How to Use STL Stack Container in C++

1. Use std::stack to Declare Stack Container Object in C++
2. Use the top() Function to Access the Most Recently Added Element in C++
3. Use the swap() Function to Exchange Contents of Two Stacks in C++

This article will demonstrate multiple methods about how to use STL stack container in C++.

Use std::stack to Declare Stack Container Object in C++

std::stack is called a container adapter, which can act as wrappers of standard containers but provide limited and specialized functionality. As an example, std::stack class provides LIFO (last-in, firs-out) data structure and it can be mapped to a std::vector or std::deque container underneath. std::stack can be initialized with another stack object or compatible sequence containers like deque, vector and list. Note, though, the default container used to store the stack elements is deque. Also, there is no constructor to support passing the initializer list or elements directly. We need to use the push method once the stack object has been declared.

Notice that, stack object can not be iterated through using the range-based loop. Thus, we implement a special while loop to print each element to the cout stream. printStack function takes stack argument, but st1 that was initialized by the vector object can’t be passed to it, as types of these two objects differ and compiler throws the error.

#include <iostream>
#include <stack>
#include <vector>

using std::cout;
using std::endl;
using std::stack;
using std::vector;

template <typename T>
void printStack(stack<T> s) {
  while (!s.empty()) {
    cout << s.top() << "; ";
    s.pop();
  }
  cout << endl;
}

int main() {
  vector<int> vec1 = {1, 2, 3, 4, 11};
  stack st1{vec1};
  stack<int> st2;

  for (int i = 1; i <= 10; ++i) {
    st2.push(i * 12);
  }

  //    printStack(st1); Error - no matching function
  printStack(st2);

  return EXIT_SUCCESS;
}

Use the top() Function to Access the Most Recently Added Element in C++

The top() function is a member function that returns the top element in the stack. Note that this function does not automatically remove a returned element from the stack object. One should call the pop member function to remove it.

#include <iostream>
#include <stack>

using std::cout;
using std::endl;
using std::stack;

int main() {
  stack<int> st2;

  for (int i = 1; i <= 10; ++i) {
    st2.push(i * 12);
  }

  cout << "top of the stack st2: ";
  cout << st2.top() << endl;

  return EXIT_SUCCESS;
}

Output:

top of the stack st2: 120

Use the swap() Function to Exchange Contents of Two Stacks in C++

The swap() function is a member function of the stack container. It takes a reference to the stack object and swaps elements from these stacks. Notice that, st1 object, which is initialized using the vector object, can’t call the swap function or be an argument to it. In order to enable the container initialized stack to be used with the swap function, it needs to be created using the std::move call, as the st3 object is initialized in the following code example. The latter one also can be passed to the printStack function without any issues. Finally, we swap elements of st2/st3 stack objects and print the results to the console.

#include <deque>
#include <iostream>
#include <stack>
#include <vector>

using std::cout;
using std::deque;
using std::endl;
using std::stack;
using std::vector;

template <typename T>
void printStack(stack<T> s) {
  while (!s.empty()) {
    cout << s.top() << "; ";
    s.pop();
  }
  cout << endl;
}

int main() {
  deque<int> deq1 = {11, 12, 13, 14};
  vector<int> vec1 = {1, 2, 3, 4, 11};
  stack st1{vec1};
  stack<int> st2;

  for (int i = 1; i <= 10; ++i) {
    st2.push(i * 12);
  }

  //    st2.swap(st1); Error
  stack<int> st3{std::move(deq1)};
  printStack(st2);
  printStack(st3);
  st2.swap(st3);
  printStack(st2);
  printStack(st3);

  return EXIT_SUCCESS;
}

Output:

120; 108; 96; 84; 72; 60; 48; 36; 24; 12;
14; 13; 12; 11;
14; 13; 12; 11;
120; 108; 96; 84; 72; 60; 48; 36; 24; 12;