Issue 2714: complex stream extraction underspecified

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2714. `complex` stream extraction underspecified

Section: 28.4.6 [complex.ops] Status: New Submitter: Tim Song Opened: 2016-05-23 Last modified: 2018-10-16

Priority: 3

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Discussion:

The specification of operator>>(istream&, complex<T>&) is extremely short on details. It currently reads, in its entirety (28.4.6 [complex.ops]/12-15):

template<class T, class charT, class traits>
basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, complex<T>& x);
Effects: Extracts a complex number x of the form: u, (u), or (u,v), where u is the real part and v is the imaginary part (31.7.5.3 [istream.formatted]).
Requires: The input values shall be convertible to T.
If bad input is encountered, calls is.setstate(ios_base::failbit) (which may throw ios::failure (31.5.4.4 [iostate.flags])).
Returns: is.
Remarks: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions.

It is completely unclear:

which "simpler extractions" are performed,
how the extracted characters are matched to the special characters '(', ')' and ',' (by ==, or by traits::eq),
what is left in the stream on failure. (For example, with "(0, 0]", libstdc++ extracts the ] while libc++ leaves it in the stream.)

Previous resolution [SUPERSEDED]:

Drafting note: the following wording is based on:

Characters are extracted using operator>> and compared using traits::eq.

Mismatched characters are returned to the stream.

This wording is relative to N4582.
Replace 28.4.6 [complex.ops]/12-15 with the following paragraphs:
template<class T, class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, complex<T>& x);
-?- Effects: First, extracts a character from is.

If the character extracted is equal to is.widen('('), extracts an object u of type T from is, then extracts a character from is.

If this character is equal to is.widen(')'), then assigns complex<T>(u) to x.

Otherwise, if this character is equal to is.widen(','), extracts an object v of type T from is, then extracts a character from is. If this character is equal to is.widen(')'), then assigns complex<T>(u, v) to x; otherwise returns the character to is and the extraction fails.

Otherwise, returns the character to is and the extraction fails.

Otherwise, returns the character to is, extracts an object u of type T from is, and assigns complex<T>(u) to x.

In the description above, characters are extracted from is as if by operator>> (31.7.5.3.3 [istream.extractors]), and returned to the stream as if by basic_istream::putback (31.7.5.4 [istream.unformatted]). Character equality is determined using traits::eq. An object t of type T is extracted from is as if by is >> t.
If any extraction operation fails, no further operation is performed and the whole extraction fails.
On failure, calls is.setstate(ios_base::failbit) (which may throw ios::failure (31.5.4.4 [iostate.flags])).
-?- Returns: is.
-?- [Note: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions. — end note]

[2017-12-13 Tim Song adjusts the P/R to avoid relying on putback.]

Proposed resolution:

Drafting note: the following wording assumes that:

Characters are extracted using operator>> and compared using traits::eq.

Mismatched characters are not extracted.

x is assigned a value-initialized complex on failure for consistency with the arithmetic extractors (compare LWG 696).

This wording is relative to N4778.

Replace 28.4.6 [complex.ops]/12-16 with the following paragraphs:
```
template<class T, class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, complex<T>& x);
```
-?- Effects: Let PEEK(is) be a formatted input function (31.7.5.3.1 [istream.formatted.reqmts]) of is that returns the next character that would be extracted from is by operator>>. [Note: The sentry object is constructed and destroyed, but the returned character is not extracted from the stream. — end note]
- If PEEK(is) is not equal to is.widen('('), extracts an object u of type T from is, and assigns complex<T>(u) to x.
- Otherwise, extracts that character from is, then extracts an object u of type T from is, then:
  - If PEEK(is) is equal to is.widen(')'), then extracts that character from is and assigns complex<T>(u) to x.
  - Otherwise, if it is equal to is.widen(','), then extracts that character from is and then extracts an object v of type T from is, then:
    
    If PEEK(is) is equal to is.widen(')'), then extracts that character from is and assigns complex<T>(u, v) to x.
    
    Otherwise, the extraction fails.
  - Otherwise, the extraction fails.
In the description above, characters are extracted from is as if by operator>> (31.7.5.3.3 [istream.extractors]), character equality is determined using traits::eq, and an object t of type T is extracted from is as if by is >> t.
If any extraction operation fails, no further operation is performed and the whole extraction fails.
On failure, assigns complex<T>() to x and calls is.setstate(ios_base::failbit) (which may throw ios::failure (31.5.4.4 [iostate.flags])).
-?- Returns: is.
-?- [Note: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions. — end note]

2714. complex stream extraction underspecified

2714. `complex` stream extraction underspecified