Sorting between signed and unsigned zeros c++ -

i have able sort negative zeros , zeros assignment doing @ university using c++, tell me why following code produce negative zero? i'm stuck , not sure why works...

        cout << "enter number of elements want add vector:\n";     cin >> x;     cout << "enter integers: \n" << endl;     (int = 0; < x; i++)     {         cin >> y;         y = y - 0.0;         cout << y;           array.push_back(y);     } 

if there better way of producing negative 0 when sorting above vector please advise. many thanks!

first of all, there need not negative zeroes in standard c++, assume talking negative 0 ieee-754, (i never encountered exception) c++ implementations base floating point math on.

in case, expression

y = y - 0.0; 

will yield -0.0 if either y == -0.0 before assignment or if set rounding mode "round towards -infinity", won't.

to produce double value -0.0, can assign desired value:

double d = -0.0; 

now d == -0.0 in ieee floating point math.

however,

comparisons shall ignore sign of zero

(ieee 754-1985, 5.7. comparison), -0.0 < 0.0 yield false, if want sort negative 0 before positive zero, need write own comparator, possibly using std::signbit.

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appendix: relevant standard quote:

when sum of 2 operands opposite signs (or difference of 2 operands signs) zero, sign of sum (or difference) shall + in rounding modes except round toward –infinity, in mode sign shall –.

ieee 754-1985, 6.3 (the sign bit)