std::optional<T>::emplace

 

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Technical Specifications

 

Utilities library

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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

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

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

make_optional

swap

Deduction guides

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

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

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 only participates in overload resolution 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))  { std::cout << " constructed\n"; }
    ~A() { std::cout << " destructed\n"; }
    A(const A& o) : s(o.s) { std::cout << " copy constructed\n"; }
    A(A&& o) : s(std::move(o.s)) { std::cout << " move constructed\n"; }
    A& operator=(const A& other) {
        s = other.s;
        std::cout << " copy assigned\n";
        return *this;
    }
    A& operator=(A&& other) {
        s = std::move(other.s);
        std::cout << " move assigned\n";
        return *this;
    }
};
 
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
 move constructed
 destructed
Emplace:
 destructed
 constructed
End example
 destructed

See also

operator=

assigns contents (public member function)