Compile error trying to put a unique_ptr into a map - c++

Question

I have a Load-Method which builds my unique_ptr (will be more than one later on) and a method to add these unique_ptr to my unordered map. But the code does not compile and I guess it has something to do with scoping...
Here is the code:
#include <unordered_map>
#include <memory>
class MyClass
{
public:
std::string Name;
};
using Map = std::unordered_map<std::string,std::unique_ptr<MyClass>>;
class MyContainer
{
private:
Map myMap;
void AddItem(std::unique_ptr<MyClass> item)
{
myMap.emplace("test", item);
}
public:
void LoadItems()
{
//Read a file ... do something before etc..
std::unique_ptr<MyClass> someItem(new MyClass);
someItem->Name = "FooBar";
AddItem(someItem);
}
};
This is one of the g++ error messages:
error: use of deleted function 'std::unique_ptr<_Tp,
_Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = MyClass; _Dp = std::default_delete]'
What is the best way to get this working? I tried changing the signature of the AddItem-method like so:
void AddItem(std::unique_ptr<MyClass>& item) //takes a reference now...
This leads to a real cryptic error message:
In instantiation of 'constexpr std::pair<_T1, _T2>::pair(_U1&&, const
_T2&) [with _U1 = const char (&)[5]; = void; _T1 = const std::basic_string; _T2 = std::unique_ptr]': e:\devtools\winbuilds\include\c++\4.8.3\bits\hashtable_policy.h:177:55:
required from 'std::__detail::_ ...
I suggest trying this piece of code on the fly here, to see the error messages:
http://cpp.sh/

Answer 1

You cannot copy a unique_ptr, because then it will not be unique. You have to move it - AddItem(std::move(someItem)); and myMap.emplace("test", std::move(item));.

Answer 2

You are trying to copy unique_ptr which is not allowed (that constructor is deleted as gcc says in the error). Instead of that you can try with std::move:
#include <unordered_map>
#include <memory>
#include <utility>
class MyClass
{
public:
std::string Name;
};
using Map = std::unordered_map<std::string,std::unique_ptr<MyClass>>;
class MyContainer
{
private:
Map myMap;
void AddItem(std::unique_ptr<MyClass> item)
{
myMap.emplace("test", std::move(item));
}
public:
void LoadItems()
{
//Read a file ... do something before etc..
std::unique_ptr<MyClass> someItem(new MyClass);
someItem->Name = "FooBar";
AddItem(std::move(someItem));
}
};
Be aware, do not use the moved object afterwards.
You can consider to use shared_ptr instead.