Difference between boost::chrono::time_point<> and boost::chrono::steady_clock::time_point

Question

What is the difference between boost::chrono::steady_clock::time_point and boost::chrono::time_point<boost::chrono::steady_clock,Duration> , and why can't they be converted between each other?

It seems extremely redundant.

Answer 1

They can be converted between each other if their durations are the same. For example, the following would be fine:

boost::chrono::steady_clock::time_point tp1 = boost::chrono::steady_clock::now();
boost::chrono::time_point<boost::chrono::steady_clock, 
                              boost::chrono::nanoseconds>
tp2 = tp1;

You can also use time_point_cast to cast one time_point to another if their durations are different.

Example

boost::chrono::steady_clock::time_point tp1 = boost::chrono::steady_clock::now();
boost::chrono::time_point<boost::chrono::steady_clock,
                              boost::chrono::milliseconds> 
tp2 = boost::chrono::time_point_cast<boost::chrono::milliseconds>(tp1);

Also, I'm not sure why you think they are redundant. If you look at the docs , steady_clock has the following member:

typedef chrono::time_point<steady_clock> time_point;

So, boost::chrono::steady_clock::time_point is just a typedef for boost::chrono::time_point<steady_clock> .

Answer 2

std::chrono::time_point (based on Boost) is a template that can be used to define a class that can, in turn, be used to define a point in time. std::chrono::steady_clock::time_point is an instantiation of that template (ie, a class) that can be used to define a point in time. So they're two rather different things with the same name. Sigh.

Various instantiations of std::chrono::time_point can, as @JesseGood pointed out, be inter-converted with time_point_cast .