std::optional<T>::emplace

	(1)
template< class... Args > T& emplace( Args&&... args );			(since C++17) (until C++20)
template< class... Args > constexpr T& emplace( Args&&... args );			(since C++20)
		(2)
template< class U, class... Args > T& emplace( std::initializer_list<U> ilist, Args&&... args );				(since C++17) (until C++20)
template< class U, class... Args > constexpr T& emplace( std::initializer_list<U> ilist, Args&&... args );				(since C++20)

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 <optional>
#include <iostream>
 
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 operations can be constexpr in C++20	made constexpr

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

Member functions
optional::optional (C++17)
optional::~optional (C++17)
optional::operator= (C++17)
Observers
optional::operator->optional::operator* (C++17)(C++17)
optional::operator booloptional::has_value (C++17)(C++17)
optional::value (C++17)
optional::value_or (C++17)
Monadic operations
optional::and_then (C++23)
optional::transform (C++23)
optional::or_else (C++23)
Modifiers
optional::emplace (C++17)
optional::swap (C++17)
optional::reset (C++17)
Non-member functions
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
make_optional (C++17)
swap (C++17)
Deduction guides
Helper classes
hash (C++17)
nullopt_t (C++17)
in_place_t (C++17)
bad_optional_access (C++17)
Helper objects
nullopt (C++17)
in_place (C++17)