STL: Avoid declaring multiple variables on a single line

[StephanTLavavej]

In the STL, we conventionally avoid declaring multiple variables on a single line. That is:

int cats, dogs; // we avoid this style

int meow; // we prefer this style
int woof;

Our reasons for this convention are:

1. Being aware of the variables in a class or function is important, because humans have difficulty reasoning about program state (especially when it's modifiable). Consistently declaring one variable per line makes it clear when variables are being introduced, and whether they're initialized. (It also makes it "feel costly" to declare many variables, which is subtle encouragement to avoid introducing unnecessary state.)
2. C++'s rules for multi-variable declarations are surprising to newer programmers. (In particular, the rules when pointers are involved.) Avoiding such declarations avoids the need to learn/teach/read that bit of complexity.

We're pretty consistent about following this convention, but there are a few files that should be changed. I observe the following occurrences (there may be more):

STL/stl/inc/random

Line 622 in fffbd8f

const _Ty *_First, *_Last, *_Other;

STL/stl/inc/xiosbase

Line 511 in fffbd8f

_Iosarray *_Ptr1, *_Ptr2;

STL/stl/inc/xiosbase

Line 532 in fffbd8f

_Iosarray *_Ptr1, *_Ptr2;

STL/stl/inc/xiosbase

Line 540 in fffbd8f

_Fnarray *_Pfa1, *_Pfa2;

STL/stl/inc/cvt/wbuffer

Line 264 in fffbd8f

_Elem _Ch, *_Dest;

STL/stl/src/filesys.cpp

Line 317 in fffbd8f

_ULARGE_INTEGER _Available, _Capacity, _Free;

STL/stl/src/xstol.cpp

Line 21 in fffbd8f

char *se, sign;

STL/stl/src/xstoll.cpp

Line 19 in fffbd8f

char *se, sign;

STL/stl/src/xstoul.cpp

Lines 35 to 36 in fffbd8f

	const char *sc, *sd;
	const char *s1, *s2;

STL/stl/src/xstoul.cpp

Line 40 in fffbd8f

unsigned long x, y;

STL/stl/src/xstoull.cpp

Lines 27 to 29 in fffbd8f

	const char *sc, *sd;
	const char *s1, *s2;
	char dig, sign;

STL/stl/src/xstoull.cpp

Line 31 in fffbd8f

unsigned long long x, y;

STL/stl/src/xxxprec.h

Line 70 in fffbd8f

short errx, xexp;

STL/stl/src/xxxprec.h

Line 105 in fffbd8f

short errx, xexp;

STL/stl/src/xxxprec.h

Line 212 in fffbd8f

int j, k;

• These occurrences are weird-looking, but should be treated similarly:

STL/stl/inc/iostream

Lines 21 to 29 in fffbd8f

	__PURE_APPDOMAIN_GLOBAL extern istream cin, *_Ptr_cin;
	__PURE_APPDOMAIN_GLOBAL extern ostream cout, *_Ptr_cout;
	__PURE_APPDOMAIN_GLOBAL extern ostream cerr, *_Ptr_cerr;
	__PURE_APPDOMAIN_GLOBAL extern ostream clog, *_Ptr_clog;
	__PURE_APPDOMAIN_GLOBAL extern wistream wcin, *_Ptr_wcin;
	__PURE_APPDOMAIN_GLOBAL extern wostream wcout, *_Ptr_wcout;
	__PURE_APPDOMAIN_GLOBAL extern wostream wcerr, *_Ptr_wcerr;
	__PURE_APPDOMAIN_GLOBAL extern wostream wclog, *_Ptr_wclog;

STL/stl/inc/iostream

Lines 33 to 41 in fffbd8f

	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT istream cin, *_Ptr_cin;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT ostream cout, *_Ptr_cout;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT ostream cerr, *_Ptr_cerr;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT ostream clog, *_Ptr_clog;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT wistream wcin, *_Ptr_wcin;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT wostream wcout, *_Ptr_wcout;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT wostream wcerr, *_Ptr_wcerr;
	__PURE_APPDOMAIN_GLOBAL extern _CRTDATA2_IMPORT wostream wclog, *_Ptr_wclog;

STL/stl/inc/ymath.h

Line 48 in fffbd8f

extern _CRTIMP2_PURE_IMPORT /* const */ _Dconst _Denorm, _Hugeval, _Inf, _Nan, _Snan;

STL/stl/inc/ymath.h

Line 56 in fffbd8f

extern _CRTIMP2_PURE_IMPORT /* const */ _Dconst _FDenorm, _FInf, _FNan, _FSnan;

STL/stl/inc/ymath.h

Line 64 in fffbd8f

extern _CRTIMP2_PURE_IMPORT /* const */ _Dconst _LDenorm, _LInf, _LNan, _LSnan;

• Finally, the occurrences in xcharconv_ryu.h should not be changed at this time (as this code is kept in sync with the upstream Ryu project):

STL/stl/inc/xcharconv_ryu.h

Line 1061 in fffbd8f

uint32_t __vr, __vp, __vm;

STL/stl/inc/xcharconv_ryu.h

Line 1751 in fffbd8f

uint64_t __vr, __vp, __vm;

I found all of these occurrences by using VSCode to regex-search for (\w+)(, \*?\w+)+; excluding files in tests/*.

[JeanPhilippeKernel]

you can assign me this ?

[StephanTLavavej]

Sure - we look forward to a PR. (We're currently working through our PR backlog in addition to the LWG issues and features that were recently voted in at the Prague meeting, which is why I haven't reviewed your other PR #497 again - hopefully I will have time soon! :-)