60. What is a formatted input function?

Section: 30.7.4.2.1 [istream.formatted.reqmts] Status: TC1 Submitter: Matt Austern Opened: 1998-08-03 Last modified: 2016-02-10

Priority: Not Prioritized

View all other issues in [istream.formatted.reqmts].

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Duplicate of: 162, 163, 166

Discussion:

Paragraph 1 of 27.6.1.2.1 contains general requirements for all formatted input functions. Some of the functions defined in section 27.6.1.2 explicitly say that those requirements apply ("Behaves like a formatted input member (as described in 27.6.1.2.1)"), but others don't. The question: is 27.6.1.2.1 supposed to apply to everything in 27.6.1.2, or only to those member functions that explicitly say "behaves like a formatted input member"? Or to put it differently: are we to assume that everything that appears in a section called "Formatted input functions" really is a formatted input function? I assume that 27.6.1.2.1 is intended to apply to the arithmetic extractors (27.6.1.2.2), but I assume that it is not intended to apply to extractors like

    basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));

and

    basic_istream& operator>>(basic_streammbuf*);

There is a similar ambiguity for unformatted input, formatted output, and unformatted output.

Comments from Judy Ward: It seems like the problem is that the basic_istream and basic_ostream operator <<()'s that are used for the manipulators and streambuf* are in the wrong section and should have their own separate section or be modified to make it clear that the "Common requirements" listed in section 27.6.1.2.1 (for basic_istream) and section 27.6.2.5.1 (for basic_ostream) do not apply to them.

Additional comments from Dietmar Kühl: It appears to be somewhat nonsensical to consider the functions defined in [istream::extractors] paragraphs 1 to 5 to be "Formatted input function" but since these functions are defined in a section labeled "Formatted input functions" it is unclear to me whether these operators are considered formatted input functions which have to conform to the "common requirements" from 30.7.4.2.1 [istream.formatted.reqmts]: If this is the case, all manipulators, not just ws, would skip whitespace unless noskipws is set (... but setting noskipws using the manipulator syntax would also skip whitespace :-)

It is not clear which functions are to be considered unformatted input functions. As written, it seems that all functions in 30.7.4.3 [istream.unformatted] are unformatted input functions. However, it does not really make much sense to construct a sentry object for gcount(), sync(), ... Also it is unclear what happens to the gcount() if eg. gcount(), putback(), unget(), or sync() is called: These functions don't extract characters, some of them even "unextract" a character. Should this still be reflected in gcount()? Of course, it could be read as if after a call to gcount() gcount() return 0 (the last unformatted input function, gcount(), didn't extract any character) and after a call to putback() gcount() returns -1 (the last unformatted input function putback() did "extract" back into the stream). Correspondingly for unget(). Is this what is intended? If so, this should be clarified. Otherwise, a corresponding clarification should be used.

Proposed resolution:

In 27.6.1.2.2 [lib.istream.formatted.arithmetic], paragraph 1. Change the beginning of the second sentence from "The conversion occurs" to "These extractors behave as formatted input functions (as described in 27.6.1.2.1). After a sentry object is constructed, the conversion occurs"

In 27.6.1.2.3, [lib.istream::extractors], before paragraph 1. Add an effects clause. "Effects: None. This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 2. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 4. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 12. Change the first two sentences from "If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). Extracts characters from *this..." to "Behaves as a formatted input function (as described in 27.6.1.2.1). If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). After a sentry object is constructed, extracts characters from *this...".

In 27.6.1.3, [lib.istream.unformatted], before paragraph 2. Add an effects clause. "Effects: none. This member function does not behave as an unformatted input function (as described in 27.6.1.3, paragraph 1)."

In 27.6.1.3, [lib.istream.unformatted], paragraph 3. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"

In 27.6.1.3, [lib.istream.unformatted], paragraph 5. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"

In 27.6.1.3, [lib.istream.unformatted], paragraph 7. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 10, because it refers to paragraph 7.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 12. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 15.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 17. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 23.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 24. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

In 27.6.1.3, [lib.istream.unformatted], before paragraph 27. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, reads but does not extract the current input character."

In 27.6.1.3, [lib.istream.unformatted], paragraph 28. Change the first sentence of the Effects clause from "If !good() calls" to Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"

In 27.6.1.3, [lib.istream.unformatted], paragraph 30. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"

In 27.6.1.3, [lib.istream.unformatted], paragraph 32. Change the first sentence of the Effects clause from "If !good() calls..." to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls..." Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"

In 27.6.1.3, [lib.istream.unformatted], paragraph 34. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls". Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"

In 27.6.1.3, [lib.istream.unformatted], paragraph 36. Change the first sentence of the Effects clause from "If !rdbuf() is" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if rdbuf() is"

In 27.6.1.3, [lib.istream.unformatted], before paragraph 37. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount()." Change the first sentence of paragraph 37 from "if fail()" to "after constructing a sentry object, if fail()".

In 27.6.1.3, [lib.istream.unformatted], paragraph 38. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()

In 27.6.1.3, [lib.istream.unformatted], paragraph 40. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()

In 27.6.2.5.2 [lib.ostream.inserters.arithmetic], paragraph 1. Change the beginning of the third sentence from "The formatting conversion" to "These extractors behave as formatted output functions (as described in 27.6.2.5.1). After the sentry object is constructed, the conversion occurs".

In 27.6.2.5.3 [lib.ostream.inserters], before paragraph 1. Add an effects clause: "Effects: None. Does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 2. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 4. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 6. Change the first sentence from "If sb" to "Behaves as a formatted output function (as described in 27.6.2.5.1). After the sentry object is constructed, if sb".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 2. Change the first sentence from "Inserts the character" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, inserts the character".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 5. Change the first sentence from "Obtains characters" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, obtains characters".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 7. Add a new sentence at the end of the paragraph: "Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1)."

Rationale:

See J16/99-0043==WG21/N1219, Proposed Resolution to Library Issue 60, by Judy Ward and Matt Austern. This proposed resolution is section VI of that paper.