std::start_lifetime_as, std::start_lifetime_as_array

Defined in header `<memory>`
start_lifetime_as
`template< class T > T* start_lifetime_as( void* p ) noexcept;`	(1)	(since C++23)
`template< class T > const T* start_lifetime_as( const void* p ) noexcept;`	(2)	(since C++23)
`template< class T > volatile T* start_lifetime_as( volatile void* p ) noexcept;`	(3)	(since C++23)
`template< class T > const volatile T* start_lifetime_as( const volatile void* p ) noexcept;`	(4)	(since C++23)
start_lifetime_as_array
`template< class T > T* start_lifetime_as_array( void* p, std::size_t n ) noexcept;`	(5)	(since C++23)
`template< class T > const T* start_lifetime_as_array( const void* p, std::size_t n ) noexcept;`	(6)	(since C++23)
`template< class T > volatile T* start_lifetime_as_array( volatile void* p, std::size_t n ) noexcept;`	(7)	(since C++23)
`template< class T > const volatile T* start_lifetime_as_array( const volatile void* p, std::size_t n ) noexcept;`	(8)	(since C++23)

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

#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]

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

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