Styjun

In the STD library file ios_base.h we see the following:

 enum _Ios_Fmtflags 
 
 _S_boolalpha = 1L << 0,
 _S_dec = 1L << 1,
 _S_fixed = 1L << 2,
 _S_hex = 1L << 3,
 _S_internal = 1L << 4,
 _S_left = 1L << 5,
 _S_oct = 1L << 6,
 _S_right = 1L << 7,
 _S_scientific = 1L << 8,
 _S_showbase = 1L << 9,
 _S_showpoint = 1L << 10,
 _S_showpos = 1L << 11,
 _S_skipws = 1L << 12,
 _S_unitbuf = 1L << 13,
 _S_uppercase = 1L << 14,
 _S_adjustfield = _S_left ;

But after that we also see:

 /// Insert/extract @c bool in alphabetic rather than numeric format.
static const fmtflags boolalpha = _S_boolalpha;

/// Converts integer input or generates integer output in decimal base.
static const fmtflags dec = _S_dec;

/// Generate floating-point output in fixed-point notation.
static const fmtflags fixed = _S_fixed;

/// Converts integer input or generates integer output in hexadecimal base.
static const fmtflags hex = _S_hex;

Why are they using static const in addition to the enum values to represent the same values? Why can't we just use the enum values? And isn't using static wasteful in this case considering those are const values?

Thanks,
Ofer

The goal is to separate the interface and implementation. _Ios_Fmtflags is implementation-defined and can be changed in the future, ios_base should not be significantly dependent on _Ios_Fmtflags, but must provide a set of constants as part of the documented interface see comment. How could we avoid code dependency on the internal _Ios_Fmtflags implementation? We can use synonyms for _Ios_Fmtflags type and constant objects of this type which is done further:

typedef _Ios_Fmtflags fmtflags;
static const fmtflags boolalpha = _S_boolalpha;
static const fmtflags dec = _S_dec;

Now only these lines depend on the specific _Ios_Fmtflags implementation. For example, we can rename _S_boolalpha and it will not affect the code except only one line static const fmtflags boolalpha = _S_boolalpha; Or as another example, we can replace enum _Ios_Fmtflags by integer or some else suitable type. A very useful technique that makes the code maintaining much easier, although it increases the code size.