std :: advance（）导致无限循环

Question

I have a code fragment that uses std::advance() .

How to avoid endless loop when using std::advance() ?

std::list<xxx>::iterator i = ppp.begin();
std::advance(i, yyy);

Answer 1

Maybe you mean, how should you avoid running past the end() iterator.

In that case, just either check

std::advance(i, std::min(yyy, std::distance(i, ppp.end()));

Or, write a wrapper around std::advance/std::next that checks for end iterators more efficienttly: http://ideone.com/7DYSSn

#include <list>
#include <cassert>

template <typename It>
   It safe_next(It it, std::size_t steps, It end)
{
    while (it!=end && steps--)
        it++;

    return it;
}

int main()
{
    std::list<int> l { 1,2,3,4,5,6,7,8 };
    auto it = begin(l);

    assert(safe_next(it, 3, end(l)) == std::next(it, 3));
    assert(safe_next(it, 30, end(l)) == end(l));

    // the `distance` trick also works:
    assert(next(it, std::min(30l, std::distance(it, end(l)))) == end(l));
}

Note that running past the end is Undefined Behaviour , which is a completely different thing than an infinite loop. It might have the same "apparent" effect (but that's the nature of UB, of course).