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Difference between revisions of "cpp/memory/start lifetime as"

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< cpp‎ | memory
(Add a note about which types each function template handles)
(...)
Line 83: Line 83:
 
     // std::cout.write(p, sizeof val); // UB: p does not point to a char array
 
     // std::cout.write(p, sizeof val); // UB: p does not point to a char array
  
−
     auto p = std::start_lifetime_as<char[sizeof(val)]>(&val, sizeof val);
+
     auto p = std::start_lifetime_as<char[sizeof(val)]>(&val);
 
     std::cout.write(p, sizeof val); // OK
 
     std::cout.write(p, sizeof val); // OK
 
}
 
}

Revision as of 21:48, 24 June 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)



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 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.

Contents

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 <iostream>
#include <memory>
 
int main()
{
    unsigned long long val = 0x5f4f4c4c45485f;
 
    // auto p = reinterpret_cast<char*>(&val);
    // std::cout.write(p, sizeof val); // UB: p does not point to a char array
 
    auto p = std::start_lifetime_as<char[sizeof(val)]>(&val);
    std::cout.write(p, sizeof val); // OK
}

Possible output: 

_HELLO_

References

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

See also

(C++20)
 reinterpret the object representation of one type as that of another
(function template) [edit]
(C++17)
 pointer optimization barrier
(function template) [edit]
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