value construction from optional

fix boostorg#531

include/boost/url/detail/optional_string.hpp

@@ -0,0 +1,99 @@
+//
+// Copyright (c) 2022 Alan de Freitas ([email protected])
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/url
+//
+
+#ifndef BOOST_URL_DETAIL_OPTIONAL_STRING_HPP
+#define BOOST_URL_DETAIL_OPTIONAL_STRING_HPP
+
+#include <boost/url/string_view.hpp>
+#include <boost/type_traits/make_void.hpp>
+
+namespace boost {
+namespace urls {
+
+#ifndef BOOST_URL_DOCS
+struct no_value_t;
+#endif
+
+namespace detail {
+struct optional_string
+{
+    string_view s;
+    bool b = false;
+};
+
+template <class String>
+typename std::enable_if<
+    std::is_convertible<String, string_view>::value,
+    optional_string>::type
+get_optional_string(
+    String const& s)
+{
+    optional_string r;
+    r.s = s;
+    r.b = true;
+    return r;
+}
+
+template <class T, class = void>
+struct is_dereferenceable : std::false_type
+{};
+
+template <class T>
+struct is_dereferenceable<
+    T,
+    boost::void_t<
+        decltype(*std::declval<T>())
+        >> : std::true_type
+{};
+
+template <class OptionalString>
+typename std::enable_if<
+    !std::is_convertible<OptionalString, string_view>::value,
+    optional_string>::type
+get_optional_string(
+    OptionalString const& opt)
+{
+    // If this goes off, it means the rule
+    // passed in did not meet the requirements.
+    // Please check the documentation of functions
+    // that call get_optional_string.
+    static_assert(
+        is_dereferenceable<OptionalString>::value &&
+        std::is_constructible<bool, OptionalString>::value &&
+        !std::is_convertible<OptionalString, string_view>::value &&
+        std::is_convertible<typename std::decay<decltype(*std::declval<OptionalString>())>::type, string_view>::value,
+        "OptionalString requirements not met");
+    optional_string r;
+    r.s = opt ? string_view(*opt) : string_view{};
+    r.b = static_cast<bool>(opt);
+    return r;
+}
+
+inline
+optional_string
+get_optional_string(
+    std::nullptr_t)
+{
+    return {};
+}
+
+inline
+optional_string
+get_optional_string(
+    no_value_t const&)
+{
+    return {};
+}
+
+
+} // detail
+} // urls
+} // boost
+
+#endif