c++: modifying a vector within a function

[neurojim]

Hi,
a question probably posed many times butI couldn't find on the web...
1) i gather that for a vector, v, if v.size() exceeds v.capacity() during a call to v.push_back(x) the allocation of increased space can result in a change in the position of that vector in memory
2) thus if that push_back(x) call occurs in a function where a reference to that vector has been passed, and a reallocation occurs, is that reference broken?

void pushme(vector<int>& w) {
...
w.push_back(32);
...
some other manipulation of w
}

int main() {
vector<int> v;
pushme(v);
}

Thanks!

[Banfa]

No the memory allocated to the vector to store it's contents is not the same as the vector object itself and since the reference is to the vector object and not it's contained memory it remains valid. If you took a reference or pointer to a member of the vector then that could become invalid after an operation that could result in a memory reallocation like so

void pushme(vector<int>& w) {
    ...
    w.push_back(32);
    w.push_back(64);
    w.push_back(128);
    ...
    int &w2 = w[2];
    ...
    w.push_back(256);
    w.push_back(512);
    w.push_back(1024);
    ...
    // At this point w is still valid but w2 may be invalid
}
 

[neurojim]

Thanks for a very clear response! :)