C++ 设置迭代器错误：集合中的“operator+=”不匹配

Question

auto str=lower_bound(s.begin(),s.end(),n);


cout<<std::distance(s.begin()+1,str)+1;

Why +1 not working after s.begin() ? It shows that there is no match for operator+ (operand types are 'std::set::iterator'...)

Answer 1

std::set iterators are bi-directional iterators, which are required only to implement the -- and ++ operators. Random-access iterators implement + / - and += / -= operators.

You can use std::advance() or std::next() to move an iterator forward when it doesn't implement operator+ , eg:

auto str = std::lower_bound(s.begin(), s.end(), n);
auto iter = s.begin();
std::advance(iter, 1);
cout << std::distance(iter, str) + 1;

auto str = std::lower_bound(s.begin(), s.end(), n);
cout << std::distance(std::next(s.begin()), str) + 1;

Answer 2

A std::set::iterator does not support operator+ (it's a LegacyBidirectionalIterator supporting operator-- and operator++ ), so it fails here:

s.begin() + 1

Skip +1 and it'll work, but it'll not be a single operation. I'll have to step forward from s.begin() to str in order to calculate the distance:

std::cout << std::distance(s.begin(), str);

Note: In order to get the same result as std::distance(s.begin() + 1, str) + 1;would have given if it had worked, you must skip the +1 you have after std::distance() too.