613. max_digits10 missing from numeric_limits

Section: [numeric.special] Status: CD1 Submitter: Bo Persson Opened: 2006-11-20 Last modified: 2016-02-10

Priority: Not Prioritized

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Section [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 [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 [numeric.special], p2:

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

Change [numeric.special], p3:

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