I’m trying to learn the use of std::reference_wrapper and std::ref and the use cases of it.
Here is a sample code I wrote, please help me solve the compilation error and it would be great if you explain the issue in depth.

#include <iostream>
#include <limits>
#include <unordered_map>
#include <functional>

class MyClass
{
public:
  struct Data
  {
    int id;

    Data() : id(std::numeric_limits<int>::max()) {}
  };

  std::unordered_map<std::string, Data> keyToData_;

  Data& getOrCreateData(const std::string &key) 
  {
    auto it = keyToData_.find(key);
    if (it == keyToData_.end())
    {
      it = keyToData_.insert({key, Data()}).first;
    }
    return it->second;
  }

  void addBuffer(const std::string &key, std::reference_wrapper<Data> && r)
  {
    buffer_[key] = r;
  }

private:
  std::unordered_map<std::string, std::reference_wrapper<Data>> buffer_;
};

int main(int, char **argv)
{
  MyClass dataManager{};

  auto &r1 = dataManager.getOrCreateData("key1");
  r1.id = 1;

  dataManager.addBuffer("key1", std::ref(r1));
}

I want to store the reference of the data, the data will keep updating in the meantime of the code, it will be like the dirty bit reference of the data, at the end of my program I would just need to update these value without iterating the whole unordered_map. I don’t want to create any copy in this process to avoid performance issues and I’m also avoiding shared_ptr as I think using raw pointer would be just better in this case(I know that the object is not going out of scope). Is there any better approach?

Here’s the error –

.sample.cpp:30:16:   required from here
C:/msys64/mingw64/include/c++/11.3.0/tuple:1824:9: error: no matching function for call to 'std::reference_wrapper<MyClass::Data>::reference_wrapper()'
 1824 |         second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from C:/msys64/mingw64/include/c++/11.3.0/functional:58,
                 from .sample.cpp:4:
C:/msys64/mingw64/include/c++/11.3.0/bits/refwrap.h:321:9: note: candidate: 'template<class _Up, class, class> std::reference_wrapper<_Tp>::reference_wrapper(_Up&&) [with _Up = _Up; <template-parameter-2-2> = <template-parameter-1-2>; <template-parameter-2-3> = <template-parameter-1-3>; _Tp = MyClass::Data]'
  321 |         reference_wrapper(_Up&& __uref)
      |         ^~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/11.3.0/bits/refwrap.h:321:9: note:   template argument deduction/substitution failed:
In file included from C:/msys64/mingw64/include/c++/11.3.0/bits/hashtable_policy.h:34,
                 from C:/msys64/mingw64/include/c++/11.3.0/bits/hashtable.h:35,
                 from C:/msys64/mingw64/include/c++/11.3.0/unordered_map:46,
                 from .sample.cpp:3:
C:/msys64/mingw64/include/c++/11.3.0/tuple:1824:9: note:   candidate expects 1 argument, 0 provided
 1824 |         second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from C:/msys64/mingw64/include/c++/11.3.0/functional:58,
                 from .sample.cpp:4:
C:/msys64/mingw64/include/c++/11.3.0/bits/refwrap.h:326:7: note: candidate: 'constexpr std::reference_wrapper<_Tp>::reference_wrapper(const std::reference_wrapper<_Tp>&) [with _Tp = MyClass::Data]'
  326 |       reference_wrapper(const reference_wrapper&) = default;
      |       ^~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/11.3.0/bits/refwrap.h:326:7: note:   candidate expects 1 argument, 0 provided

I have tried changing the function declaration like this –

 void addBuffer(const std::string &key, Data &r)
  {
    buffer_[key] = std::ref(r);
  }