How does duration_cast round

问题

If I convert to a coarser unit of time (say std::chrono::minutes to std::chrono::hours) how will duration_cast round? For example, what value will std::chrono::minutes(91) become if converted to std::chrono::hours? 2h, 1h?

回答1:

duration_cast always rounds towards zero. I.e. positive values round down and negative values round up.

For other rounding options see:

http://howardhinnant.github.io/duration_io/chrono_util.html

floor, ceil, and round are currently in the draft C++ 1z (hopefully C++17) draft working paper. In the meantime feel free to use the code at chrono_util.html, and please let me know if you have any problems with it.

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C++ 17 update

• std::chrono::floor
• std::chrono::ceil
• std::chrono::round

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std::chrono::floor<std::chrono::seconds>(1400ms)  ==  1s
std::chrono::floor<std::chrono::seconds>(1500ms)  ==  1s
std::chrono::floor<std::chrono::seconds>(1600ms)  ==  1s
std::chrono::floor<std::chrono::seconds>(-1400ms) == -2s
std::chrono::floor<std::chrono::seconds>(-1500ms) == -2s
std::chrono::floor<std::chrono::seconds>(-1600ms) == -2s

来源：https://stackoverflow.com/questions/36122428/how-does-duration-cast-round