std::optional::optional(3)

NAME
std::optional::optional - std::optional::optional

Synopsis
   constexpr optional() noexcept; (since
   constexpr optional( std::nullopt_t ) (1) C++17)
   noexcept;
   constexpr optional( const optional& other (2) (since
   ); C++17)
   constexpr optional( optional&& other ) (3) (since
   noexcept(/* see below */); C++17)
                        (since C++17)
   template < class U > (until C++20)
   optional( const optional<U>& other ); (conditionally
                        explicit)
   template < class U > (since C++20)
   constexpr optional( const optional<U>& (conditionally
   other ); explicit)
                        (since C++17)
   template < class U > (until C++20)
   optional( optional<U>&& other ); (conditionally
                        explicit)
   template < class U > (since C++20)
   constexpr optional( optional<U>&& other ); (conditionally
                        (4) explicit)
   template< class... Args >
   constexpr explicit optional( (6) (since C++17)
   std::in_place_t, Args&&... args ); (5)
   template< class U, class... Args >
   constexpr explicit optional(
   std::in_place_t, (7) (since C++17)
   std::initializer_list<U> ilist,
   Args&&... args );
   template < class U = T > (since C++17)
   constexpr optional( U&& value ); (8) (conditionally
                        explicit)
   Constructs a new optional object.
   1) Constructs an object that does not contain a value.
   2) Copy constructor: If other contains a value, initializes the contained value as
   if direct-initializing (but not direct-list-initializing) an object of type T with
   the expression *other. If other does not contain a value, constructs an object that
   does not contain a value.
     * This constructor is defined as deleted if std::is_copy_constructible_v<T> is
       false.
     * It is a trivial constructor if std::is_trivially_copy_constructible_v<T> is
       true.
   3) Move constructor: If other contains a value, initializes the contained value as
   if direct-initializing (but not direct-list-initializing) an object of type T with
   the expression std::move(*other) and does not make other empty: a moved-from
   optional still contains a value, but the value itself is moved from. If other does
   not contain a value, constructs an object that does not contain a value.
     * This constructor does not participate in overload resolution unless
       std::is_move_constructible_v<T> is true.
     * It is a trivial constructor if std::is_trivially_move_constructible_v<T> is
       true.
   4) Converting copy constructor: If other doesn't contain a value, constructs an
   optional object that does not contain a value. Otherwise, constructs an optional
   object that contains a value, initialized as if direct-initializing (but not
   direct-list-initializing) an object of type T with the expression *other.
     * This constructor does not participate in overload resolution unless the
       following conditions are met:
          * std::is_constructible_v<T, const U&> is true.
          * T is not constructible or convertible from any expression of type (possibly
            const) std::optional<U>, i.e., the following 8 type traits are all false:
               * std::is_constructible_v<T, std::optional<U>&>
               * std::is_constructible_v<T, const std::optional<U>&>
               * std::is_constructible_v<T, std::optional<U>&&>
               * std::is_constructible_v<T, const std::optional<U>&&>
               * std::is_convertible_v<std::optional<U>&, T>
               * std::is_convertible_v<const std::optional<U>&, T>
               * std::is_convertible_v<std::optional<U>&&, T>
               * std::is_convertible_v<const std::optional<U>&&, T>
     * This constructor is explicit if and only if std::is_convertible_v<const U&, T>
       is false.
   5) Converting move constructor: If other doesn't contain a value, constructs an
   optional object that does not contain a value. Otherwise, constructs an optional
   object that contains a value, initialized as if direct-initializing (but not
   direct-list-initializing) an object of type T with the expression std::move(*other).
     * This constructor does not participate in overload resolution unless the
       following conditions are met:
          * std::is_constructible_v<T, U&&> is true.
          * T is not constructible or convertible from any expression of type (possibly
            const) std::optional<U>, i.e., the following 8 type traits are all false:
               * std::is_constructible_v<T, std::optional<U>&>
               * std::is_constructible_v<T, const std::optional<U>&>
               * std::is_constructible_v<T, std::optional<U>&&>
               * std::is_constructible_v<T, const std::optional<U>&&>
               * std::is_convertible_v<std::optional<U>&, T>
               * std::is_convertible_v<const std::optional<U>&, T>
               * std::is_convertible_v<std::optional<U>&&, T>
               * std::is_convertible_v<const std::optional<U>&&, T>
     * This constructor is explicit if and only if std::is_convertible_v<U&&, T> is
       false.
   6) Constructs an optional object that contains a value, initialized as if
   direct-initializing (but not direct-list-initializing) an object of type T from the
   arguments std::forward<Args>(args)....
     * If the selected constructor of T is a constexpr constructor, this constructor is
       a constexpr constructor.
     * The function does not participate in the overload resolution unless
       std::is_constructible_v<T, Args...> is true
   7) Constructs an optional object that contains a value, initialized as if
   direct-initializing (but not direct-list-initializing) an object of type T from the
   arguments ilist, std::forward<Args>(args)....
     * If the selected constructor of T is a constexpr constructor, this constructor is
       a constexpr constructor.
     * The function does not participate in the overload resolution unless
       std::is_constructible_v<T, std::initializer_list<U>&, Args&&...> is true
   8) Constructs an optional object that contains a value, initialized as if
   direct-initializing (but not direct-list-initializing) an object of type T with the
   expression std::forward<U>(value).
     * If the selected constructor of T is a constexpr constructor, this constructor is
       a constexpr constructor.
     * This constructor does not participate in overload resolution unless
       std::is_constructible_v<T, U&&> is true and
       std::decay_t<U>
       (until C++20)
       std::remove_cvref_t<U>
       (since C++20) is neither std::in_place_t nor std::optional<T>.
     * This constructor is explicit if and only if std::is_convertible_v<U&&, T> is
       false.

Parameters
   other - another optional object whose contained value is copied
   value - value with which to initialize the contained value
   args... - arguments with which to initialize the contained value
   ilist - initializer list with which to initialize the contained value

Exceptions
   2) Throws any exception thrown by the constructor of T.
   3) Throws any exception thrown by the constructor of T. Has the following
   noexcept specification:
   noexcept(std::is_nothrow_move_constructible<T>::value)
   .
   4-8) Throws any exception thrown by the constructor of T.
  Deduction guides

Example
// Run this code
 #include <optional>
 #include <iostream>
 #include <string>
 int main()
 {
     std::optional<int> o1, // empty
                        o2 = 1, // init from rvalue
                        o3 = o2; // copy-constructor
     // calls std::string( initializer_list<CharT> ) constructor
     std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'});
     // calls std::string( size_type count, CharT ch ) constructor
     std::optional<std::string> o5(std::in_place, 3, 'A');
     // Move-constructed from std::string using deduction guide to pick the type
     std::optional o6(std::string{"deduction"});
     std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << ' ' << *o5 << ' ' << *o6 << '\n';
 }

Output:
 1 1 abc AAA deduction
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   P0602R4 C++17 copy/move constructors may not be trivial required to propagate
                      even if underlying constructor is trivial triviality
                      converting constructors from another
   P2231R1 C++20 optional was not constexpr made constexpr
                      while the required operations can be in
                      C++20

See also
   make_optional creates an optional object
   (C++17) (function template)