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      operator==, !=, <, <=, >, >=, <=>(std::optional)

      From cppreference.com
      <cpp‎ |utility‎ |optional
       
       
      Utilities library
       
       
      Defined in header<optional>
      Compare twooptional objects
      template<class T,class U>
      constexprbool operator==(const optional<T>& lhs,const optional<U>& rhs);
      		 (1)(since C++17)
      template<class T,class U>
      constexprbool operator!=(const optional<T>& lhs,const optional<U>& rhs);
      		 (2)(since C++17)
      template<class T,class U>
      constexprbool operator<(const optional<T>& lhs,const optional<U>& rhs);
      		 (3)(since C++17)
      template<class T,class U>
      constexprbool operator<=(const optional<T>& lhs,const optional<U>& rhs);
      		 (4)(since C++17)
      template<class T,class U>
      constexprbool operator>(const optional<T>& lhs,const optional<U>& rhs);
      		 (5)(since C++17)
      template<class T,class U>
      constexprbool operator>=(const optional<T>& lhs,const optional<U>& rhs);
      		 (6)(since C++17)
      template<class T,std::three_way_comparable_with<T> U>

      constexprstd::compare_three_way_result_t<T, U>

          operator<=>(const optional<T>& lhs,const optional<U>& rhs);
      		 (7)(since C++20)
      Compare anoptional object with anullopt
      template<class T>
      constexprbool operator==(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (8)(since C++17)
      template<class T>
      constexprbool operator==(std::nullopt_t,const optional<T>& opt)noexcept;
      		 (9)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator!=(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (10)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator!=(std::nullopt_t,const optional<T>& opt)noexcept;
      		 (11)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator<(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (12)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator<(std::nullopt_t,const optional<T>& opt)noexcept;
      		 (13)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator<=(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (14)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator<=(std::nullopt_t,const optional<T>& opt)noexcept;
      		 (15)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator>(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (16)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator>(std::nullopt_t,const optional<T>& opt)noexcept;
      		 (17)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator>=(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (18)(since C++17)
      (until C++20)
      template<class T>
      constexprbool operator>=(std::nullopt_t,const optional<T>& opt)noexcept;
      		 (19)(since C++17)
      (until C++20)
      template<class T>

      constexprstd::strong_ordering

          operator<=>(const optional<T>& opt,std::nullopt_t)noexcept;
      		 (20)(since C++20)
      Compare anoptional object with a value
      template<class T,class U>
      constexprbool operator==(const optional<T>& opt,const U& value);
      		 (21)(since C++17)
      template<class U,class T>
      constexprbool operator==(const U& value,const optional<T>& opt);
      		 (22)(since C++17)
      template<class T,class U>
      constexprbool operator!=(const optional<T>& opt,const U& value);
      		 (23)(since C++17)
      template<class U,class T>
      constexprbool operator!=(const U& value,const optional<T>& opt);
      		 (24)(since C++17)
      template<class T,class U>
      constexprbool operator<(const optional<T>& opt,const U& value);
      		 (25)(since C++17)
      template<class U,class T>
      constexprbool operator<(const U& value,const optional<T>& opt);
      		 (26)(since C++17)
      template<class T,class U>
      constexprbool operator<=(const optional<T>& opt,const U& value);
      		 (27)(since C++17)
      template<class U,class T>
      constexprbool operator<=(const U& value,const optional<T>& opt);
      		 (28)(since C++17)
      template<class T,class U>
      constexprbool operator>(const optional<T>& opt,const U& value);
      		 (29)(since C++17)
      template<class U,class T>
      constexprbool operator>(const U& value,const optional<T>& opt);
      		 (30)(since C++17)
      template<class T,class U>
      constexprbool operator>=(const optional<T>& opt,const U& value);
      		 (31)(since C++17)
      template<class U,class T>
      constexprbool operator>=(const U& value,const optional<T>& opt);
      		 (32)(since C++17)
      template<class T,std::three_way_comparable_with<T> U>

      constexprstd::compare_three_way_result_t<T, U>

          operator<=>(const optional<T>& opt,const U& value);
      		 (33)(since C++20)

      Performs comparison operations onoptional objects.

