std::start_lifetime_as, std::start_lifetime_as_array

 

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Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17*) owner_less (C++11) owner_less<void> (C++17) owner_hash (C++26) owner_equal (C++26) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) out_ptr_t (C++23) inout_ptr_t (C++23) Low level memory management operator new operator new[] operator delete operator delete[] get_new_handler (C++11) set_new_handler launder (C++17) bad_alloc bad_array_new_length (C++11) nothrow_t align_val_t (C++17) destroying_delete_t (C++20) new_handler nothrow Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C Library malloc calloc realloc    aligned_alloc (C++17) free

 

Defined in header <memory>
std::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)
std::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

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 14882:2024):

• 20.2.6 Explicit lifetime management [obj.lifetime]

See also

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)