std::iterator segmentation fault when using it as a class variable

Question

Consider the following snippet

class ItrTest {
private:
    std::map<int, long> testMap;
    std::map<int, long>::iterator itr;

public:
    ItrTest(std::map<int, long> testMap): testMap(testMap) {
        itr = testMap.begin();
    }

    void printNext() {
        // itr = testMap.begin();
        for (; itr != testMap.end(); itr++) {
            cout<<"Key: "<<itr->first<<" Value:"<<itr->second<<endl;
        }
    }
};

int main() {
    std::map<int, long> m{
        { 0, 100l },
        { 1, 200l },
        { 2, 300l },
        { 3, 400l },
        { 4, 500l },
    };

    ItrTest t(std::move(m));
    t.printNext();
}

This throws a segmentation fault when trying to access the values inside the iterator. It is obvious this happens because the iterator becomes invalid (for me after a couple of iterations. Maybe it behaves differently on different systems). If I uncomment the first line of the function printNext() it works fine. I would like to get an explanation for this behaviour.

testMap(testMap)?! Why would anyone do that?!

David Schwartz
– David Schwartz

2019-09-06 16:34:35 +00:00
Commented Sep 6, 2019 at 16:34 — David Schwartz
– David Schwartz, Commented Sep 6, 2019 at 16:34

songyuanyao · Accepted Answer · 2019-09-06 16:28:04Z

3

In the constructor, for itr = testMap.begin();, testMap refer to the function parameter, not the data member. It'll be destryoed when get out of the constructor, left itr dangled. Any dereference on it later leads to UB.

You could change it to

itr = this->testMap.begin();

answered Sep 6, 2019 at 16:28

songyuanyao

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