[英]Why does std::chrono::time_point not behave as expected?
#include <chrono>
int main()
{
using clock = std::chrono::system_clock;
using time_point = std::chrono::time_point<clock>;
auto tp_now = clock::now();
auto tp_min = time_point::min();
bool b1 = tp_now > tp_min;
bool b2 = (tp_now - tp_min) > std::chrono::seconds{ 0 };
cout << boolalpha << b1 << endl << b2 << endl;
}
The expected output is: 预期的输出是:
true
真正
true
真正
But the actual output is: 但是实际输出是:
true
真正
false
假
Why does std::chrono::time_point
not behave as expected? 为什么
std::chrono::time_point
行为不符合预期?
With: 带有:
using clock = std::chrono::system_clock;
using time_point = std::chrono::time_point<clock>;
time_point
is implemented as if it stores a value of type Duration indicating the time interval from the start of the Clock's epoch. 实现
time_point
就好像它存储了一个类型为Duration的值,该值指示从时钟纪元开始的时间间隔。 (See std::chrono::time_point
) (请参阅
std::chrono::time_point
)
The duration
member type of clock
(and of time_point
) is capable of representing negative durations. clock
(和time_point
)的duration
成员类型能够表示负的持续时间。
Thus duration
in your implementation may be implemented with a back-end signed integer, (it can be implemented with unsigned integer but with a complicated comparison). 因此,实现的
duration
可以使用后端有符号整数来实现(可以使用无符号整数来实现,但是需要进行复杂的比较)。
In that particular implementation, 在该特定实施中,
time_point::min();
time_point t(clock::duration::min());
time_point t(clock::duration(std::numeric_limits<Rep>::lowest()));
and tp_now
is greater than zero
, thus when you subtract them, you get an integer overflow because the result is larger than std::numeric_limits<Rep>::max()
. 并且
tp_now
大于zero
,因此,当它们相减时,会得到整数溢出,因为结果大于std::numeric_limits<Rep>::max()
。 In implementation with signed back-end, it's undefined behavior, in implementation with unsigned back-end, I don't know about it, but I guess its special comparison will make its false
. 在带签名后端的实现中,它是未定义的行为,在带签名后端的实现中,我对此一无所知,但是我猜想它的特殊比较将使其为
false
。
In this example , tp_min
is -9223372036854775808
ticks from its epoch, that number is the same with std::numeric_limits<duration::rep>::lowest()
在此示例中 ,
tp_min
从其纪元tp_min
是-9223372036854775808
滴答,该数字与std::numeric_limits<duration::rep>::lowest()
TL;DR; TL; DR; It's integer overflow.
这是整数溢出。 Don't use
不要使用
(tp1 - tp2) > std::chrono::duration<whatever_rep>::zero
Instead, use 相反,使用
tp1 > tp2
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