# Stack in C++ STL

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The **Stack** in the STL of C++ is a dynamically resizing container. Stack class is an container adapter. Containers or container classes store objects and data. There are in total seven standard **"first-class"** container classes and three container adaptor classes and only seven header files that provide access to these containers or container adaptors. **deque** container is provided as a default container.

Stack is a data structure that collects data, but has specific operations, which operates in LIFO (Last in First out) context.Stack elements are inserted as well as get removed from **only one end**.

push() and pop() member functions are used for inserting and removing elements from stack.

### Definition of std::stack from <stack> header file

```
template <class T, class Container = deque<T> > class stack;
```

where,

**T = Type of the element of any type(user-defined / in-built).**

**Container = Type of the underlying container object.**

### Creation of Stack Objects

```
stack <data_type> object_name;
```

### Functions

Assume the stack structure to be :

Assume **s1 & s2** are the two objects of Stack class.

**Current stack : D C B A**

Following fuctions are defined under the **<stack> header** file :

**1. stack::emplace : Function inserts a new element into the stack container, the new element is added on top of the stack.**

For the above stack **E** gets inserted.

**Syntax :**

```
s1.emplace(E);
```

**Time Complexity :**

**Θ(1)**

**Stack after Operation : E D C B A**

**2. stack::empty : Function checks if the stack container is empty or not.**

For the above stack it returns **false**.

**Syntax :**

```
s1.empty(E);;
```

**Time Complexity :**

**Θ(1)**

**3. stack::operator= : Functions assigns new contents to the stack by replacing old ones.**

Above function called will copy all the elements of s1 into s2.

**Syntax :**

```
s2 = s1;
```

**Time Complexity :**

**Θ(n)**

**4. stack::pop() : Function removes top element from the stack and reduces size of stack by one.**

Above function called will pop the **E** element.

**Syntax : **

```
s1.pop();
```

**Time Complexity :**

**Θ(1)**

**Stack after Operation : D C B A**

**4. stack::push() : Funtion inserts new element at the top of the stack and increases size of stack by one.**

Above function called will push the **E** element.

**Syntax :**

```
s1.push();
```

**Time Complexity :**

**Θ(1)**

**Stack after Operation : E D C B A**

**5. stack::size() : Function returns the total number of element present in the stack.**

Above function called will give the size of stack s1 as **5**.

**Syntax :**

```
s1.size();
```

**Time Complexity :**

**Θ(1)**

**Output after Operation: 5**

**6. stack::swap() : Funtion exchanges the contents of the two stacks and changes the size of stacks if necessary.**

Above function called will swap the contents of Stack s1 and Stack s2.

**Syntax :**

```
s1.swap(s2);
```

**Time Complexity :**

**Θ(n)**

**7. stack::top() : Function returns top element of the stack.**

Above function called returns the top element present on the stack.

**Syntax :**

```
s1.top();
```

**Time Complexity :**

**Θ(1)**

**Output after Operation: E**

### Note

**In stack container, the elements are printed in reverse order because the top is printed first then moving on to other elements.**

### Implementation

```
#include <iostream>
#include <stack>
void displayStack(std::stack <int> s)
{
while (!s.empty())
{
std::cout << s.top() << '\n';
s.pop();
}
}
int main()
{
std::stack <int> s1;
s1.push(50);
s1.push(40);
s1.push(30);
s1.push(20);
s1.push(10);
std::cout << "\nS1 stack : \n";
displayStack(s1);
std::cout << "\nS1 size() : " << s1.size();
std::cout << "\nS1 top() : " << s1.top();
std::cout << "\nS1 pop() operation done.";
s1.pop();
std::cout << "\nS1 stack (after pop): \n";
displayStack(s1);
std::cout << "\nS1 empty() : ";
bool t_ = s1.empty();
std::cout << t_;
std::cout << "\nS1 emplace() operation done.";
s1.emplace(10);
std::cout << "\nS1 stack (after emplacing element : 10): \n";
displayStack(s1);
std::stack <int> s2;
s2.push(500);
s2.push(400);
s2.push(300);
s2.push(200);
s2.push(100);
std::cout << "\nS2 stack : \n";
displayStack(s2);
std::cout << "\nS2 size() : " << s2.size();
s2 = s1;
std::cout << "\nS2 stack (using = operator overloading): \n";
displayStack(s2);
std::stack <int> s3;
s3.push(55);
s3.push(44);
s3.push(33);
s3.swap(s1);
std::cout << "\nStack s1 Elements after swapping : \n";
displayStack(s1);
std::cout << "\nStack s3 Elements after swapping : \n";
displayStack(s3);
}
```

Output:

```
S1 stack :
10
20
30
40
50
S1 size() : 5
S1 top() : 10
S1 pop() operation done.
S1 stack (after pop):
20
30
40
50
S1 empty() : 0
S1 emplace() operation done.
S1 stack (after emplacing element : 10):
10
20
30
40
50
S2 stack :
100
200
300
400
500
S2 size() : 5
S2 stack (using = operator overloading):
10
20
30
40
50
Stack s1 Elements after swapping :
33
44
55
Stack s3 Elements after swapping :
10
20
30
40
50
```