Boost C++ Libraries: Ticket #10103: setprecision(0) does displays too many digits of a float128

status changed

John Maddock — Sat, 07 Jun 2014 15:38:03 GMT

status new → assigned

This is indeed a bug, but I don't know how to fix it without breaking existing code: we've used a precision of 0 to mean "as many digits as you've got", and this is pervasive/consistent throughout the library. I guess we should have used -1 as the "special" value, but it's too late now...

Sat, 07 Jun 2014 17:00:04 GMT

Perhaps define an enum for this special value and advertise that it will change in future versions.

Tue, 15 Jul 2014 03:41:15 GMT

Perhaps boost can fruitfully distinguish the cases

cout << fixed << setprecision(0) << x;

and

cout << scientific << setprecision(0) << x;

with the former printing x as the nearest integer (ties go to even) and the latter retaining its current interpretation of printing "as many digits as you've got". This would certainly satisfy my requirements and, typically, the "as many digits as you've got" mode implies scientific notation.

Wed, 14 Oct 2015 14:40:49 GMT

With C++11 manipulators, I recommend that

cout << scientific << setprecision(0) << x;
cout << hexfloat << setprecision(0) << x;

should print lossless representations of x. While

cout << fixed << setprecision(0) << x;
cout << defaultfloat << setprecision(0) << x;

should mimic whatever the standard library does with doubles.