std::pmr:: polymorphic_allocator
|
Defined in header
<memory_resource>
|
||
|
template
<
class
T
>
class polymorphic_allocator ; |
(since C++17)
(until C++20) |
|
|
template
<
class
T
=
std::
byte
>
class polymorphic_allocator ; |
(since C++20) | |
The class template
std::pmr::polymorphic_allocator
is an
Allocator
which exhibits different allocation behavior depending upon the
std::pmr::memory_resource
from which it is constructed. Since
memory_resource
uses runtime polymorphism to manage allocations, different container instances with
polymorphic_allocator
as their static allocator type are interoperable, but can behave as if they had different allocator types.
All specializations of
polymorphic_allocator
meet the
allocator completeness requirements
.
The
polymorphic_allocator::construct
member function does
uses-allocator construction
, so that the elements of a container using a
polymorphic_allocator
will use that same allocator for their own allocations. For example, a
std::
pmr
::
vector
<
std::
pmr
::
string
>
will use the same
memory_resource
for the
vector
's storage and each
string
's storage.
For non-polymorphic allocators, similar propagation can be achieved with the help of std::scoped_allocator_adaptor .
Member types
| Member type | definition |
value_type
|
T
|
Member functions
constructs a
polymorphic_allocator
(public member function) |
|
|
(destructor)
(implicitly declared)
|
implicitly declared destructor
(public member function) |
|
operator=
[deleted]
|
copy assignment operator is deleted
(public member function) |
Public member functions |
|
|
allocate memory
(public member function) |
|
|
deallocate memory
(public member function) |
|
|
constructs an object in allocated storage
(public member function) |
|
|
(deprecated in C++20)
(undeprecated in C++26)
|
destroys an object in allocated storage
(public member function) |
|
(C++20)
|
allocate raw aligned memory from the underlying resource
(public member function) |
|
(C++20)
|
free raw memory obtained from
allocate_bytes
(public member function) |
|
(C++20)
|
allocates raw memory suitable for an object or an array
(public member function) |
|
(C++20)
|
frees raw memory obtained by
allocate_object
(public member function) |
|
(C++20)
|
allocates and constructs an object
(public member function) |
|
(C++20)
|
destroys and deallocates an object
(public member function) |
create a new
polymorphic_allocator
for use by a container's copy constructor
(public member function) |
|
|
returns a pointer to the underlying memory resource
(public member function) |
|
Non-member functions
|
(removed in C++20)
|
compare two
polymorphic_allocator
s
(function) |
Notes
polymorphic_allocator
does not propagate on container copy assignment, move assignment, or swap. As a result, move assignment of a
polymorphic_allocator
-using container can throw, and swapping two
polymorphic_allocator
-using containers whose allocators do not compare equal results in undefined behavior.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_polymorphic_allocator
|
201902L | (C++20) |
std::pmr::polymorphic_allocator<>
as a vocabulary type
|
See also
|
(C++17)
|
an abstract interface for classes that encapsulate memory resources
(class) |