Re: [datetime] conversion operators are all wrong

Jeff Schwab <>
Tue, 19 Feb 2008 01:06:58 CST
Alberto Ganesh Barbati wrote:


(this post should be directed to comp.std.c++, but since it's still

referring to the latest public draft, paper N2521, the new section
[datetime] contains specifications for types nanoseconds, microseconds,
etc. All those types (except nanoseconds) have conversion operators to
types with greater resolution (i.e.: hours to minutes, minutes to
microseconds, etc.) Unfortunately all those methods are specified
incorrectly. For example method microseconds::operator nanoseconds() is
specified as:

  Returns: count() * (nanoseconds::ticks_per_second /

while it should be

  Returns: nanoseconds(count() * (nanoseconds::ticks_per_second /

The latter should have the same semantics as the former, assuming
tick_type (which is implementation-defined) is implicitly convertible to
long long. Adding the explicit type conversion implies that tick_type
need not be implicitly convertible to long long. As a simplified
example, given the following approximation to the draft:

     typedef struct tick {
         tick(long) { }
         operator long() { return 1L; }
     } tick_type;

     struct nanoseconds {
         nanoseconds(long) { }

     struct microseconds {
         typedef tick tick_type;
         operator nanoseconds() { return tick_type(1000L); }

The following does not compile:

     int main() {
         microseconds m;
         nanoseconds n = m;

Adding the explicit type conversion would make the code compile. Is the
implementation-defined tick_type required to be implicitly convertible
to long long?

Similarly, for all other conversion operators.

Moreover, method hours::operator minutes() is specified as:

  Returns: count() * (hours::seconds_per_tick * minutes::seconds_per_tick)

given that hours::seconds_per_tick is 3600 and minutes::seconds_per_tick
 is 60, even the computation is wrong!

Nice catch.

The correct specification should be:

  Returns: minutes(count() * (hours::seconds_per_tick /

Same as above re. the explicit type conversion.

Similarly, the specifications of hours::operator seconds() and
minutes::operator seconds() are also wrong, although in those cases the
computation is accidentally correct because seconds::seconds_per_tick == 1.

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