use polymorphic_allocator for json in embedded system

[yushb0602]

We are using C++20 in embedded system with 128MB memory, so we are worrying about the dynamical memory allocation/memory segmentation. And want to use pmr and customize json library as :

using json_pmr = nlohmann::basic_json<std::pmr::map, std::pmr::vector, std::pmr::string, bool, std::int64_t, std::uint64_t,
                         float, std::pmr::polymorphic_allocator>;

pmr::monotonic_buffer_resource res(10000);
pmr::string s1{&res};
json_pmr obj(&res);

It seems that this feature is not fully finished.

[nlohmann]

Can you share more information? Compiler errors?

[yushb0602]

@nlohmann Thanks for your quick reply. There is no constructor function like basic_json( Allocator& alloc )) , the alloc is not passed to children elements.

when using gcc 10.3, the compile error message is:

/root/.conan/data/nlohmann_json/3.11.2///package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/nlohmann/json.hpp:333:11: error: wrong number of template arguments (4, should be at least 2)
333 | using object_t = ObjectType<StringType,
| ^~~~~~~~
In file included from /opt/cloudlink_x86/cppsrc/base_includes.h:2,
from /opt/cloudlink_x86/cppsrc/main.cpp:3:
/usr/include/c++/11/map:78:13: note: provided for 'template<class _Key, class _Tp, class _Cmp> using map = std::map<_Key, _Tp, _Cmp, std::pmr::polymorphic_allocator<std::pair<const Key, Tp> > >'
78 | using map
| ^~~
In file included from /opt/cloudlink_x86/cppsrc/jsonext.h:14,
from /opt/cloudlink_x86/cppsrc/base_includes.h:17,
from /opt/cloudlink_x86/cppsrc/main.cpp:3:
/root/.conan/data/nlohmann_json/3.11.2///package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/nlohmann/json.hpp:341:11: error: wrong number of template arguments (2, should be 1)
341 | using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
| ^~~~~~~
In file included from /opt/cloudlink_x86/cppsrc/base_includes.h:5,
from /opt/cloudlink_x86/cppsrc/main.cpp:3:
/usr/include/c++/11/vector:86:13: note: provided for 'template using vector = std::vector<_Tp, std::pmr::polymorphic_allocator<_CharT> >
86 | using vector = std::vector<_Tp, polymorphic_allocator<_Tp>>;
| ^~~~~~
In file included from /opt/cloudlink_x86/cppsrc/jsonext.h:14,

when use Vistual studio 2022, the translated error message is:

Error C2977 “std::pmr::map”: too many tempalte paramters ConsoleApplication3 C:\Users\yushb\source\repos\ConsoleApplication3\packages\nlohmann.json.3.11.2\build\native\include\nlohmann\json.hpp 19511
Error C2977 “std::pmr::vector”: too many tempalte paramters ConsoleApplication3 C:\Users\yushb\source\repos\ConsoleApplication3\packages\nlohmann.json.3.11.2\build\native\include\nlohmann\json.hpp 19519

[yushb0602]

I have tried json 3.11.2 on Vistual studio and g++ 10.3 with C++20 standard