std::start_lifetime_as, std::start_lifetime_as_array

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Dynamic memory management

Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) voidify (C++20) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::construct_at (C++20) ranges::destroy (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::uninitialized_default_construct_n (C++20)   ranges::uninitialized_value_construct_n (C++20) Allocators allocator allocator_traits (C++11) allocation_result (C++23) allocator_arg (C++11) uses_allocator (C++11) uses_allocator_construction_args (C++20) make_obj_using_allocator (C++20) uninitialized_construct_using_allocator (C++20) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17)   pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17) Garbage collection support declare_reachable (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) get_pointer_safety (C++11)(until C++23) Uninitialized storage raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Explicit lifetime management start_lifetime_asstart_lifetime_as_array (C++23)(C++23)

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>
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 object array of type T with 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	-	possible cv-qualified void pointer, denotes the address of the region consisting objects
n	-	std::size_t, denotes the size of the object 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) std::byte[size] works as an untyped version of this operation. 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()
{
    // Assume the byte order has been adjusted already
    alignas(std::complex<float>) unsigned char network_data[] = {
        0xcd, 0xcc, 0xcc, 0x3d, 0xcd, 0xcc, 0x4c, 0x3e
    };
    static_assert(sizeof(network_data) >= sizeof(std::complex<float>));
 
//  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
}

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)