Question

I have a question regarding vector [] operator in C++.

vector<stack<T>> myStackVec;
ops...
...
...

This code does not modify myStackVec:

stack<T> temp = myStackVec.at(stackPos);
cout << "removing " << temp.top() << endl;

This code does modify myStackVec through modifying temp:

stack<T> *temp = &myStackVec[stackPos];
temp->push(item);

[] operator returns a reference, why code snippet 1 does not work? Is temp in code1 a copy?

Answer 1

stack<T> temp = myStackVec.at(stackPos);

will make a copy of stack, then temp.push_back will operate on the copied stack, you need reference instead:

stack<T> &temp = myStackVec.at(stackPos);
temp.push_back(item);

or simply:

myStackVec.at(stackPos).push_back(item);