backup: add result<T> class

core/src/filed/backup.cc

@@ -51,8 +51,52 @@
 #include "lib/serial.h"
 #include "lib/compression.h"
 
+#include <variant>
+
 namespace filedaemon {
 
+// fixme(ssura): replace by std::expected<T, PoolMem> (C++23)
+//               or std::expected<T, std::string>
+template <typename T> class result {
+ public:
+  constexpr result() : data{1, "Not Initialized"} {}
+
+  template <typename Arg0, typename... Args>
+  constexpr result(Arg0 arg0, Args... args)
+      : data(std::forward<Arg0>(arg0), std::forward<Args>(args)...)
+  {
+  }
+
+  bool holds_error() const { return data.index() == 1; }
+  PoolMem* error()
+  {
+    if (holds_error()) {
+      return &std::get<1>(data);
+    } else {
+      return nullptr;
+    }
+  }
+
+  T* value()
+  {
+    if (!holds_error()) {
+      return &std::get<0>(data);
+    } else {
+      return nullptr;
+    }
+  }
+
+  PoolMem& error_unchecked() { return std::get<1>(data); }
+
+  T& value_unchecked() { return std::get<0>(data); }
+
+ private:
+  // we are using PoolMem here, since it is easier to format into a PoolMem.
+  // Once we have access to std::format everywhere, this should change
+  // to std::string.
+  std::variant<T, PoolMem> data;
+};
+
 #ifdef HAVE_DARWIN_OS
 const bool have_darwin_os = true;
 #else