For fixed-width floating-point types, this is the number of fractional significand bits.
- iOS 7.0+
- macOS 10.9+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
- Xcode 8.0+
- Core Graphics
For extensible floating-point types,
significand should be the maximum allowed significand width (without counting any leading integral bit of the significand). If there is no upper limit, then
significand should be
Float80 is 63, even though 64 bits are used to store the significand in the memory representation of a
Float80 (unlike other floating-point types,
Float80 explicitly stores the leading integral significand bit, but the
Binary APIs provide an abstraction so that users don’t need to be aware of this detail).