std::start_lifetime_as,std::start_lifetime_as_array(3)

NAME
std::start_lifetime_as,std::start_lifetime_as_array - std::start_lifetime_as,std::start_lifetime_as_array

Synopsis
   Defined in header <memory>
   start_lifetime_as
   template< class T > (1) (since C++23)
   T* start_lifetime_as( void* p ) noexcept;
   template< class T > (2) (since C++23)
   const T* start_lifetime_as( const void* p ) noexcept;
   template< class T > (3) (since C++23)
   volatile T* start_lifetime_as( volatile void* p ) noexcept;
   template< class T >
   const volatile T* start_lifetime_as( const volatile void* p ) (4) (since C++23)
   noexcept;
   start_lifetime_as_array
   template< class T > (5) (since C++23)
   T* start_lifetime_as_array( void* p, std::size_t n ) noexcept;
   template< class T >
   const T* start_lifetime_as_array( const void* p, (6) (since C++23)
                        std::size_t n ) noexcept;
   template< class T >
   volatile T* start_lifetime_as_array( volatile void* p, (7) (since C++23)
                        std::size_t n ) noexcept;
   template< class T >
   const volatile T* start_lifetime_as_array( const volatile void* p, (8) (since C++23)
                        std::size_t n )
   noexcept;
   1-4) Implicitly creates a complete object of type T (whose address is p) and objects
   nested within it. The value of each created object obj of TriviallyCopyable type U
   is determined in the same manner as for a call to std::bit_cast<U>(E) except that
   the storage is not actually accessed, where E is the lvalue of type U denoting obj.
   Otherwise, the values of such created objects are unspecified.
     * T shall be an ImplicitLifetimeType and shall be a complete type. Otherwise, the
       program is ill-formed.
     * The behavior is undefined if:
     * [p, (char*)p + sizeof(T)) does not denote a region of allocated storage that is
       a subset of the region of storage reachable through p, or
     * the region is not suitably aligned for the T.
     * Note that the unspecified value can be indeterminate.
   5-8) Implicitly creates an array with element type T and length n. To be precise, if
   n > 0 is true, it is equivalent to std::start_lifetime_as<U>(p) where U is the type
   "array of n Ts". Otherwise, the function has no effects.
     * T shall be a complete type. Otherwise, the program is ill-formed.
     * The behavior is undefined if:
     * Non-null p is not suitably aligned for an array of T, or
     * n <= std::size_t(-1) / sizeof(T) is false, or
     * n > 0 and [(char*)p, (char*)p + (n * sizeof(T))) does not denote a region of
       allocated storage that is a subset of the region of storage reachable through p.

Parameters
   p - the address of the region consisting objects
   n - the number of the element of the array to be created

Return value
   1-4) A pointer to the complete object as described above.
   5-8) A pointer to the first element of the created array, if any; otherwise, a
   pointer that compares equal to p.

Notes
   new (void_ptr) unsigned char[size] or new (void_ptr) std::byte[size] works as an
   untyped version of std::start_lifetime_as, but it does not keep the object
   representation.
   std::start_lifetime_as handles non-array types as well as arrays of known bound,
   while std::start_lifetime_as_array handles arrays of unknown bound.
       Feature-test macro Value Std Feature
   __cpp_lib_start_lifetime_as 202207L (C++23) Explicit lifetime management

Example
// Run this code
 #include <complex>
 #include <iostream>
 #include <memory>
 int main()
 {
     alignas(std::complex<float>) unsigned char network_data[sizeof(std::complex<float>)]{
         0xcd, 0xcc, 0xcc, 0x3d, 0xcd, 0xcc, 0x4c, 0x3e
     };
 // auto d = *reinterpret_cast<std::complex<float>*>(network_data);
 // std::cout << d << '\n'; // UB: network_data does not point to a complex<float>
 // auto d = *std::launder(reinterpret_cast<std::complex<float>*>(network_data));
 // std::cout << d << '\n'; // Possible UB, related to CWG1997:
 // the implicitly created complex<float> may hold indeterminate value
     auto d = *std::start_lifetime_as<std::complex<float>>(network_data);
     std::cout << d << '\n'; // OK
 }

Possible output:
 (0.1,0.2)

References
     * C++23 standard (ISO/IEC 14882:2023):
     * 20.2.6 Explicit lifetime management [obj.lifetime]

See also
   bit_cast reinterpret the object representation of one type as that of
   (C++20) another
                     (function template)
   as_bytes converts a span into a view of its underlying bytes
   as_writable_bytes (function template)
   (C++20)