Error returning a reference of an std::unordered_map with an std::unique_ptr

Question

I'm wondering why do I have this error compiling the following example code:

#include <unordered_map>
#include <memory>

class OneClass
{

};

using TestUnorderedMap = std::unordered_map<int, std::unique_ptr<OneClass>>;

class TestClass
{
public:
    TestUnorderedMap& getMap() { return m_map; }
private:
    TestUnorderedMap m_map;
};

void simpleTestFunction()
{
    TestClass t;
    TestUnorderedMap map = t.getMap();
}

I'm getting this error when compiling with Visual Studio 2022, can I not return a reference to an unordered_map that uses an std::unique_ptr as an object ?

D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\xutility(241): error C2280: 'std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>::pair(const std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>> &)': attempting to reference a deleted function
  D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\utility(253): note: see declaration of 'std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>::pair'
  D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\utility(253): note: 'std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>::pair(const std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>> &)': function was implicitly deleted because a data member invokes a deleted or inaccessible function 'std::unique_ptr<OneClass,std::default_delete<OneClass>>::unique_ptr(const std::unique_ptr<OneClass,std::default_delete<OneClass>> &)'
  D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\memory(3288): note: 'std::unique_ptr<OneClass,std::default_delete<OneClass>>::unique_ptr(const std::unique_ptr<OneClass,std::default_delete<OneClass>> &)': function was explicitly deleted
  D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\xutility(241): note: the template instantiation context (the oldest one first) is
  D:\Users\Rui Barreiros\Documents\Projectos\Audio\rSensors\rSensorServer_V2\test.cpp(20): note: see reference to class template instantiation 'std::unordered_map<int,std::unique_ptr<OneClass,std::default_delete<OneClass>>,std::hash<int>,std::equal_to<int>,std::allocator<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>>>' being compiled
  D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\unordered_map(110): note: while compiling class template member function 'std::unordered_map<int,std::unique_ptr<OneClass,std::default_delete<OneClass>>,std::hash<int>,std::equal_to<int>,std::allocator<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>>>::unordered_map(const std::unordered_map<int,std::unique_ptr<OneClass,std::default_delete<OneClass>>,std::hash<int>,std::equal_to<int>,std::allocator<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>>> &)'
  D:\Users\Rui Barreiros\Documents\Projectos\Audio\rSensors\rSensorServer_V2\test.cpp(27): note: see the first reference to 'std::unordered_map<int,std::unique_ptr<OneClass,std::default_delete<OneClass>>,std::hash<int>,std::equal_to<int>,std::allocator<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>>>::unordered_map' in 'simpleTestFunction'
  D:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\include\unordered_map(111): note: see reference to function template instantiation 'std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>::_Hash<std::allocator<std::_List_node<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>,std::_Default_allocator_traits<_Alloc>::void_pointer>>>(const std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>> &,const _Any_alloc &)' being compiled
          with
          [
              _Kty=int,
              _Ty=std::unique_ptr<OneClass,std::default_delete<OneClass>>,
              _Hasher=std::hash<int>,
              _Keyeq=std::equal_to<int>,
              _Alloc=std::allocator<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>>,
              _Any_alloc=std::allocator<std::_List_node<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>,std::_Default_allocator_traits<std::allocator<std::pair<const int,std::unique_ptr<OneClass,std::default_delete<OneClass>>>>>::void_pointer>>
          ]
  ...snip...

You can return a reference to unique_ptr. You cannot create a copy out of that reference, and that's what simpleTestFunction tries to do. Did you mean TestUnorderedMap& map = t.getMap(); instead? — Yksisarvinen
– Yksisarvinen, Commented May 10, 2024 at 14:53
unique_ptr is not copyable and TestUnorderedMap map = t.getMap(); causes a copy — NathanOliver
– NathanOliver, Commented May 10, 2024 at 14:53
I'm confused.... what is the diference between 'TestUnorderedMap& map = t.getMap();' and 'TestUnorderedMap map = t.getMap();' shouldn't it be the same ? I also tried 'auto map = t.getMap();' and the error remains. — rbarreiros
– rbarreiros, Commented May 10, 2024 at 15:07
TestUnorderedMap& map = t.getMap(); is a good option to use. — NathanOliver
– NathanOliver, Commented May 10, 2024 at 15:10
No, it shouldn't be, and isn't the same. TestUnorderedMap& map = t.getMap(); defines map as a reference, which is initialized from the returned reference. TestUnorderedMap map = t.getMap(); defines map as a standalone unordered_map in its own right that is to be initialized as a copy of the one referenced by the returned reference, only the copy can't be made. — Avi Berger
– Avi Berger, Commented May 10, 2024 at 15:29

Remy Lebeau · Accepted Answer · 2024-05-10 20:22:55Z

2

Figured out what was happening, thanks to @Yksisarvinen and @NathanOliver, I was creating a copy of the unordered_map object by using:

TestUnorderedMap map = t.getMap();

It should be using this instead:

TestUnorderedMap& map = t.getMap();

By using the first one, it would create a copy of the object into a new memory location, which unique_ptr does not allow. The second is just a reference.

edited May 10, 2024 at 20:22

Remy Lebeau

609k36 gold badges516 silver badges875 bronze badges

answered May 10, 2024 at 15:32

rbarreiros

612 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Error returning a reference of an std::unordered_map with an std::unique_ptr

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related