CODEWITHSUNDEEP
c++c++20undefined-behaviorlifetimestdlaunder
Dr. Gut
Dr. Gut

Reputation: 2856

Constructing a const object over itself

In the following example I ended the life of s (by calling its dtor.), and after that at the same location I created a new object of the same type S. Since I violated [basic.life]/8.3 (s is const), in order to access the newly created S I have to std::launder its address (see Note 3).

#include <new>
#include <utility>

struct S {
    int x;
};

template <class T, class... Pars>
void Recreate(const T& value, Pars&&... pars) {
    value.~T();
    new(const_cast<T*>(&value)) T(std::forward<Pars>(pars)...);
}

int main() {
    const S s(1);
    Recreate(s, 2);
    return std::launder(&s)->x;
}

Live demo.

Does this program have to return 1 or 2 or is it UB? Or how to std::launder properly? Or is it just ambiguously specified? Notes are non-normative.

Upvotes: 3

Views: 49

Answers (0)

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