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### 23. Num_get overflow result

Section: 28.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.

Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.

Further comments from Dietmar:

I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:

Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:

- According to 28.4.3.2.3 [facet.num.get.virtuals] paragraph 11 `failbit` is set if `scanf()` would return an input error; otherwise a value is converted to the rules of `scanf`.
- `scanf()` is defined in terms of `fscanf()`.
- `fscanf()` returns an input failure if during conversion no character matching the conversion specification could be extracted before reaching EOF. This is the only reason for `fscanf()` to fail due to an input error and clearly does not apply to the case of overflow.
- Thus, the conversion is performed according to the rules of `fscanf()` which basically says that `strtod`, `strtol()`, etc. are to be used for the conversion.
- The `strtod()`, `strtol()`, etc. functions consume as many matching characters as there are and on overflow continue to consume matching characters but also return a value identical to the maximum (or minimum for signed types if there was a leading minus) value of the corresponding type and set `errno` to `ERANGE`.
- Thus, according to the current wording in the standard, overflows can be detected! All what is to be done is to check `errno` after reading an element and, of course, clearing `errno` before trying a conversion. With the current wording, it can be detected whether the overflow was due to a positive or negative number for signed types.

Further discussion from Redmond:

The basic problem is that we've defined our behavior, including our error-reporting behavior, in terms of C90. However, C90's method of reporting overflow in scanf is not technically an "input error". The `strto_*` functions are more precise.

There was general consensus that `failbit` should be set upon overflow. We considered three options based on this:

1. Set failbit upon conversion error (including overflow), and don't store any value.
2. Set failbit upon conversion error, and also set `errno` to indicated the precise nature of the error.
3. Set failbit upon conversion error. If the error was due to overflow, store +-numeric_limits<T>::max() as an overflow indication.

Straw poll: (1) 5; (2) 0; (3) 8.

Discussed at Lillehammer. General outline of what we want the solution to look like: we want to say that overflow is an error, and provide a way to distinguish overflow from other kinds of errors. Choose candidate field the same way scanf does, but don't describe the rest of the process in terms of format. If a finite input field is too large (positive or negative) to be represented as a finite value, then set failbit and assign the nearest representable value. Bill will provide wording.

Discussed at Toronto: N2327 is in alignment with the direction we wanted to go with in Lillehammer. Bill to work on.

Proposed resolution:

Change 28.4.3.2.3 [facet.num.get.virtuals], end of p3:

> Stage 3: The result of stage 2 processing can be one of The sequence of `char`s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header `<cstdlib>`:

> - A sequence of `char`s has been accumulated in stage 2 that is converted (according to the rules of `scanf`) to a value of the type of val. This value is stored in val and `ios_base::goodbit` is stored in err. For a signed integer value, the function `strtoll`.
> - The sequence of `char`s accumulated in stage 2 would have caused `scanf` to report an input failure. `ios_base::failbit` is assigned to err. For an unsigned integer value, the function `strtoull`.
> - For a floating-point value, the function `strtold`.

> The numeric value to be stored can be one of:

> - zero, if the conversion function fails to convert the entire field. `ios_base::failbit` is assigned to err.
> - the most positive representable value, if the field represents a value too large positive to be represented in val. `ios_base::failbit` is assigned to err.
> - the most negative representable value (zero for unsigned integer), if the field represents a value too large negative to be represented in val. `ios_base::failbit` is assigned to err.
> - the converted value, otherwise.

> The resultant numeric value is stored in val.

Change 28.4.3.2.3 [facet.num.get.virtuals], p6-p7:

>     iter_type do_get(iter_type in, iter_type end, ios_base& str,
>                      ios_base::iostate& err, bool& val) const;

> > -6- Effects: If `(str.flags()&ios_base::boolalpha)==0` then input proceeds as it would for a `long` except that if a value is being stored into val, the value is determined according to the following: If the value to be stored is 0 then `false` is stored. If the value is 1 then `true` is stored. Otherwise `err|=ios_base::failbit` is performed and no value `true` is stored. and `ios_base::failbit` is assigned to err.

> > -7- Otherwise target sequences are determined "as if" by calling the members `falsename()` and `truename()` of the facet obtained by `use_facet<numpunct<charT> >(str.getloc())`. Successive characters in the range `[in,end)` (see 23.1.1) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator in is compared to end only when necessary to obtain a character. If and only if a target sequence is uniquely matched, val is set to the corresponding value. Otherwise `false` is stored and `ios_base::failbit` is assigned to err.