C++ STL - Cheat Sheet

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This C++ STL cheatsheet covers a wide range of topics from basic STL like vectors, hashmaps, sets, etc., to advanced concepts like functors, iterators, and so on. It is designed for programmers who want to quickly read through key concepts along with the syntax and related examples.

Introduction

The Standard Template Library (STL) is a C++ library that contains all class and function templates. It is used for implementing the basic Data Structures (like Hashset, Heap, List, etc.) and functions (like math, algorithms, etc.), and contains all containers available in C++ programming language.

Components of STL

Standard Template Library contains of several containers and functions, and can be categorized in the following manner −

• Containers
• Functors
• Algorithms
• Iterators

STL Containers

The STL container template class contains data structures like Dynamic Arrays (or Vectors, Lists), Hashmaps, Hashsets, Trees, Linked Lists, etc. These are used for storing and performing operations on the data.

The container template has the following components −

• Sequential Containers
• Container Adapters
• Associative Containers
• Unordered Containers

Each container has its respective header file, which can be included at the start of the program. For example, the std::vector is included in the #include<vector> library.

Sequential Containers

The sequential containers implement the data structures with sequential access. These include −

• Vector
• List
• Deque
• Array
• Forward List

Container Adapters

The container adapters implement data structures like queues, stacks, etc by providing different interfaces for sequential containers. These include −

• Stack
• Queue
• Priority Queue

Associative Containers

The associative containers are used to store ordered data that can be quickly searched using their key value. These include −

• Set
• Multiset
• Map
• Multimap

Unordered Containers

The unordered containers are similar to associative containers but the only difference is that they don’t store sorted data. Still, these containers provide quick search time using key-value pairs. They are −

• Unordered Set
• Unordered Multiset
• Unordered Map
• Unordered Multimap

Now that we have established a learning graph of all containers, we will briefly explain each container in the STL with an exemplar code −

Vector in STL

The vector is initialized as a dynamic array at runtime, and its size is variable. It can be found in the C++ <vector> header file.

Syntax

vector<data_type> vec1; //1D vector
vector<vector<data_type>> vec2; //2D vector
vector<container_type<data_type>> vec3; //2D vector of other container
vector<data_type> vec1(n); //vector of size n
vector<data_type> vec1(n,k); // vector of size n, with each element=k

There are different functions in the vector template. These are explained briefly in the table below −

Here, TC indicates time complexity of different member functions of the vector template. For more information on time complexity, visit this article − Time complexity.

Example

// C++ program to illustrate the vector container
#include <iostream>
#include <vector>
using namespace std;

int main(){
   int n=2;
   vector<int> vec1 = { 1, 2, 3, 4, 5 };
   vector<int> vec2(n,0);

   vector<vector<int>> vec3(n,vector<int>(2*n,1));

   vec1.push_back(6);

   cout << "vec1: ";
   for (int i = 0; i < vec1.size(); i++) {
      cout << vec1[i] << " ";
   }
   cout << endl<<"vec2: ";
   for (int i = 0; i < vec2.size(); i++) {
      cout << vec2[i] << " ";
   }
   cout << endl;

   vec1.erase(vec1.begin() + 4);

   cout << "vec1 after erasing: ";
   for (auto i = vec1.begin(); i != vec1.end(); i++) {
      cout << *i << " ";
   }
   cout << endl;

   cout << "vec3:-" << endl;
   for (auto i : vec3) {
      for (auto j : i) {
         cout << j << " ";
      }
      cout << endl;
   }
   cout << endl;

   vector<pair<int,int>> vec4;
   vec4.push_back({2,3});
   vec4.push_back({4,3});

   cout<<"vector of pairs, vec4 : "<<endl;
   for(auto i: vec4){
      cout<<i.first<<" "<<i.second<<endl;
   }
   return 0;
}

Output

vec1: 1 2 3 4 5 6 
vec2: 0 0 
vec1 after erasing: 1 2 3 4 6 
vec3:-
1 1 1 1 
1 1 1 1 

vector of pairs, vec4 : 
2 3
4 3

List in STL

The list container is initialized as a doubly linked list, whereas for implementing a singly linked list, we use a forward_list. It can be found in the C++ <list> header file.

