c++ - Is it well-defined to compare with a value-initialized iterator? -

c++ - Is it well-defined to compare with a value-initialized iterator? -

does next programme invoke undefined behavior?

#include <iostream> #include <iterator> int main(int argc, char* argv[]) { (auto = std::istream_iterator<std::string>(std::cin); != std::istream_iterator<std::string>(); ++it) { std::cout << *it << " "; } homecoming 0; }

this 4 year old question says can't compared:

iterators can have singular values not associated container. [example: after declaration of uninitialized pointer x (as int* x;), x must assumed have singular value of pointer. ] results of expressions undefined singular values; excep- tion assignment of non-singular value iterator holds singular value.

but reply says c++14 standard:

however, value-initialized iterators may compared , shall compare equal other value-initialized iterators of same type.

you conflating 2 different issues.

istream_iterator input iterator, not forwards iterator, c++14 alter cited doesn't apply @ all. allowed compare istream_iterators in manner because explicitly specified allow such comparisons. standard says (§24.6.1 [istream.iterator])

the constructor no arguments istream_iterator() constructs end-of-stream input iterator object, legitimate iterator used end condition. [...]

two end-of-stream iterators equal. end-of-stream iterator not equal non-end-of-stream iterator. 2 non-end-of-stream iterators equal when constructed same stream.

for forwards iterators (which includes bidirectional , random access ones) in general, value-initialized iterators made comparable each other in c++14. if standard library implements it, can compare 2 value-initialized iterators. allows create empty range without underlying container. however, still not allowed compare non-singular iterator value-initialized iterator. next code has undefined behavior in c++14:

std::list<int> l; if(l.begin() == std::list<int>::iterator()) foo(); else bar();

c++ c++11