c++ - How to compare (the order) of two bidirectional iterators? -

i wondering if there build-in way in c++ compare order of 2 bidirectional iterators. example, have sum function calculate sum between 2 iterators in same list:

double sum(std::list::const_iterator start, std::list::const_iterator end){       double sum=0;       (start;start!=end;start++)            sum+=*start;       return sum; } 

then: sum(my_list.begin(),my_list.end()); fine, sum(my_list.end(),my_list.begin()); cause runtime error.

i thinking puttingif (start>end) return 0; prevent error. seems cannot compare iterators this.

you should read introduction stl explains various refinements of iterator concept.

only randomaccessiterators support comparison < because not efficient operation non-randomaccessiterators.

the way tell if bidirectionaliterator i less-than another, j, incrementing i 1 step @ time , seeing if ever reach j, never happen if j not reachable i, , error if i not incrementable, e.g. because past-the-end iterator range.

alternatively, decrement i , see if reach j, in case know j less-than i, won't work if i begin iterator, because can't iterate before beginning of range.

so in general there no way know whether 1 non-randomaccessiterator comes before or after another, because can't know whether start iterating forwards or backwards reach other, , wouldn't know when safe keep going or when reach end of valid range.

so convention pass iterators in same order, begin iterator comes first, , past-the-end iterator comes second, , past-the-end iterator can reached incrementing begin iterator 0 or more times.

then: sum(my_list.begin(),my_list.end()); fine, sum(my_list.end(),my_list.begin()); cause runtime error.

don't then!

it caller's responsibility call function correctly, , why caller not know iterator start , end?