C++利用两个栈实现队列的方法

#include <iostream>
#include <stack>
#include <exception>

template<class T> class MyQueue {
 public:
 void push(const T& num); // 入队列
 T pop(); // 出队列
 T top();
 private:
 std::stack<T> pushStack;
 std::stack<T> popStack;
};
template<typename T>
void MyQueue<T>::push(const T& num) {
 pushStack.push(num);
}
template<typename T>
T MyQueue<T>::pop() {
 if (pushStack.empty() && popStack.empty()) { // 如果二个栈都为空
 throw std::runtime_error("queue is empty");
 } else if (popStack.empty()) { // 如果popStack为空,将pushStack全部元素倒popStack
 while (!pushStack.empty()) {
 T data = pushStack.top(); // 获取pushStack栈顶元素
 pushStack.pop(); // 出栈
 popStack.push(data);
 }
 }
 T data = popStack.top();
 popStack.pop();
 return data;
}
template<typename T>
T MyQueue<T>::top() {
 if (pushStack.empty() && popStack.empty()) { // 如果二个栈都为空
 throw std::runtime_error("queue is empty");
 } else if (popStack.empty()) { // 如果popStack为空,将pushStack全部元素倒popStack
 while (!pushStack.empty()) {
 T data = pushStack.top(); // 获取pushStack栈顶元素
 pushStack.pop(); // 出栈
 popStack.push(data);
 }
 } else { // 如果popStack不为空的话直接返回popStack栈顶
 T data = popStack.top();
 return data;
 }
}
int main() {
 MyQueue<int> myQueue1;
 myQueue1.push(1);
 myQueue1.push(2);
 myQueue1.push(3);
 myQueue1.push(4);
 std::cout << "current pop is:" << myQueue1.pop() << std::endl;
 std::cout << "current pop is:" << myQueue1.pop() << std::endl;
 std::cout << "current pop is:" << myQueue1.pop() << std::endl;
 std::cout << "current pop is:" << myQueue1.pop() << std::endl;
 std::cout << "current pop is:" << myQueue1.pop() << std::endl;

 return 0;
}