feat: add initial experimental Twitch Eventsub support (#5837)

Co-authored-by: nerix <nerixdev@outlook.de>

lib/twitch-eventsub-ws/include/twitch-eventsub-ws/string.hpp

@@ -0,0 +1,67 @@
+#pragma once
+
+#include <boost/json.hpp>
+#include <QString>
+
+#include <string>
+#include <variant>
+
+namespace chatterino::eventsub::lib {
+
+template <class>
+constexpr bool DEPENDENT_FALSE = false;  // workaround before CWG2518/P2593R1
+
+/// String is a struct that holds either an std::string or a QString
+///
+/// The intended use is for it to receive an std::string that has been built by
+/// boost::json, and once it's been passed into a GUI appliciaton, it can use
+/// the `qt` function to convert the backing string to a QString,
+/// while we ensure the conversion is only done once.
+struct String {
+    explicit String(std::string &&v)
+        : backingString(std::move(v))
+    {
+    }
+
+    ~String() = default;
+
+    String(const String &s) = delete;
+    String(String &&s) = default;
+
+    String &operator=(const String &) = delete;
+    String &operator=(String &&) = default;
+
+    /// Returns the string as a QString, modifying the backing string to ensure
+    /// the copy only happens once.
+    QString qt() const
+    {
+        return std::visit(
+            [this](auto &&arg) -> QString {
+                using T = std::decay_t<std::remove_cvref_t<decltype(arg)>>;
+                if constexpr (std::is_same_v<T, std::string>)
+                {
+                    auto qtString = QString::fromStdString(arg);
+                    this->backingString = qtString;
+                    return qtString;
+                }
+                else if constexpr (std::is_same_v<T, QString>)
+                {
+                    return arg;
+                }
+                else
+                {
+                    static_assert(DEPENDENT_FALSE<T>,
+                                  "unknown type in variant");
+                }
+            },
+            this->backingString);
+    }
+
+private:
+    mutable std::variant<std::string, QString> backingString;
+};
+
+boost::json::result_for<String, boost::json::value>::type tag_invoke(
+    boost::json::try_value_to_tag<String>, const boost::json::value &jvRoot);
+
+}  // namespace chatterino::eventsub::lib