c++ reference in recursion (reverse a stack)

Question

I am doing a very simple exercise on C++ recursion for reversing a stack recursively. The code is attached below. My problem is about whether I should use reference T& or simply T in my recursive call putToBottom(stack&, const T&).

#include <iostream>
#include <stack>
using namespace std;

template <typename T>
void putToBottom(stack<T>& myStack, const T& topOne)
{
    if (myStack.empty()) 
    {
        myStack.push(topOne);
        return;
    }

//Should I use T& or T here???
    ***T temp = myStack.top();***
    myStack.pop();
    putToBottom(myStack, topOne);
    myStack.push(temp);
}

template <typename T>
void reverseStack(stack<T>& myStack)
{
    if (myStack.empty()) return;
    T& topOne = myStack.top();
    myStack.pop();
    reverseStack(myStack);
    putToBottom(myStack, topOne);
}

int main()
{
    stack<int> myStack;
    myStack.push(1);
    myStack.push(2);
    myStack.push(3);
    myStack.push(4);
    reverseStack(myStack);
    while (!myStack.empty())
    {
        cout << myStack.top() << "\n";
        myStack.pop();
    }
    cout << endl;
    return 0;
}

If I write T temp = myStack.top(); , I get the right answer (the stack is now 1 (top) -> 2 -> 3 -> 4). However, if I write T& temp = myStack.top(); or const T& temp = myStack.top(); , I get the wrong one (the stack becomes 4 (top) -> 4 -> 4 -> 4). Any ideas would help! Thank you!

Answer 1

If you use T& temp = myStack.top() , then once you do myStack.pop() , temp becomes a dangling reference and it is undefined behavior to use it further.

If you are using C++11 then you probably want move semantics:

T temp = std::move(myStack.top());
myStack.pop();
putToBottom(myStack, topOne);
myStack.push(std::move(temp));

And similarly in reverseStack .

Although I have to note, this is a particularly inefficient way to reverse a stack. Better would be simply to pop everything off of the original stack while pushing it onto a second stack.