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Library relocate
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// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4158.pdf | |
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0023r0.pdf | |
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1029r3.pdf | |
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1144r5.html | |
#include <type_traits> | |
#include <utility> | |
struct relocates{}; | |
template<class T> constexpr bool is_relocatable_v = std::is_constructible_v<T, relocates, T*>; | |
template<class T> | |
T relocate(T* ptr) noexcept(is_relocatable_v<T> || std::is_nothrow_move_constructible_v<T>) { | |
if constexpr (is_relocatable_v<T>) { | |
return T(relocates(), ptr); // magic: ends life of *ptr w/o calling ~T() | |
} | |
else { | |
T ret(std::move(*ptr)); | |
ptr->~T(); | |
return ret; // NRVO | |
} | |
} | |
#include <cassert> | |
#include <iostream> | |
#include <memory> | |
#include <new> | |
#include <string> | |
template<class T> | |
class nonnull_ptr { | |
std::unique_ptr<T> ptr; | |
public: | |
nonnull_ptr(std::unique_ptr<T> ptr) : ptr(std::move(ptr)) { assert(this->ptr); } | |
nonnull_ptr(nonnull_ptr&&) = delete; | |
nonnull_ptr(relocates, nonnull_ptr* r) : ptr(relocate(&r->ptr)) {} | |
T& operator*() { return *ptr; } | |
T* operator->() { return ptr; } | |
}; | |
template<class T> | |
class optional { | |
union { char disengaged; T value; }; | |
bool engaged = false; | |
public: | |
optional() : disengaged{} {} | |
optional(optional&& rhs) { | |
if ((engaged = std::exchange(rhs.engaged, false))) | |
new (&value) T(relocate(&rhs.value)); | |
} | |
~optional() { | |
if (engaged) | |
value.~T(); | |
} | |
template<class... Args> | |
T& emplace(Args&&... args) { | |
if (std::exchange(engaged, true)) | |
value.~T(); | |
new (&value) T(std::forward<Args>(args)...); | |
return value; | |
} | |
T& operator*() { return value; } | |
}; | |
int main() { | |
optional<std::string> o; | |
o.emplace("hello"); | |
optional<nonnull_ptr<std::string>> p; | |
p.emplace(std::make_unique<std::string>("world")); | |
auto q = std::move(o); | |
auto r = std::move(p); | |
std::cout << *q << ' ' << **r << std::endl; | |
} |
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