Terse conversion from double seconds to std::chrono::steady_clock::duration? - c++

Question

I can do this:
double period_in_seconds = 3.4;
auto as_duration = std::chrono::duration_cast<std::chrono::steady_clock::duration>(std::chrono::duration<double>(period_in_seconds));
But I'm sure you'll agree that is quite ridiculous. Is there a terser way?
Edit: Beside the obvious using namespace std::chrono.

Answer 1

There is good reason to be fanatic about discouraging use of using namespaces at file scope, especially in headers. However at function scope, especially with the <chrono> lib, using namespace std::chrono is your friend.
Just this step gets you from:
double period_in_seconds = 3.4;
auto as_duration = std::chrono::duration_cast<std::chrono::steady_clock::duration>(std::chrono::duration<double>(period_in_seconds));
to:
double period_in_seconds = 3.4;
using namespace std::chrono;
auto as_duration = duration_cast<steady_clock::duration>(duration<double>(period_in_seconds));
I recommend the use of round when casting from a floating point duration to an integral duration. Not only will this give the "expected" answer more often, but it is slightly less verbose than duration_cast. std::chrono::round is new with C++17. But you can grab it from various places like here or here and start using it today. Also if you use {} for constructors it helps distinguish object construction from function calls, making the code a little easier to read:
double period_in_seconds = 3.4;
using namespace std::chrono;
auto as_duration = round<steady_clock::duration>(duration<double>{period_in_seconds});
Finally, don't hesitate to create a custom duration, even if in a local scope to improve readability:
double period_in_seconds = 3.4;
using namespace std::chrono;
using dsec = duration<double>;
auto as_duration = round<steady_clock::duration>(dsec{period_in_seconds});
Edit: And if for some reason (say a misplaced style guide) you can't using a function-local using namespace std::chrono, then you can make up for that with more specific type aliases and using declarations:
double period_in_seconds = 3.4;
using dsec = std::chrono::duration<double>;
using duration = std::chrono::steady_clock::duration;
using std::chrono::round;
auto as_duration = round<duration>(dsec{period_in_seconds});
If this code is intended to be at file scope and you don't want these usings at file scope (which is perfectly understandable), just create a function to encapsulate them:
constexpr
std::chrono::steady_clock::duration
get_period(double period_in_seconds)
{
using dsec = std::chrono::duration<double>;
using duration = std::chrono::steady_clock::duration;
using std::chrono::round;
return round<duration>(dsec{period_in_seconds});
}
auto constexpr as_duration = get_period(3.4);
Above I've even demonstrated that this particular example can all be done at compile time. clang -O3 -S reduces the above down to the following assembly:
.globl _as_duration ## #as_duration
.p2align 3
_as_duration:
.quad 3400000000 ## 0xcaa7e200