c++ - 递归 boost::variant 类型不能用 "-std=c++11 -stdlib=libc++"编译

Question

考虑以下代码:

#include <vector>
#include <boost/variant.hpp>

struct foo;

typedef boost::variant<foo> bar;

struct foo
{
    std::vector<bar> baz;
};

int main ()
{
    foo f;
    return 0;
}

使用 Xcode 4.4 在 Mac OS X 上构建(我通过 Homebrew 安装了 boost 1.50.0):

• clang++ test.cc:没有错误。
• clang++ -stdlib=libc++ test.cc:没有错误。
• clang++ -std=c++11 test.cc:没有错误。
• clang++ -std=c++11 -stdlib=libc++ test.cc:很多错误!

/usr/local/include/boost/type_traits/has_nothrow_constructor.hpp:24:40: error: incomplete type 'foo' used in type trait expression
   BOOST_STATIC_CONSTANT(bool, value = BOOST_HAS_NOTHROW_CONSTRUCTOR(T));
                                       ^
...snip...
test.cc:10:19: note: in instantiation of template class '...snip...' requested here
        std::vector<bar> baz;
                         ^
test.cc:8:8: note: definition of 'foo' is not complete until the closing '}'
struct foo
       ^

/usr/local/include/boost/mpl/next_prior.hpp:31:22: error: type 'int' cannot be used prior to '::' because it has no members
    typedef typename T::next type;
                     ^
test.cc:10:19: note: in instantiation of template class '...snip...' requested here
        std::vector<bar> baz;
                         ^

/usr/local/include/boost/mpl/sizeof.hpp:27:20: error: invalid application of 'sizeof' to an incomplete type 'foo'
    : mpl::size_t< sizeof(T) >
                   ^~~~~~~~~

test.cc:10:19: note: in instantiation of template class '...snip...' requested here
        std::vector<bar> baz;
                         ^
test.cc:8:8: note: definition of 'foo' is not complete until the closing '}'
struct foo
       ^

...big snip...

8 errors generated.

这是怎么回事？为什么我不能用指定的选项编译它？有什么办法可以解决吗？

Answer 1

boost::recursive_wrapper<foo>是处理不完整类型的工具，同时仍然保持例如的错觉。访问者认为一个变体真的持有foo .