lifetime.hpp

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#pragma once
 
/// @file
/// @ingroup memory
/// Fallback implementations for `std::start_lifetime_as`.
 
#include <memory>
 
#if __cpp_lib_start_lifetime_as >= 202207L
 
namespace guanaqo {
using std::start_lifetime_as;
using std::start_lifetime_as_array;
} // namespace guanaqo
 
#else
 
#include <cstring>
#include <new>
#include <type_traits>
 
namespace guanaqo {
 
/// @addtogroup memory
/// @{
 
template <class T>
    requires std::is_trivially_copyable_v<T>
T *start_lifetime_as_array(void *p, size_t n) noexcept {
#if __cpp_lib_is_implicit_lifetime >= 202302L
    static_assert(std::is_implicit_lifetime_v<T>);
#endif
    return std::launder(static_cast<T *>(std::memmove(p, p, n * sizeof(T))));
}
 
template <class T>
    requires std::is_trivially_copyable_v<T>
const T *start_lifetime_as_array(const void *p, size_t n) noexcept {
#if __cpp_lib_is_implicit_lifetime >= 202302L
    static_assert(std::is_implicit_lifetime_v<T>);
#endif
    static_cast<void>(n); // TODO
    // best we can do without compiler support
    return std::launder(static_cast<const T *>(p));
}
 
template <class T>
    requires std::is_trivially_copyable_v<T>
T *start_lifetime_as(void *p) noexcept {
    return start_lifetime_as_array<T>(p, 1);
}
 
template <class T>
    requires std::is_trivially_copyable_v<T>
const T *start_lifetime_as(const void *p) noexcept {
    return start_lifetime_as_array<T>(p, 1);
}
 
/// @}
 
} // namespace guanaqo
 
#endif