std::optional::emplace(3)

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

Synopsis
   template< class... Args > (1) (since C++17)
   T& emplace( Args&&... args ); (constexpr since C++20)
   template< class U, class... Args > (since C++17)
   T& emplace( std::initializer_list<U> ilist, Args&&... (2) (constexpr since C++20)
   args );
   Constructs the contained value in-place. If *this already contains a value before
   the call, the contained value is destroyed by calling its destructor.
   1) Initializes the contained value by direct-initializing (but not
   direct-list-initializing) with std::forward<Args>(args)... as parameters.
   2) Initializes the contained value by calling its constructor with ilist,
   std::forward<Args>(args)... as parameters. This overload participates in overload
   resolution only if std::is_constructible<T, std::initializer_list<U>&,
   Args&&...>::value is true.

Parameters
   args... - the arguments to pass to the constructor
   ilist - the initializer list to pass to the constructor

Type requirements
   -
   T must be constructible from Args... for overload (1)
   -
   T must be constructible from std::initializer_list and Args... for overload (2)

Return value
   A reference to the new contained value.

Exceptions
   Any exception thrown by the selected constructor of T. If an exception is thrown,
   *this does not contain a value after this call (the previously contained value, if
   any, had been destroyed).

Example
// Run this code
 #include <iostream>
 #include <optional>
 struct A
 {
     std::string s;
     A(std::string str) : s(std::move(str)), id{n++} { note("+ constructed"); }
     ~A() { note("~ destructed"); }
     A(const A& o) : s(o.s), id{n++} { note("+ copy constructed"); }
     A(A&& o) : s(std::move(o.s)), id{n++} { note("+ move constructed"); }
     A& operator=(const A& other)
     {
         s = other.s;
         note("= copy assigned");
         return *this;
     }
     A& operator=(A&& other)
     {
         s = std::move(other.s);
         note("= move assigned");
         return *this;
     }
     inline static int n{};
     int id{};
     void note(auto s) { std::cout << " " << s << " #" << id << '\n'; }
 };
 int main()
 {
     std::optional<A> opt;
     std::cout << "Assign:\n";
     opt = A("Lorem ipsum dolor sit amet, consectetur adipiscing elit nec.");
     std::cout << "Emplace:\n";
     // As opt contains a value it will also destroy that value
     opt.emplace("Lorem ipsum dolor sit amet, consectetur efficitur.");
     std::cout << "End example\n";
 }

Output:
 Assign:
   + constructed #0
   + move constructed #1
   ~ destructed #0
 Emplace:
   ~ destructed #1
   + constructed #2
 End example
   ~ destructed #2
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   P2231R1 C++20 emplace was not constexpr while the required made constexpr
                      operations can be constexpr in C++20

See also
   operator= assigns contents
             (public member function)