Using std::allocate_shared with polymorphic resource allocators - c++

Question

I am attempting to create shared pointers with a std::pmr::monotonic_buffer_resource, and I cannot make it compile. What am I missing?
https://godbolt.org/z/R9jdju
#include <memory>
#include <memory_resource>
int main() {
char buffer[100];
std::pmr::monotonic_buffer_resource mbr(buffer, 100);
std::shared_ptr<double> sp = std::allocate_shared<double>(mbr);
}
In file included from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/ext/alloc_traits.h:34,
from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/stl_uninitialized.h:67,
from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/memory:66,
from <source>:1:
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/alloc_traits.h: In substitution of 'template<class _Alloc, class _Up> using __alloc_rebind = typename std::__allocator_traits_base::__rebind<_Alloc, _Up>::type [with _Alloc = std::pmr::monotonic_buffer_resource; _Up = std::_Sp_counted_ptr_inplace<double, std::pmr::monotonic_buffer_resource, __gnu_cxx::_S_atomic>]':
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h:542:13: required from 'class std::_Sp_counted_ptr_inplace<double, std::pmr::monotonic_buffer_resource, __gnu_cxx::_S_atomic>'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h:679:43: required from 'std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = double; _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h:1371:71: required from 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}; _Tp = double; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr.h:408:59: required from 'std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}; _Tp = double]'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr.h:859:14: required from 'std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = double; _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}]'
<source>:7:66: required from here
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/alloc_traits.h:78:11: error: no type named 'type' in 'struct std::__allocator_traits_base::__rebind<std::pmr::monotonic_buffer_resource, std::_Sp_counted_ptr_inplace<double, std::pmr::monotonic_buffer_resource, __gnu_cxx::_S_atomic>, void>'
78 | using __alloc_rebind
| ^~~~~~~~~~~~~~
In file included from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr.h:52,
from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/memory:84,
from <source>:1:
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h: In instantiation of 'std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = double; _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]':
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h:1371:71: required from 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}; _Tp = double; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr.h:408:59: required from 'std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}; _Tp = double]'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr.h:859:14: required from 'std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = double; _Alloc = std::pmr::monotonic_buffer_resource; _Args = {}]'
<source>:7:66: required from here
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h:682:16: error: use of deleted function 'std::pmr::monotonic_buffer_resource::monotonic_buffer_resource(const std::pmr::monotonic_buffer_resource&)'
682 | auto __pi = ::new (__mem)
| ^~~~~~~~~~~~~
683 | _Sp_cp_type(__a._M_a, std::forward<_Args>(__args)...);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from <source>:2:
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/memory_resource:604:5: note: declared here
604 | monotonic_buffer_resource(const monotonic_buffer_resource&) = delete;
| ^~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr.h:52,
from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/memory:84,
from <source>:1:
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/shared_ptr_base.h:546:33: note: initializing argument 1 of 'std::_Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp>::_Sp_counted_ptr_inplace(_Alloc, _Args&& ...) [with _Args = {}; _Tp = double; _Alloc = std::pmr::monotonic_buffer_resource; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
546 | _Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
| ~~~~~~~^~~

Answer 1

That long error message basically boils down to these 2 errors:
error: no type named 'type' in 'struct std::__allocator_traits_base::__rebind<std::pmr::monotonic_buffer_resource, std::_Sp_counted_ptr_inplace<double, std::pmr::monotonic_buffer_resource, __gnu_cxx::_S_atomic>, void>'
error: use of deleted function 'std::pmr::monotonic_buffer_resource::monotonic_buffer_resource(const std::pmr::monotonic_buffer_resource&)'
std::pmr::monotonic_buffer_resource does not satisfy the requirements that std::allocate_shared() expects, specifically:
All memory allocation is done using a copy of alloc, which must satisfy the Allocator requirements.
In particular, "a COPY of alloc", which fails since monotonic_buffer_resource's copy constructor is delete'd so it can't be copied.
As #MilesBudnek stated in comments, you can wrap the monotonic_buffer_resource inside a std::pmr::polymorphic_allocator, which is designed to be used as an Allocator for standard containers:
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.
For example:
#include <memory>
#include <memory_resource>
int main() {
char buffer[100];
std::pmr::monotonic_buffer_resource mbr(buffer, 100);
auto sp = std::allocate_shared<double, std::pmr::polymorphic_allocator<double>>(&mbr);
}
https://godbolt.org/z/-xFfFY
Alternatively:
#include <memory>
#include <memory_resource>
int main() {
char buffer[100];
std::pmr::monotonic_buffer_resource mbr(buffer, 100);
std::pmr::polymorphic_allocator<double> alloc(&mbr);
auto sp = std::allocate_shared<double>(alloc);
}
https://godbolt.org/z/GLE4-5