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

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< cpp‎ | memory
(→‎Example: Explain why std::launder may be insufficient to avoid UB.)
m (→‎Example: in-house fmt: +'\n')
 
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{{dcl begin}}
 
{{dcl begin}}
 
{{dcl header|memory}}
 
{{dcl header|memory}}
−
{{dcl h|start_lifetime_as}}
+
{{dcl h|{{tt|std::start_lifetime_as}}}}
 
{{dcl|num=1|since=c++23|1=
 
{{dcl|num=1|since=c++23|1=
 
template< class T >
 
template< class T >
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const volatile T* start_lifetime_as( const volatile void* p ) noexcept;
 
const volatile T* start_lifetime_as( const volatile void* p ) noexcept;
 
}}
 
}}
−
{{dcl h|start_lifetime_as_array}}
+
{{dcl h|{{tt|std::start_lifetime_as_array}}}}
 
{{dcl|num=5|since=c++23|1=
 
{{dcl|num=5|since=c++23|1=
 
template< class T >
 
template< class T >
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@1-4@ [[cpp/language/object#Object creation|Implicitly creates]] a complete object of type {{tt|T}} (whose address is {{c|p}}) and objects nested within it. The value of each created object {{tti|obj}} of {{named req|TriviallyCopyable}} type {{tt|U}} is determined in the same manner as for a call to {{c|std::bit_cast<U>(E)}} except that the storage is not actually accessed, where {{tt|E}} is the lvalue of type {{tt|U}} denoting {{tti|obj}}. Otherwise, the values of such created objects are unspecified.
 
@1-4@ [[cpp/language/object#Object creation|Implicitly creates]] a complete object of type {{tt|T}} (whose address is {{c|p}}) and objects nested within it. The value of each created object {{tti|obj}} of {{named req|TriviallyCopyable}} type {{tt|U}} is determined in the same manner as for a call to {{c|std::bit_cast<U>(E)}} except that the storage is not actually accessed, where {{tt|E}} is the lvalue of type {{tt|U}} denoting {{tti|obj}}. Otherwise, the values of such created objects are unspecified.
−
* {{tt|T}} shall be an {{named req|ImplicitLifetimeType}} and shall be a [[cpp/language/type#Incomplete type|complete type]], otherwise the program is ill-formed.
+
* {{tt|T}} shall be an {{named req|ImplicitLifetimeType}} and shall be a [[cpp/language/type#Incomplete type|complete type]]. Otherwise, the program is ill-formed.
 
* The behavior is undefined if:
 
* The behavior is undefined if:
 
:* {{range|p|(char*)p + sizeof(T)}} does not denote a region of allocated storage that is a subset of the region of storage reachable through {{c|p}}, or
 
:* {{range|p|(char*)p + sizeof(T)}} does not denote a region of allocated storage that is a subset of the region of storage reachable through {{c|p}}, or
 
:* the region is not suitably aligned for the {{tt|T}}.
 
:* the region is not suitably aligned for the {{tt|T}}.
 
* Note that the unspecified value can be indeterminate.
 
* Note that the unspecified value can be indeterminate.
−
@5-8@ [[cpp/language/object#Object creation|Implicitly creates]] an object array of type {{tt|T}} with length {{c|n}}. To be precise, if {{c|n > 0}} is {{c|true}}, it is equivalent to {{c|std::start_lifetime_as<U>(p)}} where {{tt|U}} is the type "array of {{c|n}} {{tt|T}}s". Otherwise, the function has no effects.
+
@5-8@ [[cpp/language/object#Object creation|Implicitly creates]] an array with element type {{tt|T}} and length {{c|n}}. To be precise, if {{c|n > 0}} is {{c|true}}, it is equivalent to {{c|std::start_lifetime_as<U>(p)}} where {{tt|U}} is the type "array of {{c|n}} {{tt|T}}s". Otherwise, the function has no effects.
−
* {{tt|T}} shall be a [[cpp/language/type#Incomplete type|complete type]], otherwise the program is ill-formed.
+
* {{tt|T}} shall be a [[cpp/language/type#Incomplete type|complete type]]. Otherwise, the program is ill-formed.
 
* The behavior is undefined if:
 
* The behavior is undefined if:
 
:* Non-null {{c|p}} is not suitably aligned for an array of {{tt|T}}, or
 
:* Non-null {{c|p}} is not suitably aligned for an array of {{tt|T}}, or
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===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par|p|possible cv-qualified {{c|void}} pointer, denotes the address of the region consisting objects}}
+
{{par|p|the address of the region consisting objects}}
−
{{par|n|{{c|std::size_t}}, denotes the size of the object array to be created}}
+
{{par|n|the number of the element of the array to be created}}
 
{{par end}}
 
{{par end}}
  
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===Notes===
 
===Notes===
−
{{c|new (void_ptr) std::byte[size]}} works as an untyped version of this operation. {{c|std::start_lifetime_as}} handles non-array types as well as arrays of known bound, while {{c|std::start_lifetime_as_array}} handles arrays of unknown bound.
+
{{c|new (void_ptr) unsigned char[size]}} or {{c|new (void_ptr) std::byte[size]}} works as an untyped version of {{tt|std::start_lifetime_as}}, but it does not keep the object representation.
 +
 
 +
{{c|std::start_lifetime_as}} handles non-array types as well as arrays of known bound, while {{c|std::start_lifetime_as_array}} handles arrays of unknown bound.
  
 
{{feature test macro|__cpp_lib_start_lifetime_as|std=C++23|value=202207L|Explicit lifetime management}}
 
{{feature test macro|__cpp_lib_start_lifetime_as|std=C++23|value=202207L|Explicit lifetime management}}
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int main()
 
int main()
 
{
 
{
−
    // Assume the byte order has been adjusted already
+
     alignas(std::complex<float>) unsigned char network_data[sizeof(std::complex<float>)]
−
     alignas(std::complex<float>) unsigned char network_data[] = {
+
    {
 
         0xcd, 0xcc, 0xcc, 0x3d, 0xcd, 0xcc, 0x4c, 0x3e
 
         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);
 
//  auto d = *reinterpret_cast<std::complex<float>*>(network_data);
 
//  std::cout << d << '\n'; // UB: network_data does not point to a complex<float>
 
//  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));
+
//  auto d = *std::launder(reinterpret_cast<std::complex<float>*>(network_data));
−
//  std::cout << d << '\n'; // Possible UB, related to CWG1997:
+
//  std::cout << d << '\n'; // Possible UB, related to CWG1997:
−
//      the implicitly created complex<float> may hold indeterminate values
+
//      the implicitly created complex<float> may hold indeterminate value
  
 
     auto d = *std::start_lifetime_as<std::complex<float>>(network_data);
 
     auto d = *std::start_lifetime_as<std::complex<float>>(network_data);
 
     std::cout << d << '\n'; // OK
 
     std::cout << d << '\n'; // OK
 
}
 
}
 +
|p=true
 
|output=
 
|output=
 
(0.1,0.2)
 
(0.1,0.2)

Latest revision as of 03:42, 29 August 2024

 
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>
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( 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)
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( 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

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)

[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]
(C++20)
 converts a span into a view of its underlying bytes
(function template) [edit]
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