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### 613. max_digits10 missing from numeric_limits

Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Bo Persson Opened: 2006-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Section 17.3.5.3 [numeric.special] starts out by saying that "All members shall be provided for all specializations."

Then it goes on to show specializations for float and bool, where one member is missing (max_digits10).

Maarten Kronenburg adds:

I agree, just adding the comment that the exact number of decimal digits is digits * ln(radix) / ln(10), where probably this real number is rounded downward for digits10, and rounded upward for max_digits10 (when radix=10, then digits10=max_digits10). Why not add this exact definition also to the standard, so the user knows what these numbers exactly mean.

Howard adds:

For reference, here are the correct formulas from N1822:

>     digits10 = floor((digits-1) * log10(2))
>     max_digits10 = ceil((1 + digits) * log10(2))

We are also missing a statement regarding for what specializations this member has meaning.

Proposed resolution:

Change and add after 17.3.5.2 [numeric.limits.members], p11:

>     static const int max_digits10;

> > -11- Number of base 10 digits required to ensure that values which differ by only one epsilon are always differentiated.

> > -12- Meaningful for all floating point types.

Change 17.3.5.3 [numeric.special], p2:

>     template<> class numeric_limits<float> {
>     public:
>       static const bool is_specialized = true;
>       ...
>       static const int digits10 = 6;
>       static const int max_digits10 = 9;
>       ...

Change 17.3.5.3 [numeric.special], p3:

>     template<> class numeric_limits<bool> {
>     public:
>       static const bool is_specialized = true;
>       ...
>       static const int digits10 = 0;
>       static const int max_digits10 = 0;
>       ...