Proposed Approach for Locale Control in Streams
	X3J16/93-0111
	WG21/N0318
	29 July 1993
	
	P.J. Plauger
	
	There is strong sentiment for associating tailored locales with
	at least certain I/O streams. The LWG has agreed to explore adding
	a kind of stream class that has locale-dependent behavior. It too
	is a candidate for having its own tailored locale. (It is also a
	candidate for connecting to a streambuf, of course.) The idea is
	to have member functions that manipulate these objects switch
	part or all of the current locale to the tailored locale while
	performing locale-dependent operations on them, then switch back
	when the operation is complete.
	
	I have written enough code using such machinery that I think I now
	know how best to go about it. (It was spelled out in the draft I
	brought to Munich.) Briefly:
	
	>> Add the bitmask type ios::localecat
	
	The C Standard is vague about the nature of the category argument
	to setlocale, but X/OPEN has made an important clarification. They
	insist that a category is a bitmask, so that one scalar quantity
	can represent an arbitrary combination of categories. I think the
	C++ Standard should endorse that position and add the bitmask
	type localecat to ios, with elements that name all the individual
	categories. The constant all would have the value ~0.
	
	>> Add ios::localecat ios::locale(const char *s, ios::localecat cat)
	
	This function memorizes the locale s, for categories specified by cat,
	as the tailored locale for the controlled stream. The actual work
	should be done by a virtual member of streambuf, because locales
	should be tailored that way. The (derived) streambuf may control a
	C file with its own locale, or a locale-dependent string object.
	What you control and how you control it is thus dependent on the
	actual streambuf. Still, the access function more naturally lives
	in ios, as the LWG group concluded some time ago.
	
	All this function returns is the current category profile. (If s
	is null, the call is purely a query.) Thus, the underlying machinery
	is at liberty to store a more encoded representation of the locale,
	not the arbitrary length string s which may require considerable
	interpretation when used to specify a locale. That lets implementations
	optimize heavily if they see extensive use of locales, while simpler
	implmenentations can simply store and use the string, at whatever
	cost in performance.
	
	The default for each streambuf should be no locale (category is zero).
	Hence, the stream follows the current convention of using the current
	locale for all operations.
	
	>> Add ios::setlocale(int changefl = 0)
	
	The call ios::setlocale(1), typically within ipfx() or opfx(),
	memorizes the current locale and switches the current locale to
	match the memorized profile. The call ios::setlocale(0) reverts
	to the memorized locale. (Such a call also occurs in setstate/clear,
	before an exception is thrown or rethrown.) This is the mechanism
	used by all the member functions to switch the desired locale in
	and out. It too actually calls a virtual in streambuf, for the
	same reasons as cited above.
	
	If this approach is found acceptable, I can provide a more complete
	specification on short notice.