C ++ std :: vector <std::string> 迭代器段故障

Question

I encountered this segfault while iterating through a vector of filenames. The std::vector is populated by another function reading csv in a pretty messy code. So I narrowed it down to the below code causing the problem.

The iterator for vector segfaults after yielding first (sometimes later) item of the vector with 4 items. Pushing the 5th item fixes the problem. Strange? Iterator for vector works fine.

#include <iostream>
#include <vector>

using namespace std;

std::vector<int> popbar() {
    // populate vector of integers
    //
    std::vector<int> bar;

    for(int i = 1; i < 6; i++)
        bar.push_back(i);

    return bar;
}

std::vector<std::string> popxar() {
    // populate vector of strings
    //
    std::vector<std::string> xar;

    xar.push_back("one");
    xar.push_back("two");
    xar.push_back("three");
    xar.push_back("four");
    // this line fixes segfault
    //xar.push_back("five");

    return xar;
}

void foo () {
    // yield next vector item
    //
    //auto bar = popbar();
    auto bar = popxar();

    //static auto itx = bar.begin();
    static vector<string>::iterator itx = bar.begin();

    if (itx == bar.end()) {
        cout << "end of line" << endl;
        itx = bar.begin();
    }

    cout << *itx++ << endl;
}

int main() {
    for(int i = 0; i < 11; i++) {
        foo();
    }
}

The expected output is

one
two
three
four
end of line
one
two
three
four
end of line
one
two
three

The output I get is

one
Segmentation fault

also seen

one
two
three
Segmentation fault

and

one
three
three
���1������������1one1fourSegmentation fault

if that makes it more interesting. Does it? Please consider this for vector as well .

Answer 1

You defined a static iterator to a local variable . What did you expect was going to happen?

When foo returns, the local vector xar is going to get destroyed, which invalidates all your iterators. Re-entering foo creates a brand new vector, and then you try to use an invalid iterator. Undefined behaviour ensues.

Answer 2

It's because you have a static iterator pointing into a non-static local variable. When the foo function returns, bar is destructed. This leads to undefined behavior .