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### 596. 27.8.1.3 Table 112 omits "a+" and "a+b" modes

Section: 31.10.2.4 [filebuf.members] Status: CD1 Submitter: Thomas Plum Opened: 2006-09-26 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In testing 31.10.2.4 [filebuf.members], Table 112 (in the latest N2009 draft), we invoke

>        ostr.open("somename", ios_base::out | ios_base::in | ios_base::app)

and we expect the open to fail, because out|in|app is not listed in Table 92, and just before the table we see very specific words:

> If mode is not some combination of flags shown in the table then the open fails.

But the corresponding table in the C standard, 7.19.5.3, provides two modes "a+" and "a+b", to which the C++ modes out|in|app and out|in|app|binary would presumably apply.

We would like to argue that the intent of Table 112 was to match the semantics of 7.19.5.3 and that the omission of "a+" and "a+b" was unintentional. (Otherwise there would be valid and useful behaviors available in C file I/O which are unavailable using C++, for no valid functional reason.)

We further request that the missing modes be explicitly restored to the WP, for inclusion in C++0x.

[ Martin adds: ]

...besides "a+" and "a+b" the C++ table is also missing a row for a lone app bit which in at least two current implementation as well as in Classic Iostreams corresponds to the C stdio "a" mode and has been traditionally documented as implying ios::out. Which means the table should also have a row for in|app meaning the same thing as "a+" already proposed in the issue.

Proposed resolution:

Add to the table "File open modes" in 31.10.2.4 [filebuf.members]:

> File open modes
> `ios_base` Flag combination | `stdio` equivalent
> --- | ---
> `binary` | `in` | `out` | `trunc` | `app` | ``
> |  | `+` |  |  | `"w"`
> |  | `+` |  | `+` | `"a"`
> |  |  |  | `+` | `"a"`
> |  | `+` | `+` |  | `"w"`
> | `+` |  |  |  | `"r"`
> | `+` | `+` |  |  | `"r+"`
> | `+` | `+` | `+` |  | `"w+"`
> | `+` | `+` |  | `+` | `"a+"`
> | `+` |  |  | `+` | `"a+"`
> `+` |  | `+` |  |  | `"wb"`
> `+` |  | `+` |  | `+` | `"ab"`
> `+` |  |  |  | `+` | `"ab"`
> `+` |  | `+` | `+` |  | `"wb"`
> `+` | `+` |  |  |  | `"rb"`
> `+` | `+` | `+` |  |  | `"r+b"`
> `+` | `+` | `+` | `+` |  | `"w+b"`
> `+` | `+` | `+` |  | `+` | `"a+b"`
> `+` | `+` |  |  | `+` | `"a+b"`

[ Kona (2007) Added proposed wording and moved to Review. ]