Constructing class with member tuple containing rvalue references

Question

When trying to construct a class which is supposed to hold a tuple created by calling std::forward_as_tuple I ran into the following error when compiling with clang(187537) and libc++:

/usr/include/c++/v1/tuple:329:11: error: rvalue reference to type 'int' cannot bind to lvalue of type 'int' : value(__t.get()) ^ ~~~~~~~~~ /usr/include/c++/v1/tuple:447:8: note: in instantiation of member function 'std::__1::__tuple_leaf<0, int &&, false>::__tuple_leaf' requested here struct __tuple_impl<__tuple_indices<_Indx...>, _Tp...> ^ tuple.cpp:31:5: note: in instantiation of function template specialization 'make_foo2<int>' requested here make_foo2(1 + 1); ^ In file included from tuple.cpp:2: /usr/include/c++/v1/tuple:330:10: error: static_assert failed "Can not copy a tuple with rvalue reference member" {static_assert(!is_rvalue_reference<_Hp>::value, "Can not copy ...

I was able to work around the above error by declaring the return type differently, but, from my understanding, it should have the same semantics so I would not expect it to stop the error. In the below code make_foo is the workaround which does not error out and make_foo2 causes the above error. I am able to successfully compile both versions using gcc 4.8.1 and the version of clang at coliru.

#include <utility> #include <tuple> template<class Tuple> struct foo { Tuple t; foo(Tuple &&t) : t(std::move(t)) { } }; template<class... Args> using ForwardedTuple = decltype(std::forward_as_tuple(std::forward<Args>(std::declval<Args>())...)); template<class... Args> foo<ForwardedTuple<Args...>> make_foo(Args&&... args) { return {std::forward_as_tuple(std::forward<Args>(args)...)}; } template<class... Args> auto make_foo2(Args&& ...args) -> decltype(foo<decltype(std::forward_as_tuple(std::forward<Args>(args)...))>(std::forward_as_tuple(std::forward<Args>(args)...))) { return foo<decltype(std::forward_as_tuple(std::forward<Args>(args)...))>(std::forward_as_tuple(std::forward<Args>(args)...)); } int main() { make_foo(1 + 1); make_foo2(1 + 1); }

What is the difference between the above make_foo functions and is make_foo2 incorrect?

Thanks.

std::forward_as_tuple creates a tupel of rvalue references (or lvalue references). Is that what you want? — Walter
– Walter, Commented Aug 22, 2013 at 0:34
@Walter - I believe so; I'm interested in manipulating the values of the resulting tuple in the same expression it was created in so, from my understanding, the rvalue's captured in the tuple should exist for the whole expression. (ie. cout << std::get<0>(make_foo(1 + 1).t) << endl; — user2705506
– user2705506, Commented Aug 22, 2013 at 0:41
So both make_foo and make_foo2 compile at coliru's clang++ and g++ but make_foo2 does not compile on your own clang++ with libc++? — dyp
– dyp, Commented Aug 22, 2013 at 5:17
One difference between make_foo and make_foo2 is that make_foo doesn't require foo<...> to have either a copy or move ctor (none is required). See this answer and the comments to it. — dyp
– dyp, Commented Aug 22, 2013 at 5:24

Artem Tokmakov · Accepted Answer · 2013-08-22 02:01:58Z

1

Looks like you return foo<> from make_foo2. But foo doesn't have move constructor generated (Compiler won't generate it). Therefore copy constructor is called and compilation fails because of that.

answered Aug 22, 2013 at 2:01

Artem Tokmakov

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3 Comments

user2705506 Over a year ago

Thanks. When I add foo(foo&&) = default; foo& operator=(foo&&) & = default; to foo it then compiles make_foo2. Would you mind going into a little detail on why the compiler was unable to generate the move constructor for foo? I thought that since the tuple member was movable, and no copy, move, or destructor was declared that the compiler should generate the move constructor.

Artem Tokmakov Over a year ago

Well, I'm not really sure. Looks to me it should have been generated according to C++11 rules. It might just be compiler issue. Not sure about Clang (that version you're using) but one of C++11 features Visual Studio compiler is (at least was) missing is generating default move constructors.

Artem Tokmakov Over a year ago

It works fine as is with gcc in ideone.com: ideone.com/pQ3jzt so it does look like it's a compiler issue.

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