CODEWITHSUNDEEP
c++11c++-chrono
waas1919
waas1919

Reputation: 2605

C++11 Chrono - How to cast 'unsigned int' to a time_point<system_clock>?

I have a function that has this signature:

void checkTime (const std::chrono::time_point<std::chrono::system_clock> &time)
{
   //do stuff...
}

I need to call the above function like this:

void wait_some_time (unsigned int ms)
{
    //do stuff...
    checkTime(ms); //ERROR: How can I cast unsigned int to a time_point<system_clock> as now() + some milliseconds?
    //do more stuff...
}

I want to use like this: 

wait_some_time(200); //wait now + 200ms

Question:

How can I cast 'unsigned int' to a const std::chrono::time_point that has the milliseconds value ?

Thanks!

Upvotes: 3

Views: 11388

Answers (3)

vukung
vukung

Reputation: 1884

You can construct a time_pont with a duration, and you can construct a duration with unsigned int, so

using TimePoint = std::chrono::time_point<std::chrono::system_clock>; 
using Duration = std::chrono::duration<unsigned int, std::milli>;
checkTime(TimePoint(Duration(ms)));

... although I don't really see what you want to achieve :)

EDIT: If you want now + ms, you can write

std::chrono::system_clock::now() + Duration(ms)

Upvotes: 3

Jonathan Wakely
Jonathan Wakely

Reputation: 171313

How can I cast 'unsigned int' to a const std::chrono::time_point that has the milliseconds value ?

A time_point is a point in time, represented as an offset to some epoch (the "zero" value for the time_point). For system_clock the epoch is 00:00:00 Jan 1 1970.

Your unsigned int is just an offset, it can't be converted directly to a time_point because it has no epoch information associated with it.

So to answer the question "how do you convert an unsigned int to a time_point?" you need to know what the unsigned int represents. The number of seconds since the epoch started? The number of hours since you last called the function? A number of minutes from now?

If what it's meant to mean is "now + N milliseconds" then N corresponds to a duration, measured in units of milliseconds. You can convert it to that easily with std::chrono::milliseconds(ms) (where the type milliseconds is a typedef for something like std::chrono::duration<long long, std::milli> i.e. a duration represented as a signed integer type in units of 1000th of a second).

Then to get the time_point corresponding to "now + N milliseconds" you just add that duration to a time_point value for "now" obtained from the relevant clock:

    std::chrono::system_clock::now() + std::chrono::milliseconds(ms);

Upvotes: 5

hellow
hellow

Reputation: 13440

If you are using C++14 you can simplify it by using this

auto later = std::chrono::steady_clock::now() + 500ms;

But even without 14 I would alter your function definition to:

void wait_some_time (std::chrono::milliseconds ms);

and then just add the milliseconds to your steady_clock. If you really want to support an integer, you could implement the operator by yourself (see cppreference for the original source)

constexpr std::chrono::milliseconds operator ""ms(unsigned long long ms)
{
    return chrono::milliseconds(ms);
}

Upvotes: 3

Related Questions