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| Member functions | ||||
| Observers | ||||
| Iterators | ||||
(C++26) | ||||
(C++26) | ||||
| Monadic operations | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
| Modifiers | ||||
optional::emplace | ||||
| Non-member functions | ||||
| Deduction guides | ||||
| Helper classes | ||||
| Helper objects | ||||
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.
Contents |
| args... | - | the arguments to pass to the constructor |
| ilist | - | the initializer list to pass to the constructor |
| Type requirements | ||
-T must be constructible fromArgs... for overload(1) | ||
-T must be constructible fromstd::initializer_list andArgs... for overload(2) | ||
A reference to the new contained value.
Any exception thrown by the selected constructor ofT. 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) | Fullyconstexpr(1,2) |
#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;} inlinestaticint 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 #0Emplace: ~ destructed #1 + constructed #2End example ~ destructed #2
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 notconstexpr while the required operations can beconstexpr in C++20 | madeconstexpr |
| assigns contents (public member function)[edit] |
