Stack with min operation

stack-with-min.cpp

template <typename T>
class StackWithMin {
private:
    stack< pair<T, T> > S;
public:
    void push(T& x) {
        S.push(pair<T, T>(x, S.empty() ? x : min(x, S.top().second)));
    }

    T pop() {
        if (S.empty())
            throw "stack empty";
        pair<T, T> top = S.top(); S.pop();
        return top.first;
    }

    T getMin() {
        if (S.empty())
            throw "stack empty";
        return S.top().second;
    }

    int size() {
        return S.size();
    }

    bool empty() {
        return S.empty();
    }
};