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std::start_lifetime_as, std::start_lifetime_as_array

From cppreference.com
< cpp‎ | memory
 
 
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Explicit lifetime management
start_lifetime_asstart_lifetime_as_array
(C++23)(C++23)

 
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.
  • [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.

Contents

[edit] Parameters

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

[edit] 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.

[edit] 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

[edit] 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)

[edit] References

  • C++23 standard (ISO/IEC 14882:2024):
  • 20.2.6 Explicit lifetime management [obj.lifetime]

[edit] See also

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