Hành vi khác nhau giữa libstdC++ và libC++ khi xây dựng std :: function with lambda

Question

Sự cố này xuất phát từ this question.

Các mã sau đây compiles fine sử dụng kêu vang 3.4 với libstdC++:

#include <functional> 

int main() { 
    std::function<void()> f = []() {}; 
} 

Nhưng fails miserably sử dụng kêu vang 3.4 và libC++:

In file included from main.cpp:1: 
In file included from /usr/include/c++/v1/functional:465: 
In file included from /usr/include/c++/v1/memory:599: 
/usr/include/c++/v1/tuple:320:11: error: rvalue reference to type '<lambda at main.cpp:4:31>' cannot bind to lvalue of type '<lambda at main.cpp:4:31>' 
     : value(__t.get()) 
     ^ ~~~~~~~~~ 
/usr/include/c++/v1/tuple:444:8: note: in instantiation of member function 'std::__1::__tuple_leaf<0, <lambda at main.cpp:4:31> &&, false>::__tuple_leaf' requested here 
struct __tuple_impl<__tuple_indices<_Indx...>, _Tp...> 
    ^
/usr/include/c++/v1/functional:1278:26: note: in instantiation of member function 'std::__1::__function::__func<<lambda at main.cpp:4:31>, std::__1::allocator<<lambda at main.cpp:4:31> >, void()>::__func' requested here 
      ::new (__f_) _FF(_VSTD::move(__f)); 
         ^
main.cpp:4:31: note: in instantiation of function template specialization 'std::__1::function<void()>::function<<lambda at main.cpp:4:31> >' requested here 
    std::function<void()> f = []() {}; 
          ^
In file included from main.cpp:1: 
In file included from /usr/include/c++/v1/functional:465: 
In file included from /usr/include/c++/v1/memory:599: 
/usr/include/c++/v1/tuple:321:10: error: static_assert failed "Can not copy a tuple with rvalue reference member" 
     {static_assert(!is_rvalue_reference<_Hp>::value, "Can not copy a tuple with rvalue reference member");} 
     ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
/usr/include/c++/v1/tuple:320:11: error: rvalue reference to type 'allocator<[...]>' cannot bind to lvalue of type 'allocator<[...]>' 
     : value(__t.get()) 
     ^ ~~~~~~~~~ 
/usr/include/c++/v1/tuple:444:8: note: in instantiation of member function 'std::__1::__tuple_leaf<0, std::__1::allocator<<lambda at main.cpp:4:31> > &&, false>::__tuple_leaf' requested here 
struct __tuple_impl<__tuple_indices<_Indx...>, _Tp...> 
    ^
/usr/include/c++/v1/functional:1286:34: note: in instantiation of member function 'std::__1::__function::__func<<lambda at main.cpp:4:31>, std::__1::allocator<<lambda at main.cpp:4:31> >, void()>::__func' requested here 
      ::new (__hold.get()) _FF(_VSTD::move(__f), allocator<_Fp>(__a)); 
           ^
main.cpp:4:31: note: in instantiation of function template specialization 'std::__1::function<void()>::function<<lambda at main.cpp:4:31> >' requested here 
    std::function<void()> f = []() {}; 
          ^
In file included from main.cpp:1: 
In file included from /usr/include/c++/v1/functional:465: 
In file included from /usr/include/c++/v1/memory:599: 
/usr/include/c++/v1/tuple:321:10: error: static_assert failed "Can not copy a tuple with rvalue reference member" 
     {static_assert(!is_rvalue_reference<_Hp>::value, "Can not copy a tuple with rvalue reference member");} 
     ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
4 errors generated. 

Những hành vi là đúng?

Answer 1

Tôi ngạc nhiên rằng có bất kỳ nghi ngờ nào về việc triển khai nào đang thực hiện đúng và đang thực hiện sai; các đoạn mã nên tất nhiên biên dịch. libc++ có hành vi sai, và bên dưới là liên kết đến báo cáo lỗi có liên quan.

• Bug 17798 - Regression: std::function cannot be assigned a lambda function

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^{Note: Bug 17798 đã được cố định trong các phiên bản mới hơn của việc thực hiện thư viện.}