我的容器的构造函数之一default-将分配器构造为默认参数值:
template<class T, class Allocator>
struct my_container
{
my_container(int n, Allocator alloc = Allocator()) {}
};
据推测，仅当可以默认构造
Allocator时，才启用此构造函数。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，我得到了一个编译时错误，而不是false，这是我期望的:#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
代替sfinaeing感兴趣的构造函数，
std::is_constructible的实现会导致错误。my_container的构造函数的实现不正确吗？ 最佳答案
默认参数初始化似乎在my_container，[meta.unary_prop]/8初始化的直接上下文中:
T t(declval<Args>()...);
根据[expr.call]/7:
因此，可以推断出默认参数初始化发生在“立即上下文”中。我的看法是，这还不是很清楚，术语“即时上下文”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，我们可以放心地假设这是Clang错误。
关于c++ - 为什么不使用std::is_constructible编译？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/54099360/