<
-    is_charW_type<str_arg_char_t<Args...>>::value,
+    is_charW_type_v<str_arg_char_t<Args...>>,
     int>;
 
 

include/upa/url_search_params.h

@@ -47,7 +47,7 @@ using iterable_value_t = std::remove_cv_t<std::remove_reference_t<
 
 // is iterable over the std::pair values
 template<class T>
-struct is_iterable_pairs : is_pair<iterable_value_t<T>> {};
+constexpr bool is_iterable_pairs_v = is_pair<iterable_value_t<T>>::value;
 
 // enable if `Base` is not the base class of `T`
 template<class Base, class T>
@@ -120,7 +120,7 @@ class url_search_params
     template<class ConT,
         // do not hide the copy and move constructors:
         detail::enable_if_not_base_of_t<url_search_params, ConT> = 0,
-        std::enable_if_t<detail::is_iterable_pairs<ConT>::value, int> = 0
+        std::enable_if_t<detail::is_iterable_pairs_v<ConT>, int> = 0
     >
     explicit url_search_params(ConT&& cont) {
         for (const auto& p : cont) {