Casting double chrono::seconds into millisecond

September 1, 2023

im have simple function like

void foo(std::chrono::milliseconds ms) {
    std::cout << ms.count() << " milliseconds" << std::endl;
}

and next im calling them by

int main() {
    using namespace std::chrono_literals;

    boo(3s);
    boo(1h);
    boo(100ms);
}

Output is simple:

3000 milliseconds
3600000 milliseconds
100 milliseconds

But, what if im wanna to use this function with:

    boo(3.5s);
    boo(0.5s);
    boo(0.3days);

Then i have compile error.
So, i can write function who receive chrono::duration:

template<class T>
void foo(std::chrono::duration<T> duration) {
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(duration).count() << " milliseconds" << std::endl;
}

and then 3.5s will work, but 1h or 3.5h isnt work.
So, question, can im write universal function which convert any of 1s/1.s/1m/1.5m/1h/1.5h/etc into milliseconds ?
Maybe i can create overloaded operators for chrono::seconds / hours ?
Or just cast, cast, cast ?

Im trying all described below/above

>Solution :

The proper function template definition should take the period as a template parameter too:

template <class Rep, class Period>
void boo(std::chrono::duration<Rep, Period> duration) {
    
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(duration)
                     .count()
              << " milliseconds\n";
}

Or in C++20, don’t call .count() and let it print the unit automatically:

template <class Rep, class Period>
void boo(std::chrono::duration<Rep, Period> duration) {
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(duration)
              << '\n';
}

I have no clue about the days literal though. Never seen it, but the rest of them will work.

int main() {
    using namespace std::chrono_literals;

    boo(3s);
    boo(1h);
    boo(100ms);

    boo(3.5s);
    boo(0.5s);
    boo(std::chrono::days(1) * 0.3); // what I got working
}

Demo