std::optional<T>::reset

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< cpp‎ | utility‎ | optional
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::optional
 
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.

Notes

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

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

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

See also

 assigns contents
(public member function)
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