# std::start_lifetime_as, std::start_lifetime_as_array

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< cpp‎ | memory

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 `T`s". 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 | Comment
--- | --- | --- | ---
`__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 (C++20) | reinterpret the object representation of one type as that of another (function template)
as_bytes as_writable_bytes (C++20) | converts a `span` into a view of its underlying bytes (function template)

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