c++ - 为什么不使用 std::is_constructible 编译？

Question

我容器的一个构造函数默认构造一个分配器作为默认参数值:

template<class T, class Allocator>
struct my_container
{
  my_container(int n, Allocator alloc = Allocator()) {}
};

据推测，只有当 Allocator 可以默认构造时，此构造函数才会启用。

我想用 std::is_constructible 测试此构造函数是否可以与不是默认可构造的分配器一起使用:

template<class T>
struct my_not_default_constructible_allocator
{
  // no default ctor
  my_not_default_constructible_allocator(int) {}
};

但是，当我应用 std::is_constructible 时，我得到一个编译时错误，而不是 false，这正是我所期望的:

#include <type_traits>

template<class T, class Allocator>
struct my_container
{
  my_container(int n, Allocator alloc = Allocator()) {}
};

template<class T>
struct my_not_default_constructible_allocator
{
  // no default ctor
  my_not_default_constructible_allocator(int) {}
};

int main()
{
  bool result = std::is_constructible<my_container<int, my_not_default_constructible_allocator<int>>, int>::value;

  return 0;
}

编译器输出:

$ clang -std=c++14 repro.cpp 
repro.cpp:6:41: error: no matching constructor for initialization of 'my_not_default_constructible_allocator<int>'
  my_container(int n, Allocator alloc = Allocator()) {}
                                        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:976:24: note: in instantiation of default function argument expression for
      'my_container<int, my_not_default_constructible_allocator<int> >' required here
             = decltype(::new _Tp(declval<_Arg>()))>
                              ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:977:24: note: in instantiation of default argument for '__test<my_container<int,
      my_not_default_constructible_allocator<int> >, int>' required here
      static true_type __test(int);
                       ^~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:987:24: note: while substituting deduced template arguments into function template '__test' [with _Tp =
      my_container<int, my_not_default_constructible_allocator<int> >, _Arg = int, $2 = (no value)]
      typedef decltype(__test<_Tp, _Arg>(0)) type;
                       ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:144:14: note: in instantiation of template class 'std::__is_direct_constructible_impl<my_container<int,
      my_not_default_constructible_allocator<int> >, int>' requested here
    : public conditional<_B1::value, _B2, _B1>::type
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:992:14: note: in instantiation of template class 'std::__and_<std::is_destructible<my_container<int,
      my_not_default_constructible_allocator<int> > >, std::__is_direct_constructible_impl<my_container<int, my_not_default_constructible_allocator<int> >, int> >' requested here
    : public __and_<is_destructible<_Tp>,
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:1074:14: note: in instantiation of template class 'std::__is_direct_constructible_new_safe<my_container<int,
      my_not_default_constructible_allocator<int> >, int>' requested here
    : public conditional<is_reference<_Tp>::value,
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:1082:14: note: in instantiation of template class 'std::__is_direct_constructible_new<my_container<int,
      my_not_default_constructible_allocator<int> >, int>' requested here
    : public __is_direct_constructible_new<_Tp, _Arg>::type
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:1122:14: note: in instantiation of template class 'std::__is_direct_constructible<my_container<int,
      my_not_default_constructible_allocator<int> >, int>' requested here
    : public __is_direct_constructible<_Tp, _Arg>
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/type_traits:1133:14: note: in instantiation of template class 'std::__is_constructible_impl<my_container<int,
      my_not_default_constructible_allocator<int> >, int>' requested here
    : public __is_constructible_impl<_Tp, _Args...>::type
             ^
<snip>

编译器详细信息:

$ clang --version
clang version 4.0.1-6 (tags/RELEASE_401/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

std::is_constructible 的实现会导致错误，而不是 SFINAE 移除感兴趣的构造函数。

my_container 的构造函数的实现是否有误？

Answer 1

默认参数初始化似乎在 my_container 初始化的直接上下文中, [meta.unary_prop]/8 :

The predicate condition for a template specialization is_­constructible<T, Args...> shall be satisfied if and only if the following variable definition would be well-formed for some invented variable t:

T t(declval<Args>()...);

[ Note: These tokens are never interpreted as a function declaration. — end note  ] Access checking is performed as if in a context unrelated to T and any of the Args. Only the validity of the immediate context of the variable initialization is considered. [ Note: The evaluation of the initialization can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the “immediate context” and can result in the program being ill-formed. — end note  ]

根据 [expr.call]/7:

The initialization and destruction of each parameter occurs within the context of the calling function.

因此可以推断默认参数初始化发生在“直接上下文”中。我的意见是这不是很清楚，术语直接上下文没有正式定义。

另一方面，Clang 还认为默认函数参数初始化发生在初始化表达式的直接上下文中。例如，此代码使用 Clang 编译:

template<class T,class =void>
struct constructible:std::false_type{};

template<class T>
struct constructible<T,std::void_t<decltype(T{std::declval<int>()})>>:std::true_type{};

int main()
{
  static_assert(!constructible<my_container<int, my_not_default_constructible_allocator<int>>, int>::value);

  return 0;
}

因此我们可以安全地假设这是一个 Clang 错误。