binary '>=': 'std::chrono::system_clock::time_point' - c++

Question

I am not sure how to get ride of this compiler error:
error C2676: binary '>=': 'std::chrono::system_clock::time_point'
#include <ctime>
#include <chrono>
int main()
{
std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
std::time_t now_c = std::chrono::system_clock::to_time_t(now - std::chrono::hours(24));
if (std::chrono::system_clock::now() >= now_c)
{
}
}
Here is what the compiler outputs:
1>------ Build started: Project: test, Configuration: Debug x64 ------
1> Source.cpp
1>d:\dev\cpptests\test\test\source.cpp(25): error C2784: 'bool std::operator >=(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)': could not deduce template argument for 'const std::pair<_Ty1,_Ty2> &' from 'std::chrono::system_clock::time_point'
1> c:\program files (x86)\microsoft visual studio 14.0\vc\include\utility(311): note: see declaration of 'std::operator >='
1>d:\dev\cpptests\test\test\source.cpp(25): error C2784: 'bool std::chrono::operator >=(const std::chrono::duration<_Rep,_Period> &,const std::chrono::duration<_Rep2,_Period2> &)': could not deduce template argument for 'const std::chrono::duration<_Rep,_Period> &' from 'std::chrono::system_clock::time_point'
1> c:\program files (x86)\microsoft visual studio 14.0\vc\include\chrono(538): note: see declaration of 'std::chrono::operator >='
1>d:\dev\cpptests\test\test\source.cpp(25): error C2784: 'bool std::chrono::operator >=(const std::chrono::time_point<_Clock,_Duration> &,const std::chrono::time_point<_Clock,_Duration2> &)': could not deduce template argument for 'const std::chrono::time_point<_Clock,_Duration2> &' from 'time_t'
1> c:\program files (x86)\microsoft visual studio 14.0\vc\include\chrono(905): note: see declaration of 'std::chrono::operator >='
1>d:\dev\cpptests\test\test\source.cpp(25): error C2676: binary '>=': 'std::chrono::system_clock::time_point' does not define this operator or a conversion to a type acceptable to the predefined operator
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

I recommend you to use type deduction with auto for this kind of case, as it makes the code much clearer. Also, as said in the comments above, the std::chrono facilities are not directly compatible with c-style time_t. I would recommend to keep using just std::chrono since it is more type-safe than it's counterpart.
#include <ctime>
#include <chrono>
#include <iostream>
int main()
{
auto now = std::chrono::system_clock::now();
auto now_c = now - std::chrono::hours(24);
if (std::chrono::system_clock::now() >= now_c)
{
std::cout << "it works!" << std::endl;
}
return 0;
}

Answer 2

You are try to compare a C++ time_point with a C time! And there is no operator >= to compare. Then you try to compare nanosecond with second
The time_point has a function named time_since_epoch and you can use it.
Using auto can help solve the problem but not understanding what happens and what is under the hood!
So you simply can compare(not good):
if ( now.time_since_epoch().count() >= now_c)
And the better code is:
std::chrono::duration_cast< std::chrono::seconds>(now.time_since_epoch()).count()
Because time_t is per second
if ( now.time_since_epoch().count() >= now_c){
std::cout << now.time_since_epoch().count() << '\n';
std::cout << std::chrono::duration_cast< std::chrono::seconds>(now.time_since_epoch()).count() << '\n';
std::cout << now_c << '\n';
}
the output:
1487879248873636085
1487879248
1487792848