std::optional<T>::emplace

 

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std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

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

See More

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).

Feature-test macro	Value	Std	Feature
__cpp_lib_optional	202106L	(C++20) (DR20)	Fully constexpr (1,2)

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 operations can be constexpr in C++20	made constexpr

See also

operator=

assigns contents (public member function)