boostorg/locale

[Q] What does `time_zone::global` do exactly?

Flamefire opened this issue · 4 comments

Flamefire commented

While trying to fix/improve unittests I came across the date_time class and the time_zone::global() function.

I can't really figure out what they are supposed to do.

@artyom-beilis Can you explain what the observable effect of the time_zone::global call below should be?

                time_zone::global("GMT");
                date_time tp_1;
                time_zone::global("GMT+01:00");
                date_time tp_2;

I see the default ctor uses the global TZ so I would expect that e.g. tp_1 != tp_2 but that doesn't seem to be the case. In fact I fail to find any influence of the global TZ on anything observable of the date_time instances.

artyom-beilis commented

time_zone defined how to manipulate with different period values. Comparison operators relate to time as absolute time point - Unix time. See: https://www.boost.org/doc/libs/1_79_0/libs/locale/doc/html/classboost_1_1locale_1_1date__time.html#a76c5b0bbc01bd561c25cb83f26e5f535

So ignoring chances that clock second changed between two date_time constructors the universal time point is same;

#include <boost/locale.hpp>

int main()
{
    using namespace boost::locale;
    generator gen;
    std::locale::global(gen("en_US.UTF-8"));
    time_zone::global("GMT");
    date_time tp_1;
    time_zone::global("GMT+01:00");
    date_time tp_2;
    std::cout << tp_1.get(period::hour()) <<" " << tp_2.get(period::hour()) << " same time=" << (tp_1 == tp_2)<< std::endl;
}

Would print something like

3 4 same time = 1

Because they refer to same "universal time"

Flamefire commented

Thanks, added a test for that in #112: ee67d09

artyom-beilis commented

TEST(tp_gmt == tp_gmt1);

Actually there is a small chance of failing of tp_gmt and tp_gmt1 are created in different seconds. Basically if the clock accuracy is around 10um than once in 100,000 cases it will fail