std :: throw_with_nested期望多态类型在C ++ 11？ [英] std::throw_with_nested expects polymorphic type in C++11?

问题描述

为什么这不编译（尝试与Clang 3.4.2和GCC版本4.7.4，4.8.3和4.9.1）：

Why does this not compile (tried with Clang 3.4.2 and GCC versions 4.7.4, 4.8.3 and 4.9.1):

#include <exception>

struct E { E(int) {} };

int main() {
  std::throw_with_nested(E(42));
  return 0;
}

GCC错误4.9.1：

Errors from GCC 4.9.1:

In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/exception:163:0,
                from test.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h: In instantiation of 'static const std::nested_exception* std::__get_nested_helper<_Ex>::_S_get(const _Ex&) [with _Ex = E]':
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h:104:51:   required from 'const std::nested_exception* std::__get_nested_exception(const _Ex&) [with _Ex = E]'
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h:138:38:   required from 'void std::throw_with_nested(_Ex) [with _Ex = E]'
test.cpp:6:31:   required from here
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h:90:59: error: cannot dynamic_cast '& __ex' (of type 'const struct E*') to type 'const class std::nested_exception*' (source type is not polymorphic)
      { return dynamic_cast<const nested_exception*>(&__ex); }
                                                          ^

来自Clang 3.4.2的错误：

Errors from Clang 3.4.2:

In file included from test.cpp:1:
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/exception:163:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h:90:16: error: 'E' is not polymorphic
      { return dynamic_cast<const nested_exception*>(&__ex); }
              ^                                     ~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h:104:40: note: in instantiation of member function 'std::__get_nested_helper<E>::_S_get' requested here
    { return __get_nested_helper<_Ex>::_S_get(__ex); }
                                      ^
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.1/include/g++-v4/bits/nested_exception.h:138:11: note: in instantiation of function template specialization 'std::__get_nested_exception<E>' requested here
      if (__get_nested_exception(__ex))
          ^
test.cpp:6:8: note: in instantiation of function template specialization 'std::throw_with_nested<E>' requested here
  std::throw_with_nested(E(42));
      ^

std :: throw_with_nested 在C ++ 11中预期一个多态类型的参数，或者这是编译器或libstdc ++中的错误？

Does std::throw_with_nested in C++11 expect an argument with polymorphic type or is this a bug in the compiler or libstdc++?

推荐答案

当实施它时 for libstdc ++ in 2009 the spec in N2619 需要 E 是一个多态类型，但2011标准中的最终规范是不同的，libstdc ++中的实现从未改变。

When I implemented it for libstdc++ in 2009 the spec in N2619 required E to be a polymorphic type, but the final spec in the 2011 standard is different and the implementation in libstdc++ was never changed.

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