Text formatting with std::fmt about mp-units HOT 2 CLOSED

What units formatting approach is supposed to be?

A few different ones could be considered:

1. Simple one: Put only the quantity (i.e. value from unit.count()) into formatted string.
   I.e.

   std::string s = fmt::format("The speed is {} kmph", 2_kmph);
   // s == "The speed is 2 kmph"
   

   Which is simple in use and appears to be easy in implementation.
   The obvious drawback here is that the information about dimensions is not used in formatting.
   So, some useful information becomes unavailable in formatting string and it becomes possible to make a mistake in formatting string - write wrong dimensions.

2. More complex one: Give ability to refer to both quantity and dimensions from formating string.
   In that case special format specification mini language will be required.
   Probably, something similar to Date and Time formatting could be taken.

   For example, like that:

   std::string s = fmt::format("The speed is {:%q %d}", 2_kmph);
   // s == "The speed is 2 kmph"
   

   Or it could be:

   std::string s = fmt::format("The speed, {0:%d}: {0:%q}", 2_kmph);
   // s == "The speed, kmph: 2"
   

   Where %q refers to quantity, and %d refers to dimensions.

   I think, it might be also possible to make it compliant with standard fmt features. Though the syntax becomes complicated:

   std::string s = fmt::format("The speed is {:%q:+.3f: %d:>6:}", 2_kmph);
   // s == "The speed is +2.000   kmph"
   

   Format specifications are shown here enclosed in columns: :+.3f: and :>6:.

   Terminating character : is necessary to detect the end of quantity/dimensions format specifier.

   The examples above do not pretend to be the only possible option of format string. They are more the ideas/proposals how could format look like. Just to express to power of it.
   The exact format of units formatting string is an open question.
   Another open question here is dimensions localization support. Most likely the library need to be extensible by user to provide dimensions string representations in any language other than English.

3. The most complex one. Option (2) could be extended even more. To provide ability to have units conversions at formatting time.
   It has a few interesting use cases. Though it could be considered later - if option (2) is accepted.

A design note:
As formatting feature might become complicated - ideally if it will be "plug-able" to the units library.
Its implementation might be located in a separate optional header.
i.e.

#include <units/fmt.h>

The rest of library should be able to work without formatting header included.

from mp-units.

Thanks Aliaksander for investigating this subject. As of now, I did not have time to work on this. However, I strongly believe that we should strive to create something similar to http://wg21.link/P1361.

from mp-units.