Not seeing the effects of action on a vector's last element via a reference

Question

It seems I don't fully understand how exactly C++ references work. Trying to run following snippet:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> test{1,2,3};
    int& ref = test.back();
    auto lambda = [&ref, &test](){
        std::cout << "inside lambda " << ref << std::endl;
        ref += 1;
        for (auto&v : test) { std::cout << v << " "; }
    };

    lambda(); lambda(); lambda();
    test.push_back(5);
    lambda(); lambda(); lambda();
}

And got this result:

inside lambda 3                                                                                                                                                                       
1 2 4                                                                                                                                                                                 
inside lambda 4                                                                                                                                                                       
1 2 5                                                                                                                                                                                 
inside lambda 5                                                                                                                                                                       
1 2 6                                                                                                                                                                                 
inside lambda 6                                                                                                                                                                       
1 2 6 5                                                                                                                                                                               
inside lambda 7                                                                                                                                                                       
1 2 6 5                                                                                                                                                                               
inside lambda 8                                                                                                                                                                       
1 2 6 5

Why after push_back is done, none of vector elements is incremented? And where ref points to from this moment?

If the vector needs to reallocate its data storage, all iterators and pointer and references to elements become invalidated. — Some programmer dude, Mar 02 '18 at 16:43
Death by reallocation. The vector's content moved away, but the reference stayed and became dangling. — Quentin, Mar 02 '18 at 16:43
This question doesn't really have anything to do with lambdas, you would have gotten the same thing without them. — einpoklum, Mar 02 '18 at 16:47

score 3 · Accepted Answer · answered Mar 02 '18 at 16:49

3

At start, ref references the third element of the storage of test. After pushing back a new element it happens that the vector reallocated its storage, so that the old ref element is no more valid. Your code leads to undefined behaviour.

answered Mar 02 '18 at 16:49

Jean-Baptiste Yunès

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Not seeing the effects of action on a vector's last element via a reference

1 Answers1