Syntax

list<data_type> list1;

There are different functions in the list template. These are explained briefly in the table below −

Example

#include <iostream>
#include <list>
#include <vector>
using namespace std;

int main(){
   list<int> list1 = { 1, 5, 9, 1, 4, 6 };

   cout << "List1 first and last element: " << list1.front() <<" "<<list1.back()<<endl;

   // adding element
   list1.insert(list1.begin(), 5);

   // deleting element
   list1.erase(list1.begin());

   // traversing list1
   cout << "list1: ";
   for (auto i = list1.begin(); i != list1.end(); i++) {
      cout << *i << " ";
   }
   cout << endl;

   return 0;
}

Output

List1 first and last element: 1 6
list1: 1 5 9 1 4 6 

Deque in STL

The deque container is initialized as a doubly ended queue, where elements can be pushed and popped from both ends of the queue. It can be found in the C++ <deque> header file.

Syntax

deque<data_type> dq1;

There are different functions in the deque template. These are explained briefly in the table below −

Example

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

int main(){

   deque<int> dq = { 1, 2, 3, 4, 5 ,6, 8 };
   cout<<"Initial Deque: "<<endl;
   for (auto i : dq) {
      cout << i << " ";
   }
   cout<<endl;
   dq.push_front(dq.back());
   dq.pop_back();
   for (auto i : dq) {
      cout << i << " ";
   }
   cout<<endl;

   dq.push_front(dq.back());
   dq.pop_back();
   for (auto i : dq) {
      cout << i << " ";
   }
   cout<<endl;

   dq.pop_back();
   dq.pop_front();
   for (auto i : dq) {
      cout << i << " ";
   }
   cout<<endl;

   dq.push_front(11);
   dq.push_back(99);

   for (auto i : dq) {
      cout << i << " ";
   }

   return 0;
}

Output

Initial Deque: 
1 2 3 4 5 6 8 
8 1 2 3 4 5 6 
6 8 1 2 3 4 5 
8 1 2 3 4 
11 8 1 2 3 4 99 

Stack in STL

The stack container is initialized as a LIFO container, where elements can be pushed to the top, and popped from the top. Hence, the last element to enter is the first one to exit from the container. It can be found in the C++ <stack> header file.

Syntax

stack<data_type> s1;

There are different functions in the stack template. These are explained briefly in the table below −

Example

// C++ Program to illustrate the stack
#include <bits/stdc++.h>
using namespace std;

int main(){
   stack<int> s;

   s.push(2);
   s.push(9);
   s.push(3);
   s.push(1);
   s.push(6);

   cout << "Top is: " << s.top() << endl;

   while (!s.empty()) {
      cout<<"size is: "<<s.size()<<" ";
      cout << "element is: "<<s.top() << endl;
      s.pop();
   }
   return 0;
}

Output

Top is: 6
size is: 5 element is: 6
size is: 4 element is: 1
size is: 3 element is: 3
size is: 2 element is: 9
size is: 1 element is: 2

Queue in STL

The queue container is initialized as a FIFO container, where elements can be pushed to the front, and popped from the front. Hence, the first element to enter is the first one to exit from the container. It can be found in the C++ <queue> header file.

Syntax

queue<data_type> q1;

There are different functions in the queue template. These are explained briefly in the table below −

Example

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

int main(){
   queue<int> q;
   q.push(1);
   q.push(1);
   q.push(6);
   q.push(1);

   cout << "Front element: " << q.front() << endl;
   cout << "Back element: " << q.back() << endl;

   cout << "q: ";
   int size = q.size();

   for (int i = 0; i < size; i++) {
      cout << q.front() << " ";
      q.pop();
   }

   return 0;
}

Output

Front element: 1
Back element: 1
q: 1 1 6 1 

Hash-Set/Set in STL

The set container is initialized as unique element storage data structure, which also implements a sorted order, both ascending and descending. It generally implements a red-black tree as an underlying data structure. It can be found in the C++ <set> header file.

