[英]Subclassing std::chrono::duration
I have a piece of code where std::chrono::duration<int64_t, std::milli>
is subclassed, and used to create a std::chrono::time_point
, like so:我有一段代码,其中
std::chrono::duration<int64_t, std::milli>
是子类,并用于创建std::chrono::time_point
,如下所示:
#include <chrono>
class my_duration : public std::chrono::duration<int64_t, std::milli>
{ /* snip */ };
int main()
{
typedef std::chrono::time_point< std::chrono::system_clock, my_duration > my_time_point;
my_time_point t( my_duration{} );
//....
}
This was seemingly compiling and working fine when using GCC < 10. However, when using GCC 10, a static assertion in std::chrono::time_point will fail with:当使用 GCC < 10 时,这似乎编译和工作正常。但是,当使用 GCC 10 时,staticchrono 断言将失败:
/opt/wandbox/gcc-head/include/c++/11.0.0/chrono:764:37: error: static assertion failed: duration must be a specialization of std::chrono::duration
/opt/wandbox/gcc-head/include/c++/11.0.0/chrono:764:37:错误:static 断言失败:duration 必须是 std::chrono::duration 的特化
This can be seen at the following link, which also demonstrates that clang gives the same error: https://wandbox.org/permlink/CQw6fWt4kZ1xYdet这可以在以下链接中看到,这也表明 clang 给出了相同的错误: https://wandbox.org/permlink/CQw6fWt4kZ1xYdet
In std::chrono, the failing assert is due to the __is_duration
structs used by time_point
:在 std::chrono 中,失败的断言是由于
__is_duration
使用的time_point
结构:
template<typename _Tp>
struct __is_duration
: std::false_type
{ };
template<typename _Rep, typename _Period>
struct __is_duration<duration<_Rep, _Period>>
: std::true_type
{ };
//... further down, in time_point:
static_assert(__is_duration<_Dur>::value,
"duration must be a specialization of std::chrono::duration");
My understanding of it is that __is_duration<my_duration>
will be an std::false_type
causing the static assert to fail.我的理解是
__is_duration<my_duration>
将是一个std::false_type
导致 static 断言失败。
So, my question is: does this mean it isn't possible to subclass std::chrono::duration and use the derived class with time_point?所以,我的问题是:这是否意味着不可能继承 std::chrono::duration 并将派生的 class 与 time_point 一起使用? Or is there some trick that will allow the duration subclass to pass the static assertion?
或者是否有一些技巧可以让持续时间子类通过 static 断言?
std::time_point
has always required that the duration type be a specialization of std::duration
. std::time_point
始终要求持续时间类型是std::duration
的特化。 If it isn't, then the program is ill-formed (aka: compile error).如果不是,则程序格式错误(又名:编译错误)。 This may just be the first time GCC implemented that requirement.
这可能只是 GCC 第一次实现该要求。
Deriving from duration
doesn't really serve any useful purpose.从
duration
派生并没有真正起到任何有用的目的。
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