      1-7) Compares twooptional objects,lhs andrhs. The contained values are compared (using the corresponding operator ofT) only if bothlhs andrhs contain values. Otherwise,
      • lhs is consideredequal torhs if, and only if, bothlhs andrhs do not contain a value.
      • lhs is consideredless thanrhs if, and only if,rhs contains a value andlhs does not.
      1-6) Let@ denote the corresponding comparison operator, for each of these functions:

      If the corresponding expression*lhs @*rhs is ill-formed or its result is not convertible tobool, the program is ill-formed.

      		(until C++26)

      This overload participates in overload resolution only if the corresponding expression*lhs @*rhs is well-formed and its result is convertible tobool.

      		(since C++26)
      8-20) Comparesopt with anullopt. Equivalent to(1-6) when comparing to anoptional that does not contain a value.

      The<,<=,>,>=, and!= operators aresynthesized fromoperator<=> andoperator== respectively.

      		(since C++20)
      21-33) Comparesopt with avalue. The values are compared (using the corresponding operator ofT) only ifopt contains a value. Otherwise,opt is consideredless thanvalue.
      21-32) Let@ denote the corresponding comparison operator, for each of these functions:

      If the corresponding expression*opt @ value orvalue @*opt (depending on the positions of the operands) is ill-formed or its result is not convertible tobool, the program is ill-formed.

      		(until C++26)

      This overload participates in overload resolution only if all following conditions are satisfied:

      • U is not a specialization ofstd::optional.
      • The corresponding expression*opt @ value orvalue @*opt (depending on the positions of the operands) is well-formed and its result is convertible tobool.
      		(since C++26)

      Contents

      [edit]Parameters

      lhs, rhs, opt - anoptional object to compare
      value - value to compare to the contained value

      [edit]Return value

      1)lhs.has_value()!= rhs.has_value()?false:
         (lhs.has_value()==false?true:*lhs==*rhs)
      2)lhs.has_value()!= rhs.has_value()?true:
         (lhs.has_value()==false?false:*lhs!=*rhs)
      3)!rhs?false:(!lhs?true:*lhs<*rhs)
      4)!lhs?true:(!rhs?false:*lhs<=*rhs)
      5)!lhs?false:(!rhs?true:*lhs>*rhs)
      6)!rhs?true:(!lhs?false:*lhs>=*rhs)
      7)lhs&& rhs?*lhs<=>*rhs: lhs.has_value()<=> rhs.has_value()
      8,9)!opt
      10,11)opt.has_value()
      12)false
      13)opt.has_value()
      14)!opt
      15)true
      16)opt.has_value()
      17)false
      18)true
      19)!opt
      20)opt.has_value()<=>false
      21)opt.has_value()?*opt== value:false
      22)opt.has_value()? value==*opt:false
      23)opt.has_value()?*opt!= value:true
      24)opt.has_value()? value!=*opt:true
      25)opt.has_value()?*opt< value  :true
      26)opt.has_value()? value<*opt  :false
      27)opt.has_value()?*opt<= value:true
      28)opt.has_value()? value<=*opt:false
      29)opt.has_value()?*opt> value  :false
      30)opt.has_value()? value>*opt  :true
      31)opt.has_value()?*opt>= value:false
      32)opt.has_value()? value>=*opt:true
      33)opt.has_value()?*opt<=> value: std::strong_ordering::less

      [edit]Exceptions

      1-7) May throw implementation-defined exceptions.
      21-33) Throws when and what the comparison throws.

      [edit]Notes

      Feature-test macroValueStdFeature
      __cpp_lib_constrained_equality202403L(C++26)constrained comparison operators forstd::optional

      [edit]Defect reports

      The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

      DRApplied toBehavior as publishedCorrect behavior
      LWG 2945C++17order of template parameters inconsistent for compare-with-T casesmade consistent
      Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/optional/operator_cmp&oldid=180659"
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