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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(constvoid* p)noexcept; | (2) | (since C++23) |
template<class T> volatile T* start_lifetime_as(volatilevoid* p)noexcept; | (3) | (since C++23) |
template<class T> constvolatile T* start_lifetime_as(constvolatilevoid* 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(constvoid* p, | (6) | (since C++23) |
template<class T> volatile T* start_lifetime_as_array(volatilevoid* p, | (7) | (since C++23) |
template<class T> constvolatile T* start_lifetime_as_array(constvolatilevoid* p, | (8) | (since C++23) |
T (whose address isp) and objects nested within it. The value of each created objectobj ofTriviallyCopyable typeU is determined in the same manner as for a call tostd::bit_cast<U>(E) except that the storage is not actually accessed, whereE is the lvalue of typeU denotingobj. Otherwise, the values of such created objects are unspecified.T shall be anImplicitLifetimeType and shall be acomplete type. Otherwise, the program is ill-formed.[p, (char*)p+ sizeof(T)) does not denote a region of allocated storage that is a subset of the region of storage reachable throughp, orT.T and lengthn. To be precise, ifn>0 istrue, it is equivalent tostd::start_lifetime_as<U>(p) whereU is the type "array ofnTs". Otherwise, the function has no effects.T shall be acomplete type. Otherwise, the program is ill-formed.T, or[(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 throughp.Contents |
| p | - | the address of the region consisting objects |
| n | - | the number of the element of the array to be created |
new(void_ptr)unsignedchar[size] ornew(void_ptr)std::byte[size] works as an untyped version ofstd::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, whilestd::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 |
#include <complex>#include <iostream>#include <memory> int main(){ alignas(std::complex<float>)unsignedchar 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 d1 = *std::launder(reinterpret_cast<std::complex<float>*>(network_data));// std::cout << d1 << '\n'; // UB: implicitly created objects have dynamic storage// duration and have indeterminate value initially,// even when an array which provides storage for// them has determinate bytes.// See also CWG2721. auto d2=*std::start_lifetime_as<std::complex<float>>(network_data);std::cout<< d2<<'\n';// OK}
Possible output:
(0.1,0.2)
(C++20) | reinterpret the object representation of one type as that of another (function template)[edit] |
(C++20) | converts aspan into a view of its underlying bytes(function template)[edit] |
