Problem with stack of strings

[Gawaine]

So I have code that uses a stack to check whether a value exists in the stack. When the stack is a stack of int, it properly tells me whether a number is in the stack for numbers 10 and up but single digit numbers always return false. Additionally the stack of strings always returns false. What is going wrong?
Thanks

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

template <typename T>
ostream &operator <<(ostream &os, stack<T> &s) {
	stack<T> hold;
	os << "[";
	while (!s.empty()) {
		T t = s.top();
		hold.push(t);
		s.pop();
		os << t << (!s.empty() ? ", " : "");
	}
	os << "]";
	while (!hold.empty()) {
		s.push(hold.top());
		hold.pop();
	}
	return os;
}

template <typename T>
bool contains(stack<T> s, T val){
    for (int i = 0; i < s.size(); i++){
		if (s.top() == val) return true;
        s.pop();
    }
	return false;
}

int main() {
	cout << boolalpha;
    
	stack<int> si;
    
	for (int i = 0; i < 20; i++)
		si.push(i);
    
    cout << "si contains 15: " << contains(si, 15) << endl;
	cout << "si contains 23: " << contains(si, 23) << endl;
    
	cout << si << endl;
    
	while (!si.empty()) {
		si.pop();
		cout << si << endl;
	}
    
	const int ARR_SIZE = 4;
	string arr[ARR_SIZE] = {"cat", "dog", "cow", "elephant"};
    
	cout << endl;
    
	stack<string> ss;
    
	for (int i = 0; i < ARR_SIZE; i++)
		ss.push(arr[i]);
    
    cout << "si contains \"dog\": " << contains(ss, string("dog")) << endl;
	cout << "si contains \"muskrat\": " << contains(ss, string("muskrat")) << endl;
    
	cout << ss << endl;
    
	while (!ss.empty()) {
		ss.pop();
		cout << ss << endl;
	}

	return 0;
}

[Peter87]

The loop inside the contains function does not check all elements in the stack.

First iteration: i=0, s.size()=4
Second iteration: i=1, s.size()=3
The third iteration doesn't happen because then i=2 and s.size()=2

[Gawaine]

Thanks, I fixed it.