使用 lambda 构造 std::function 时 libstdc++ 和 libc++ 之间的不同行为

Question

这个问题来自this question。

以下代码compiles fine 使用带有 libstdc++ 的 clang 3.4：

#include <functional>

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

但是 fails miserably 使用 clang 3.4 和 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.

哪种行为是正确的？

Answer 1

令我惊讶的是，对于哪些实施正确，哪些实施错误存在任何疑问； sn-p 当然应该编译。 libc++ 有错误行为，下面是相关错误报告的链接。

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

---

^{注意：Bug 17798 已在较新版本的库实现中得到修复。}