c++ - Get time since epoch in milliseconds, preferably using C++11 chrono

Question

All I want is to get the time since epoch in milliseconds and store it in an unsigned long.

I found this related question. But honestly, this can't be the easiest way to perform such a simple task, is it? I am hoping for something much simpler, but can't find anything in the std::chrono reference. Any advice is most welcome. I don't necessarily have to use std::chrono, but I want it to be platform independent.

Answer 1

unsigned long milliseconds_since_epoch =
    std::chrono::system_clock::now().time_since_epoch() / 
    std::chrono::milliseconds(1);

although, especially since you want platform independence, it might be better to replace unsigned long with a type that's more likely to be large enough:

• (unsigned) long long
• std::(u)int64_t
• std::chrono::milliseconds::rep
• auto

To me, this clearly states both that you're risking loss of precision (by analogy with integer division) and that you're leaving the safety of the type system (by dividing by a typed time to give a unitless number). However, as demonstrated in the comments, some people would say that any attempt to move away from type-safety should be accompanied by a deliberate attempt to make the code look dangerous. If you need to deal with people who hold that belief, it might be simpler to use duration_cast rather than enter into an argument about irrelevant stylistic choices:

unsigned long milliseconds_since_epoch = 
    std::chrono::duration_cast<std::chrono::milliseconds>
        (std::chrono::system_clock::now().time_since_epoch()).count();