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Maria Iscariot
Maria Iscariot

Reputation: 21

C++ chrono::system_clock::now() vs time(0)

is there any significant difference between the chrono::system_clock::now() and time(0) functions, and their return value?

When should I use chrono::system_clock::now() instead of time(0), and vice versa ?

Upvotes: 2

Views: 1699

Answers (1)

Howard Hinnant
Howard Hinnant

Reputation: 218700

system_clock::now() vs time(0)

  • system_clock::now() and time(0) measure the same thing: Unix Time. This is unspecified by time(0), but true in practice. It is also unspecified by system_clock::now() in C++11/14/17, but true in practice. In C++20 it is specified by system_clock::now().

  • The precision (seconds, microseconds, whatever) reported by system_clock::now() and time(0) is unspecified. For system_clock, the precision can be discovered by your application, either at compile time or run time. For time(0), the precision must be read by a human from the documentation. The typical precision for time(0) is seconds. For LLVM on Apple OS's, the precision of sytem_clock::now() is microseconds. On gcc/Linux, nanoseconds. And on Windows 1/10 of a microsecond. Because of the design of <chrono> it is easy to write portable code that does not change with the underlying precision.

  • The range of time(0) is unspecified, and on 32 bit machines typically about +/-68 years centered on 1970 (2038 is coming up fast...). On 64 bit machines the range of time(0) is +/-292 billion years. The range of system_clock::now() is discoverable at compile-time or run-time and is never less than +/-292 years (centered on 1970). The finer the precision, the smaller the range.

  • The return type of time(0) is typically a signed integral type, which runs the risk of being confused with all of the other integral types in your application (no type safety). The return type of system_clock::now() is a class type that has the semantics of a time point. It can not be confused with integral or floating point types. It is a distinct type from a time duration. Misuses of this time point in illogical ways are caught at compile time (strong type safety).

  • time(0) interoperates with the rest of the date/time library in <time.h>.

  • sytem_clock::now() interoperates with the rest of the date/time library in <chrono>. This includes conversions to and from time_t returned by time(0). In C++11/14/17, calendrical and time zone functionality does not exist, but can be added on by free, open-source libraries.1 In C++20, full calendrical and time zone support exists within <chrono>.

1 E.g.: Howard Hinnant's date/time library.

Upvotes: 5

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