std::optional<T>::emplace
From cppreference.com
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) |
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.
Contents |
[edit] 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)
|
[edit] Return value
A reference to the new contained value.
[edit] 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) |
[edit] 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
[edit] 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 |
[edit] See also
assigns contents (public member function) |