MS VS 2013 'LDBL_DIG' const. From 'float.h' is incorrect?

Question

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C++

Visual-Studio

VisualC++

In 'float.h' from MS VS Community 2013 Update 5:

Quote:
#define LDBL_DIG DBL_DIG /* # of decimal digits of precision */

Quote:
#define DBL_DIG 15 /* # of decimal digits of precision */

I tested it using sprintf, it shows 17 meaningful digits.

It also shows 17 digits ('0.14285714285714285') in the editor ('autos' window), when i running test code in the debug mode.

I also opened 'float.h' in the MS VS 2015 Community Update 2 RC, it still same,
but now it has also a new constant:

Quote:
#define DBL_DECIMAL_DIG 17 // # of decimal digits of rounding precision

What I have tried:

Tets code:

const auto oneSeventh = 1.0L / 7.0L;
sprintf(buf, "%.*Lf", 30U, oneSeventh); // 0.142857142857142850000000000000

Posted 17-Mar-16 5:19am

Shvetsov Evgeniy

Updated 17-Mar-16 6:16am

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Richard MacCutchan 17-Mar-16 11:35am

Very interesting, do you have a question?

Shvetsov Evgeniy 17-Mar-16 12:12pm

What the point of your meaningless comment and your negative vote?
The question is in the very first line and it is "MS VS 2013 'LDBL_DIG' const. From 'float.h' is incorrect?"

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Jochen Arndt · Accepted Answer · 2016-03-17T06:16:00

A double floating point number has 53 mantissa bits (DBL_MANT_DIG) where the MSB is implicit set and 52 bits are stored. The corresponding number of decimal digits is

log10(2^53) = 53 * log10(2) = 15.95

Rounding that value down is used as DBL_DIG. So there is nearly one more decimal digit of precision.

DBL_DECIMAL_DIG specifies the number of decimal digits that can be rounded to a double precision number and back again without change to the value. It must be one more than the uprounded precision of 16 to satisfy this requirement. See also the standard at <float.h>[^].

So printing a value with 17 significant decimal digits and converting that back to double precision will result in the same value. When printing only 16 digits, the converted value might be not identical.

Microsoft has no native support for long double precision:

Fundamental Types (C++)[^]
The representation of long double and double is identical. However, long double and double are separate types.

Therefore the LDBL_xxx definitions are using the corresponding DBL_xxx definitions.