C++ on MacOS: show date and time issue - c++

Question

I bought a MacBook Pro and I've been using MacOS for the past two days. I've been trying to write this C++ code that outputs the date and time using chrono and ctime libraries.
This code was working just fine on my Windows machine and my CentOS7 server. However, on my MacBook Pro it fails to compile.
This is the error message I get when I try to compile with G++:
main.cpp:19:61: error: no viable conversion from 'time_point<std::__1::chrono::steady_clock,
duration<[...], ratio<[...], 1000000000>>>' to 'const
time_point<std::__1::chrono::system_clock, duration<[...], ratio<[...], 1000000>>>'
std::time_t date = std::chrono::system_clock::to_time_t(now);
^~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1340:28: note: candidate constructor
(the implicit copy constructor) not viable: no known conversion from
'std::__1::chrono::time_point<std::__1::chrono::steady_clock, std::__1::chrono::duration<long long,
std::__1::ratio<1, 1000000000> > >' to 'const
std::__1::chrono::time_point<std::__1::chrono::system_clock, std::__1::chrono::duration<long long,
std::__1::ratio<1, 1000000> > > &' for 1st argument
class _LIBCPP_TEMPLATE_VIS time_point
^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1340:28: note: candidate constructor
(the implicit move constructor) not viable: no known conversion from
'std::__1::chrono::time_point<std::__1::chrono::steady_clock, std::__1::chrono::duration<long long,
std::__1::ratio<1, 1000000000> > >' to 'std::__1::chrono::time_point<std::__1::chrono::system_clock,
std::__1::chrono::duration<long long, std::__1::ratio<1, 1000000> > > &&' for 1st argument
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1359:5: note: candidate template
ignored: could not match 'std::__1::chrono::system_clock' against 'std::__1::chrono::steady_clock'
time_point(const time_point<clock, _Duration2>& t,
^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:1566:53: note: passing argument to
parameter '__t' here
static time_t to_time_t (const time_point& __t) _NOEXCEPT;
^
1 error generated.
basavyr#Roberts-MacBook-Pro simpleTest %
Here is the code:
void getTime()
{
auto now = std::chrono::high_resolution_clock::now();
std::time_t date = std::chrono::system_clock::to_time_t(now);
std::cout << std::ctime(&date);
}
int main()
{
getTime();
}
I assume the issue has to do with the fact that MacOS is using CLANG compiler?
Any ideas how can I solve this?
Thank you!

Answer 1

Your code is only likely to work when std::high_resolution_clock is the same type as std::system_clock. There is no guarantee that time points from different clocks are the same or convertible.
For conversion to time_t which only has seconds resolution system_clock will be perfectly adequate:
void getTime()
{
auto now = std::chrono::system_clock::now();
std::time_t date = std::chrono::system_clock::to_time_t(now);
std::cout << std::ctime(&date);
}
int main()
{
getTime();
}