cpp/types/numeric limits/digits

The value of is the number of digits in base- that can be represented by the type  without change. For integer types, this is the number of bits not counting the sign bit and the padding bits (if any). For floating-point types, this is the digits of the mantissa (for implementations, this is the number of digits stored for the mantissa plus one, because the mantissa has an implicit leading 1 and binary point).