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std::optional<T>::reset

From cppreference.com
< cpp‎ | utility‎ | optional
 
 
Utilities library
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Relational operators (deprecated in C++20)
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void reset() noexcept;
(since C++17)
(constexpr since C++20)

If *this contains a value, destroy that value as if by value().T::~T(). Otherwise, there are no effects.

*this does not contain a value after this call.

[edit] Notes

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

[edit] Example

Run this code
#include <iostream>
#include <optional>
 
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::cout << "Create empty optional:\n";
 std::optional <A> opt;
 
 std::cout << "Construct and assign value:\n";
 opt = A("Lorem ipsum dolor sit amet, consectetur adipiscing elit nec.");
 
 std::cout << "Reset optional:\n";
 opt.reset();
 std::cout << "End example\n";
}

Output:

Create empty optional:
Construct and assign value:
 constructed
 move constructed
 destructed
Reset optional:
 destructed
End example

[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 reset was not constexpr while non-trivial destruction is allowed in constexpr in C++20 made constexpr

[edit] See also

assigns contents
(public member function) [edit]

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