Jar*_*ock 9 c++ sfinae type-traits
我的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,只有在Allocator默认构造时才启用此构造函数.
我想测试一下std::is_constructible,这个构造函数是否可以与一个不是默认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,当我申请时std::is_constructible,我得到一个编译时错误,而不是false,这是我所期望的:
#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>
编译器细节:
$ clang --version
clang version 4.0.1-6 (tags/RELEASE_401/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
而不是SFINAEing感兴趣的构造函数,实现std::is_constructible导致错误.
是不是my_container构造函数的实现不正确？
默认参数初始化似乎是在[meta.unary_prop]/8my_container初始化的直接上下文中：
\n\n\n\n
is_\xc2\xadconstructible<T, Args...>当且仅当以下变量定义对于某些发明变量 t 来说是格式良好的时，才应满足模板特化的谓词条件：
T t(declval<Args>()...);\n\n\n\n[\xe2\x80\x89注意：这些标记永远不会被解释为函数声明。\n \xe2\x80\x94\xe2\x80\x89end note\n \xe2\x80\x89]\n 访问检查的执行就像在与 T 和任何 Args 无关的上下文中。\n 仅考虑变量初始化的直接上下文的有效性。\n [\xe2\x80\x89注意：初始化的评估可能会导致副作用，例如类模板特化和函数模板特化的实例化、隐式定义函数的生成等等。\n 此类副作用不在 \xe2\x80\x9ci 直接上下文\xe2\x80\x9d 中，并且可能导致程序格式错误。\n \xe2\x80\x94\xe2\x80\x89end note\n \xe2\x80\x89]
\n
根据 [expr.call]/7：
\n\n\n\n\n每个参数的初始化和销毁都发生在调用函数的上下文中。
\n
因此可以推断默认参数初始化发生在“直接上下文”中。我的观点是，这并不是很清楚，术语“直接上下文”没有正式的定义。
\n\n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：
\n\ntemplate<class T,class =void>\nstruct constructible:std::false_type{};\n\ntemplate<class T>\nstruct constructible<T,std::void_t<decltype(T{std::declval<int>()})>>:std::true_type{};\n\nint main()\n{\n static_assert(!constructible<my_container<int, my_not_default_constructible_allocator<int>>, int>::value);\n\n return 0;\n}\n所以我们可以放心地假设这是一个 Clang bug。
\n