src/base/Optional.h

   1 /*
   2  * Copyright (C) 1996-2020 The Squid Software Foundation and contributors
   3  *
   4  * Squid software is distributed under GPLv2+ license and includes
   5  * contributions from numerous individuals and organizations.
   6  * Please see the COPYING and CONTRIBUTORS files for details.
   7  */
   8
   9 #ifndef SQUID__SRC_BASE_OPTIONAL_H
  10 #define SQUID__SRC_BASE_OPTIONAL_H
  11
  12 #include <exception>
  13
  14 /// std::bad_optional_access replacement (until we upgrade to C++17)
  15 class BadOptionalAccess: public std::exception
  16 {
  17 public:
  18     BadOptionalAccess() {}
  19     /* std::exception API */
  20     virtual const char* what() const noexcept override { return "bad-optional-access"; }
  21     virtual ~BadOptionalAccess() noexcept = default;
  22 };
  23
  24 /// (limited) std::optional replacement (until we upgrade to C++17)
  25 template <typename Value>
  26 class Optional
  27 {
  28 public:
  29     // std::optional supports non-trivial types as well, but we
  30     // do not want to fiddle with unions to disable default Value constructor
  31     // until that work becomes necessary
  32     static_assert(std::is_trivial<Value>::value, "Value is trivial");
  33
  34     constexpr Optional() noexcept {}
  35     constexpr explicit Optional(const Value &v): value_(v), hasValue_(true) {}
  36
  37     constexpr explicit operator bool() const noexcept { return hasValue_; }
  38     constexpr bool has_value() const noexcept { return hasValue_; }
  39
  40     const Value &value() const &
  41     {
  42         if (!hasValue_)
  43             throw BadOptionalAccess();
  44         return value_;
  45     }
  46
  47     template <class Other>
  48     constexpr Value value_or(Other &&defaultValue) const &
  49     {
  50         return hasValue_ ? value_ : static_cast<Value>(std::forward<Other>(defaultValue));
  51     }
  52
  53 private:
  54     Value value_; // stored value; inaccessible/uninitialized unless hasValue_
  55     bool hasValue_ = false;
  56 };
  57
  58 #endif /* SQUID__SRC_BASE_OPTIONAL_H */
  59