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2117. ios_base manipulators should have showgrouping/noshowgrouping

Section: 30.4.3.3.3 [facet.num.put.virtuals], 31.5.2.2.2 [ios.fmtflags], 31.5.5.1 [fmtflags.manip] Status: Open Submitter: Benjamin Kosnik Opened: 2011-12-15 Last modified: 2023-02-07

Priority: 3

View other active issues in [facet.num.put.virtuals].

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Discussion:

Iostreams should include a manipulator to toggle grouping on/off for locales that support grouped digits. This has come up repeatedly and been deferred. See LWG 826 for the previous attempt.

If one is using a locale that supports grouped digits, then output will always include the generated grouping characters. However, very plausible scenarios exist where one might want to output the number, un-grouped. This is similar to existing manipulators that toggle on/off the decimal point, numeric base, or positive sign.

See some user commentary here.

[21012, Kona]

Move to Open.

This is a feature request.

Walter is slightly uncomfortable with processing feature requests through the issues lists.

Alisdair says this is far from the first feature request that has come in from the issues list.

STL: The fact that you can turn off grouping on hex output is compelling.

Marshall: if we add this flag, we'll need to update tables 87-91 as well.

STL: If it has been implemented somewhere, and it works, we'd be glad to add it.

Howard: We need to say what the default is.

Alisdair sumarizes:

(1) We want clear wording that says what the effect is of turning the flag off;

(2) what the default values are, and

(3) how this fits into tables 87-90. (and 128)

[Issaquah 2014-02-10-12: Move to LEWG]

Since this issue was filed, we have grown a new working group that is better placed to handle feature requests.

We will track such issues with an LEWG status until we get feedback from the Library Evolution Working Group.

[Issaquah 2014-02-12: LEWG discussion]

Think about the ABI break for adding a flag. But this could be mitigated by putting the data into an iword instead of a flag.

This needs to change Stage 2 in [facet.num.put.virtuals].

Previous resolution, which needs the above corrections:

This wording is relative to the FDIS.

1. Insert in 30.4.3.3.3 [facet.num.put.virtuals] paragraph 5:

   Stage 1: The first action of stage 1 is to determine a conversion specifier. The tables that describe this determination use the following local variables

   fmtflags flags = str.flags() ;
   fmtflags basefield = (flags & (ios_base::basefield));
   fmtflags uppercase = (flags & (ios_base::uppercase));
   fmtflags floatfield = (flags & (ios_base::floatfield));
   fmtflags showpos = (flags & (ios_base::showpos));
   fmtflags showbase = (flags & (ios_base::showbase));
   fmtflags showgrouping = (flags & (ios_base::showgrouping));
   

2. Change header <ios> synopsis, [iostreams.base.overview] as indicated:

   #include <iosfwd>
   
   namespace std {
     […]
     // 27.5.6, manipulators:
     […]
     ios_base& showpoint     (ios_base& str);
     ios_base& noshowpoint   (ios_base& str);
     ios_base& showgrouping  (ios_base& str);
     ios_base& noshowgrouping(ios_base& str);
     ios_base& showpos       (ios_base& str);
     ios_base& noshowpos     (ios_base& str);
     […]
   }
   

3. Change class ios_base synopsis, 31.5.2 [ios.base] as indicated:

   namespace std {
     class ios_base {
     public:
     class failure;
       // 27.5.3.1.2 fmtflags
       typedef T1 fmtflags;
       […]
       static constexpr fmtflags showpoint = unspecified ;
       static constexpr fmtflags showgrouping = unspecified ;
       static constexpr fmtflags showpos = unspecified ;
       […]
     };
   }
   

4. Add a new entry to Table 122 — "fmtflags effects" as indicated:

   Table 122 — fmtflags effects

   Element	Effect(s) if set
   […]
   showpoint	generates a decimal-point character unconditionally in generated floatingpoint output
   showgrouping	generates grouping characters unconditionally in generated output
   […]

5. After [ios::fmtflags] p12 insert the following:

   ios_base& showgrouping(ios_base& str);
   

   -?- Effects: Calls str.setf(ios_base::showgrouping).

   -?- Returns: str.

   ios_base& noshowgrouping(ios_base& str);
   

   -?- Effects: Calls str.unsetf(ios_base::showgrouping).

   -?- Returns: str.

Proposed resolution: