Why does lambda init-capture not work for unique_ptr? - c++

Question

I'm trying to use the C++14 init-capture feature to move a unique_ptr inside a lambda via capture. For some reason, both gcc and clang refuse to compile my code, insisting that I'm trying to copy a unique_ptr which obviously doesn't work. I thought that avoiding the copy was exactly the point of the init-capture + std::move feature - in fact, passing a unique_ptr seems to be the prime example used by everybody.
What am I doing wrong?
#include <functional>
#include <iostream>
#include <memory>
#include <string>
void runFunc(std::function<void()>&& f) {
auto ff = std::move(f);
ff();
}
int main() {
auto ptr = std::make_unique<std::string>("hello world\n");
runFunc([captured_ptr = std::move(ptr)]() {
std::cout << *captured_ptr;
});
}
Output from gcc:
http://coliru.stacked-crooked.com/a/d91a480b2b6428ac
g++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
In file included from main.cpp:1:0:
/usr/local/include/c++/5.2.0/functional: In instantiation of 'static void std::_Function_base::_Base_manager<_Functor>::_M_clone(std::_Any_data&, const std::_Any_data&, std::false_type) [with _Functor = main()::<lambda()>; std::false_type = std::integral_constant<bool, false>]':
/usr/local/include/c++/5.2.0/functional:1746:16: required from 'static bool std::_Function_base::_Base_manager<_Functor>::_M_manager(std::_Any_data&, const std::_Any_data&, std::_Manager_operation) [with _Functor = main()::<lambda()>]'
/usr/local/include/c++/5.2.0/functional:2260:19: required from 'std::function<_Res(_ArgTypes ...)>::function(_Functor) [with _Functor = main()::<lambda()>; <template-parameter-2-2> = void; _Res = void; _ArgTypes = {}]'
main.cpp:15:6: required from here
/usr/local/include/c++/5.2.0/functional:1710:34: error: use of deleted function 'main()::<lambda()>::<lambda>(const main()::<lambda()>&)'
__dest._M_access<_Functor*>() =
^
main.cpp:13:43: note: 'main()::<lambda()>::<lambda>(const main()::<lambda()>&)' is implicitly deleted because the default definition would be ill-formed:
runFunc([captured_ptr = std::move(ptr)]() {
^
main.cpp:13:43: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = std::__cxx11::basic_string<char>; _Dp = std::default_delete<std::__cxx11::basic_string<char> >]'
In file included from /usr/local/include/c++/5.2.0/memory:81:0,
from main.cpp:3:
/usr/local/include/c++/5.2.0/bits/unique_ptr.h:356:7: note: declared here
unique_ptr(const unique_ptr&) = delete;
Output from clang:
http://coliru.stacked-crooked.com/a/4374988d875fcedc
In file included from main.cpp:1:
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/functional:1711:10: error: call to implicitly-deleted copy constructor of '(lambda at main.cpp:13:13)'
new _Functor(*__source._M_access<_Functor*>());
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/functional:1746:8: note: in instantiation of member function 'std::_Function_base::_Base_manager<(lambda at main.cpp:13:13)>::_M_clone' requested here
_M_clone(__dest, __source, _Local_storage());
^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/functional:2260:33: note: in instantiation of member function 'std::_Function_base::_Base_manager<(lambda at main.cpp:13:13)>::_M_manager' requested here
_M_manager = &_My_handler::_M_manager;
^
main.cpp:13:13: note: in instantiation of function template specialization 'std::function<void ()>::function<(lambda at main.cpp:13:13), void>' requested here
runFunc([captured_ptr = std::move(ptr)]() {
^
main.cpp:13:14: note: copy constructor of '' is implicitly deleted because field '' has a deleted copy constructor
runFunc([captured_ptr = std::move(ptr)]() {
^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/bits/unique_ptr.h:356:7: note: 'unique_ptr' has been explicitly marked deleted here
unique_ptr(const unique_ptr&) = delete;
^
1 error generated.

Answer 1

Because std::function must be copyable:
std::function satisfies the requirements of CopyConstructible and CopyAssignable.
And the constructor in question:
Initializes the target with a copy of f.
The lambda that you are constructing (completely validly) has a unique_ptr member, which makes it noncopyable. If you rewrote runFunc to take an arbitrary functor by value:
template <typename F>
void runFunc(F f) {
auto ff = std::move(f);
ff();
}
it would compile.