2011. Unexpected output required of strings

Section: 24.3.3.9 [string.io] Status: C++14 Submitter: James Kanze Opened: 2010-07-23 Last modified: 2016-02-10

Priority: Not Prioritized

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View all issues with C++14 status.

Discussion:

What should the following code output?

#include <string>
#include <iostream>
#include <iomanip>

int main() 
{ 
   std::string test("0X1Y2Z"); 
   std::cout.fill('*'); 
   std::cout.setf(std::ios::internal, std::ios::adjustfield); 
   std::cout << std::setw(8) << test << std::endl; 
} 

I would expect "**0X1Y2Z", and this is what the compilers I have access to (VC++, g++ and Sun CC) do. But according to the standard, it should be "0X**1Y2Z":

24.3.3.9 [string.io]/5:

template<class charT, class traits, class Allocator>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);

Effects: Behaves as a formatted output function (30.7.5.2.1 [ostream.formatted.reqmts]). After constructing a sentry object, if this object returns true when converted to a value of type bool, determines padding as described in 25.4.2.2.2 [facet.num.put.virtuals], then inserts the resulting sequence of characters seq as if by calling os.rdbuf()->sputn(seq, n), where n is the larger of os.width() and str.size(); then calls os.width(0).

25.4.2.2.2 [facet.num.put.virtuals]/5:

[…]

Stage 3: A local variable is initialized as

fmtflags adjustfield= (flags & (ios_base::adjustfield));

The location of any padding is determined according to Table 88.

If str.width() is nonzero and the number of charT's in the sequence after stage 2 is less than str.width(), then enough fill characters are added to the sequence at the position indicated for padding to bring the length of the sequence to str.width(). str.width(0) is called.

Table 88 — Fill padding
State Location
adjustfield == ios_base::left pad after
adjustfield == ios_base::right pad before
adjustfield == internal and a sign occurs in the representation pad after the sign
adjustfield == internal and representation after stage 1 began with 0x or 0X pad after x or X
otherwise pad before

Although it's not 100% clear what "the sequence after stage 2" should mean here, when there is no stage 2, the only reasonable assumption is that it is the contents of the string being output. In the above code, the string being output is "0X1Y2Z", which starts with "0X", so the padding should be inserted "after x or X", and not before the string. I believe that this is a defect in the standard, and not in the three compilers I tried.

[ 2010 Batavia (post meeting session) ]

Consensus that all known implementations are consistent, and disagree with the standard. Preference is to fix the standard before implementations start trying to conform to the current spec, as the current implementations have the preferred form. Howard volunteered to drught for Madrid, move to Open.

[2011-03-24 Madrid meeting]

Daniel Krügler volunteered to provide wording, interacting with Dietmar and Bill.

[2011-06-24 Daniel comments and provides wording]

The same problem applies to the output provided by const char* and similar character sequences as of 30.7.5.2.4 [ostream.inserters.character] p. 5. and even for single character output (!) as described in 30.7.5.2.4 [ostream.inserters.character] p. 1, just consider the character value '-' where '-' is the sign character. In this case Table 91 — "Fill padding" requires to pad after the sign, i.e. the output for the program

#include <iostream>
#include <iomanip>

int main() 
{ 
   char c = '-'; 
   std::cout.fill('*'); 
   std::cout.setf(std::ios::internal, std::ios::adjustfield); 
   std::cout << std::setw(2) << c << std::endl; 
} 

According to the current wording this program should output "-*", but all tested implementations output "*-" instead.

I suggest to replace the reference to 25.4.2.2.2 [facet.num.put.virtuals] in all three places. It is not very complicated to describe the padding rules for simple character sequences "inline". A similar approach is used as for the money_put functions.

[ 2011 Bloomington ]

Move to Review, the resolution seems correct but it would be nice if some factoring of the common words were proposed.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

While better factoring of the common words is desirable, it is also editorial and should not hold up the progress of this issue. As the edits impact two distinct clauses, it is not entirely clear what a better factoring should look like.

[2012, Portland: applied to WP]

Proposed resolution:

The new wording refers to the FDIS numbering.

  1. Change 24.3.3.9 [string.io]/5 as indicated:

    template<class charT, class traits, class Allocator>
      basic_ostream<charT, traits>&
        operator<<(basic_ostream<charT, traits>& os,
                   const basic_string<charT,traits,Allocator>& str);
    

    -5- Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]). After constructing a sentry object, if this object returns true when converted to a value of type bool, determines padding as described in [facet.num.put.virtuals],follows: A charT character sequence is produced, initially consisting of the elements defined by the range [str.begin(), str.end()). If str.size() is less than os.width(), then enough copies of os.fill() are added to this sequence as necessary to pad to a width of os.width() characters. If (os.flags() & ios_base::adjustfield) == ios_base::left is true, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. Tthen inserts the resulting sequence of characters seq as if by calling os.rdbuf()->sputn(seq, n), where n is the larger of os.width() and str.size(); then calls os.width(0).

  2. Change 30.7.5.2.4 [ostream.inserters.character]/1 as indicated (An additional editorial fix is suggested for the first prototype declaration):

    template<class charT, class traits>
      basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                              charT c});
    template<class charT, class traits>
      basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                              char c);
    // specialization
    template<class traits>
      basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             char c);
    // signed and unsigned
    template<class traits>
      basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             signed char c);
    template<class traits>
      basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             unsigned char c);
    

    -1- Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of out. After a sentry object is constructed it inserts characters. In case c has type char and the character type of the stream is not char, then the character to be inserted is out.widen(c); otherwise the character is c. Padding is determined as described in [facet.num.put.virtuals]follows: A character sequence is produced, initially consisting of the insertion character. If out.width() is greater than one, then enough copies of out.fill() are added to this sequence as necessary to pad to a width of out.width() characters. If (out.flags() & ios_base::adjustfield) == ios_base::left is true, the fill characters are placed after the insertion character; otherwise, they are placed before the insertion character. width(0) is called. The insertion character and any required padding are inserted into out; then calls os.width(0).

  3. Change 30.7.5.2.4 [ostream.inserters.character]/5 as indicated:

    template<class charT, class traits>
      basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                              const charT* s);
    template<class charT, class traits>
      basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                              const char* s);
    template<class traits>
      basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             const char* s);
    template<class traits>
      basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             const signed char* s);
    template<class traits>
      basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                             const unsigned char* s);
    

    […]

    -5- Padding is determined as described in [facet.num.put.virtuals]. The n characters starting at s are widened using out.widen ([basic.ios.members])follows: A character sequence is produced, initially consisting of the elements defined by the n characters starting at s widened using out.widen ([basic.ios.members]). If n is less than out.width(), then enough copies of out.fill() are added to this sequence as necessary to pad to a width of out.width() characters. If (out.flags() & ios_base::adjustfield) == ios_base::left is true, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. The widened characters and any required padding are inserted into out. Calls width(0).