floatfield

Floatfield is a bitmask used in the C++ standard library to control the floating-point formatting of output streams. Defined as part of std::ios_base::fmtflags, it identifies the set of flags that influence how floating-point values are represented when streamed.

The floatfield mask works with setf and unsetf to apply a specific floating-point notation. The primary options that can be applied through this mask are fixed, scientific, and defaultfloat. When you call setf with the floatfield mask, you typically also specify which format to apply, and the operation clears the other floating-point format flags in the stream before setting the chosen one. For example, applying fixed uses fixed-point notation, while applying scientific uses scientific notation. Defaultfloat restores the general or default representation.

In practice, fixed and scientific control the appearance of numbers with a defined precision: fixed prints a fixed number of digits after the decimal point, and scientific uses a fixed number of significant digits in scientific notation. The defaultfloat option reverts to the library’s default floating-point representation, which is a general format that chooses between fixed and scientific as appropriate. Precision interacts with these formats: in fixed and scientific, precision specifies digits after the decimal or significant digits, respectively; in defaultfloat, precision typically indicates the number of significant digits.

Floatfield is part of the broader iostream formatting facilities, and is commonly used indirectly via manipulators such as std::fixed, std::scientific, and std::defaultfloat, or through explicit setf/unsetf calls on streams like std::cout or std::stringstream.