Assigning a vector element to result of function that invokes emplace_back?

Question

The test method on the following class does not have the effect I would expect it to. I have a suspicion it is something to do with the fact that the invocation of emplace_back somehow invalidates the reference obtained via the subscript.

Either way I would expect the second print in test to result in

v[0] = 1

however both result in

v[0] = 5

suggesting that the assignment does not take place.

class FooBar {
    vector<size_t> v;
public:
    size_t add(size_t x) {
        cout << "add(" << x << ")" << endl;
        size_t K(v.size());
        v.emplace_back(x);
        return K;
    }

    void test(size_t idx) {
        cout << "v[" << idx << "] = " << v[idx] << endl;
        v[idx] = add(0);
        cout << "v[" << idx << "] = " << v[idx]<< endl;
    }
};

int main(int argc, char* argv[])
{       
    FooBar f;
    f.add(5);
    f.test(0);
}

I know that I can get around the problem by creating a temporary to store the result of add and then perform the assignment but I am interested as to why I cannot use just a straight assignment and why I do not get any kind of error when attempting to perform this.

Compiled and tested with MSVC (Visual Studio 2015).

Answer 1

The line

    v[idx] = add(0);

is cause for undefined behavior. You are modifying the contents of v in add while assuming that v[idx] will be valid.

For predictable behavior, you can use:

void test(size_t idx) {
    cout << "v[" << idx << "] = " << v[idx] << endl;
    size_t val = add(0);
    v[idx] = val;
    cout << "v[" << idx << "] = " << v[idx]<< endl;
}