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### 2243. `istream::putback` problem

Section: 31.7.5.4 [istream.unformatted] Status: C++20 Submitter: Juan Soulie Opened: 2013-03-01 Last modified: 2021-02-25

Priority: 3

View all other issues in [istream.unformatted].

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Discussion:

In 31.7.5.4 [istream.unformatted] / 34, when describing `putback`, it says that "`rdbuf->sputbackc()`" is called. The problem are not the obvious typos in the expression, but the fact that it may lead to different interpretations, since nowhere is specified what the required argument to `sputbackc` is.

It can be guessed to be "`rdbuf()->sputbackc(c)`", but "`rdbuf()->sputbackc(char_type())`" or just anything would be as conforming (or non-conforming) as the first guess.

[2017-12-12, Jonathan comments and provides wording]

Fix the bogus expression, and change `sputbackc()` to just `sputbackc` since we're talking about the function, not an expression `sputbackc()` (which isn't a valid expression any more than `rdbuf->sputbackc()` is). Make the corresponding change to the equivalent wording in p36 too.

[ 2017-12-14 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4713.

1. Change 31.7.5.4 [istream.unformatted] as shown:

   >     basic_istream<charT, traits>& putback(char_type c);

   > > -34- Effects: Behaves as an unformatted input function (as described above), except that the function first clears `eofbit`. After constructing a sentry object, if `!good()` calls `setstate(failbit)` which may throw an exception, and return. If `rdbuf()` is not null, calls `rdbuf()->sputbackc(c)`. If `rdbuf()` is null, or if `sputbackc()` returns `traits::eof()`, calls `setstate(badbit)` (which may throw `ios_base::failure` (31.5.4.4 [iostate.flags])). [Note: This function extracts no characters, so the value returned by the next call to `gcount()` is `0`. — end note]

   > > -35- Returns: `*this`.

   >     basic_istream<charT, traits>& unget();

   > > -36- Effects: Behaves as an unformatted input function (as described above), except that the function first clears `eofbit`. After constructing a sentry object, if `!good()` calls `setstate(failbit)` which may throw an exception, and return. If `rdbuf()` is not null, calls `rdbuf()->sungetc()`. If `rdbuf()` is null, or if `sungetc()` returns `traits::eof()`, calls `setstate(badbit)` (which may throw `ios_base::failure` (31.5.4.4 [iostate.flags])). [Note: This function extracts no characters, so the value returned by the next call to `gcount()` is `0`. — end note]

   > > -37- Returns: `*this`.