constexpr模板参数古怪

Question

海湾合作委员会(5.3)和Clang(3.8)声称第一行test是坏的,但第二行是好的.MSVC(2015.2)说,两者都是无效的.

template< typename N, typename T >
void f( N n, T t ) { std::get< n >( t ); }
void test() {
    std::get< std::integral_constant< size_t, 0 >() >( std::make_tuple( 123 ) ); // not ok
    f( std::integral_constant< size_t, 0 >(), std::make_tuple( 123 ) ); // ok for gcc, clang, but not msvc
}

根据标准,究竟是什么区别？此代码合法开头吗？

---

第一行的clang错误:

In file included from main.cpp:2:
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5.3.0/tuple:874:34: error: no matching function for call to '__get_helper2'
    { return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
                                 ^~~~~~~~~~~~~~~~~~~~~~~
main.cpp:10:10: note: in instantiation of function template specialization 'std::get<std::integral_constant<unsigned long, 0> (), int>' requested here
    std::get<std::integral_constant<size_t, 0>()>( std::make_tuple( 123 ) ); // not ok
         ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5.3.0/tuple:856:5: note: candidate template ignored: could not match '_Tuple_impl' against 'tuple'
    __get_helper2(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5.3.0/tuple:861:5: note: candidate template ignored: could not match '_Tuple_impl' against 'tuple'
    __get_helper2(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    ^
1 error generated.

gcc错误:

In file included from main.cpp:2:0:
/usr/local/include/c++/5.3.0/tuple: In instantiation of 'constexpr _Tp&& std::get(std::tuple<_Elements ...>&&) [with _Tp = std::integral_constant<long unsigned int, 0ul>(); _Types = {int}]':
main.cpp:10:75:   required from here
/usr/local/include/c++/5.3.0/tuple:874:57: error: no matching function for call to '__get_helper2(std::tuple<int>&)'
     { return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
                                                         ^
/usr/local/include/c++/5.3.0/tuple:856:5: note: candidate: template<class _Head, long unsigned int __i, class ... _Tail> constexpr _Head& std::__get_helper2(std::_Tuple_impl<__i, _Head, _Tail ...>&)
     __get_helper2(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
     ^
/usr/local/include/c++/5.3.0/tuple:856:5: note:   template argument deduction/substitution failed:
/usr/local/include/c++/5.3.0/tuple:874:57: note:   mismatched types 'std::integral_constant<long unsigned int, 0ul>()' and 'int'
     { return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
                                                         ^
/usr/local/include/c++/5.3.0/tuple:874:57: note:   'std::tuple<int>' is not derived from 'std::_Tuple_impl<__i, std::integral_constant<long unsigned int, 0ul>(), _Tail ...>'
/usr/local/include/c++/5.3.0/tuple:861:5: note: candidate: template<class _Head, long unsigned int __i, class ... _Tail> constexpr const _Head& std::__get_helper2(const std::_Tuple_impl<__i, _Head, _Tail ...>&)
     __get_helper2(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
     ^
/usr/local/include/c++/5.3.0/tuple:861:5: note:   template argument deduction/substitution failed:
/usr/local/include/c++/5.3.0/tuple:874:57: note:   mismatched types 'std::integral_constant<long unsigned int, 0ul>()' and 'int'
     { return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
                                                         ^
/usr/local/include/c++/5.3.0/tuple:874:57: note:   'std::tuple<int>' is not derived from 'const std::_Tuple_impl<__i, std::integral_constant<long unsigned int, 0ul>(), _Tail ...>'

Answer 1

tl; dr我认为gcc和clang在这两种情况下的行为都是正确的.

两次通话之间存在细微差别.让我添加一个别名来说明:

using Zero = std::integral_constant<size_t, 0>;
auto tuple = std::make_tuple(123);

当你写:

std::get<Zero()>(tuple);

Zero()是一种类型.这是一个返回的nullary函数Zero.因此,您正在调用的std::get类型为:std::get<T>().由于tuple该类型中没有元素,因此Zero()这是一个错误.

另一方面,当你写:

Zero n;
std::get<n>(tuple);

n不是一种类型 - 它只是一种价值.因为std::integral_constant有一个constexpr operator size_t(),一个被使用,你最终打电话std::get<I>(),这是你所期望的.

通过简单地使用大括号初始化也可以实现同样的目的:

std::get<Zero{}>(tuple);

因为Zero{}绝对不是一种类型.