How to reassign a time_point from clock::now()?

Question

Given a std::chrono::sys_seconds time, one might expect to reassign time from its clock with something like:

time = decltype(time)::clock::now();

However, this would likely fail, because decltype(time)::clock::duration has nothing to do with decltype(time). Instead it is a predefined unit(likely finer than seconds), so you would have to manually cast it to a coarser unit.

Which means you would need to write something like:

time = std::chrono::time_point_cast<decltype(time)::duration>(decltype(time)::clock::now());

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So my question is if there is a shorter syntax to do this?

Answer 1

an obvious solution is just write your own function

template<typename Clock, typename Duration>
void SetNow(std::chrono::time_point<Clock,Duration>& time){
    time = std::chrono::time_point_cast<Duration>(Clock::now());
}

// use
void foo(){
    auto time = std::chrono::system_clock::now();
    SetNow(time);
}

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you can also do some fancy overload

struct Now_t{
    template<typename Clock,typename Duration>
    operator std::chrono::time_point<Clock,Duration>()const{
        return std::chrono::time_point_cast<Duration>(Clock::now());
    }
    consteval Now_t operator()()const{return {};} // enable time = Now();
} Now;

// use
void foo() {
    auto time = std::chrono::system_clock::now();
    time = Now;
    time = Now();
}