Question

I have a structure named Foo which contains a unique_ptr

struct Foo {
    std::unique_ptr<Bar> pointer;
};

Now I’m trying to store instances of Foo in an unordered_map

std::unordered_map<int,Foo> myMap;

Technically this should be possible, since maps do not require a copy constructor, only a move constructor.

However, I can’t insert an element in my map:

myMap.insert(std::make_pair(3, Foo()));

This line will generate the following error in Visual C++ 2010 (roughly translated by me, since my compiler is not in english):

error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : unable to access private member declared in 'std::unique_ptr<_Ty>'  
with
[
             _Ty=Foo
]
c:\Softwares\Visual Studio 10.0\VC\include\memory(2347) : see declaration of 'std::unique_ptr<_Ty>::unique_ptr'
with
[
             _Ty=Foo
]
This diagnostic happened in the compiler-generated function 'Foo::Foo(const Foo&)'

So for an unknown reason, the compiler tries to generate a copy constructor for Foo instead of a move constructor, and fails.

I tried to replace std::make_pair with std::pair<int,something>, but can’t find any something which works.

---

EDIT : this works

struct Foo {
    Foo() {}
    Foo(Foo&& other) : pointer(std::move(other.pointer)) {}
    std::unique_ptr<Bar> pointer;
};

But my real structure contains a lot of members, and I don’t want to write all of them in the move constructor.

Answer 1

MSVC10 (in Visual Studio 2010) doesn’t implement implicit move constructors yet (not surprising since implicit move constructors got into the standard quite late – there was a lot of discussion about it). They won’t be in MSVC11 (in the not-yet-released Visual Studio 2012) either.

I would suggest using =default, but that’s not supported yet either.