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// Copyright (C) 2023 johannst
#include <cassert>
#include <cstdio>
#include <new>
#include <type_traits>
#include <utility>
struct S {};
struct M {
M() {
std::puts("M()");
}
M(const M&) {
std::puts("M(M&)");
}
M(M&&) {
std::puts("M(M&&)");
}
M& operator=(const M&) = delete;
M& operator=(M&&) = delete;
M(S&, int) {
std::puts("M(S&)");
}
M(S&&, int) {
std::puts("M(S&&)");
}
~M() {
std::puts("~M()");
}
};
template<typename T>
struct option {
static_assert(!std::is_reference_v<T>);
constexpr option() = default;
template<typename... Params>
constexpr option(Params&&... params) : m_has_val(true) {
// BAD: does not perfectly forward!
// eg, if option(S&&) is invoked, this would invoke M(S&).
// new (&m_val) T(params...);
// GOOD: perfectly forwards params to constructor of T.
new (m_val) T(std::forward<Params>(params)...);
}
~option() {
reset();
}
constexpr T& value() {
assert(m_has_val);
// Placement new starts a new lifetime, launder pointer returned to the
// aligned storage.
//
// [1] https://en.cppreference.com/w/cpp/utility/launder
return *__builtin_launder(reinterpret_cast<T*>(m_val));
}
private:
constexpr void reset() {
if (!m_has_val) {
return;
}
if constexpr (!std::is_trivially_destructible_v<T>) {
value().~T();
};
}
alignas(T) char m_val[sizeof(T)];
bool m_has_val{false};
};
int main() {
std::puts("==> case 1");
// invokes M(S&&, int)
option<M> opt1(S{}, 123);
std::puts("==> case 2");
// invokes M() + M(M&&)
option<M> x /* option(M&&) + M(M&&) */ = M{} /* M() */;
}
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