What does the C++ standard say about std::vector<int> v1,v2; std::distance(v1.begin(),v2.begin())?

Question

I have this code

#include <vector> #include <iostream>  int main(int argc, char* argv[]) {    std::vector<int> v1,v2;    std::cout << std::distance(v1.begin(),v2.begin());    return 0; } 

and it has a bug because it is not meaningful to compare the iterators of two different vectors.

I had a look at N3376 at 24.4.4 Iterator operations at page 815:

template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last); 

Requires: If InputIterator meets the requirements of random access iterator, last shall be reachable from first or first shall be reachable from last; otherwise, last shall be reachable from first.

Now I think that Requires is not fulfilled.

What does the standard state should happen in this case?

Answer 1

[iterator.requirements.general]:

An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j.

The problem is that once you incremented v1.begin() v1.size()-1 times, the next increment operation induces undefined behavior, so v2.begin() cannot be reached from v1.begin(). The same argument makes v1.begin() unreachable from v2.begin().

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In case your question was "What happens if a condition in a Requires section is violated?", look at [res.on.required]:

Violation of the preconditions specified in a function’s Requires: paragraph results in undefined behavior unless the function’s Throws: paragraph specifies throwing an exception when the precondition is violated.

Answer 2

In some implementations of std::distance, the first iterator is incremented until it reaches the second iterator. The iterations are counted:

unsigned int counts = 0; while (iter1 != iter2) {   ++counts;   ++iter1; } 

If the iterators point to containers in different address spaces, the loop many not terminate. Using the terms in the standard, the second iterator is not reachable.