Syntax

set<data_type> set;
set<data_type, greater<data_type>> set2; //this is a set in descending order
set<data_type, comparator/lambda_function> set3; //this is a set in custom order

There are different functions in the set template. These are explained briefly in the table below −

Example

#include <iostream>
#include <set>
#include <vector>
using namespace std;

int main(){
   set<int> set;
   set.insert(9);
   set.insert(11);
   set.insert(9);
   set.insert(11);

   bool flag=set.find(9)!=set.end();

   cout<<"Is there a 9 in the set? "<<flag<<endl;

   set.insert(21);

   for (auto i : set) {
      cout << i << " ";
   }
   cout << endl;
   return 0;
}

Output

Is there a 9 in the set? 1
9 11 21

Hash-Map/Map in STL

The map is a container which stores data in form of key-value pairs in a sorted order of keys, where each key is unique. It is implemented using the red-black tree data structure. It is included in the <map> header file.

Syntax

map<key_type,value_type> map;

There are different functions in the map template. These are explained briefly in the table below −

Example

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main(){
   map<char,int> map;
   string s="abbacdbbac";
   for(char c: s){
      map[c]++;
   }

   for(auto it:map){
      cout<<it.first<<" : "<<it.second<<endl;
   }
   cout<<"after erasing element 'c' : "<<endl;
   map.erase('c');
   for(auto it:map){
      cout<<it.first<<" : "<<it.second<<endl;
   }
   return 0;
}

Output

a : 3
b : 4
c : 2
d : 1
after erasing element 'c' : 
a : 3
b : 4
d : 1

Unordered Set in STL

The unordered_set container stores unique data values in it, but the only difference from set is that the data is not stored in any order. It is implemented using the <unordered_set> header file.

Syntax

unordered_set<data_type> set;

There are different functions in the unordered_set template. These are explained briefly in the table below −

Example

#include <iostream>
#include <unordered_set>
#include <vector>
using namespace std;

int main(){
   unordered_set<int> set;
   set.insert(19);
   set.insert(10);
   set.insert(13);
   set.insert(21);

   bool flag=set.find(9)!=set.end();

   cout<<"Is there a 9 in the set? "<<flag<<endl;

   set.insert(21);

   for (auto i : set) {
      cout << i << " ";
   }
   cout << endl;
   return 0;
}

Output

Is there a 9 in the set? 0
21 13 10 19 

Unordered Map in STL

In the unordered_map container, the data is stored similar to the map container, but the order of the data stored is random, hence, ascending or descending order is not followed. It is included in the <unordered_map> header file.

Syntax

unordered_map<key_type, value_type> map;

There are different functions in the unordered_map template. These are explained briefly in the table below −

Example

#include <iostream>
#include <unordered_map>
#include <string>
using namespace std;

int main(){
   unordered_map<char,int> map;
   string s="abbacdbbac";
   for(char c: s){
      map[c]++;
   }

   for(auto it:map){
      cout<<it.first<<" : "<<it.second<<endl;
   }
   cout<<"after erasing element 'c' : "<<endl;
   map.erase('c');
   for(auto it:map){
      cout<<it.first<<" : "<<it.second<<endl;
   }
   return 0;
}

Output

d : 1
c : 2
b : 4
a : 3
after erasing element 'c' : 
d : 1
b : 4
a : 3

Functors in C++ STL

Function objects or Functors, are objects in C++ STL that behave like methods/functions. These are included in the <functional> header file. These require the overloading of the () parenthesis operator.

The main functors in C++ STL are as follows −

• equal_to
• not_equal_to
• greater
• less
• plus
• minus

Member functions

Non-member functions

Operator classes

Example

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

int main(){
   equal_to<int> obj1;
   not_equal_to<int> obj2;
   greater<int> obj3;
   less<int> obj4;
   plus<int> obj5;
   minus<int> obj6;

   cout << "Functors and their usage: \n";
   cout << "Are these equal? " << obj1(11, 22) << endl;
   cout << "Are these different? " << obj2(11, 22) << endl;
   cout << "Is first index greater than second? " << obj3(10, 20) << endl;
   cout << "Is first index smaller than second? " << obj4(10, 2) << endl;
   cout << "After adding: " << obj5(10, 20) << endl;
   cout << "After subtracting: " << obj6(10, 8) << endl;

   return 0;
}

Output

Functors and their usage: 
Are these equal? 0
Are these different? 1
Is first index greater than second? 0
Is first index smaller than second? 0
After adding: 30
After subtracting: 2

Algorithms in C++ STL

In C++ STL, algorithms template provides a variety of operations for the user, which are crucial for implementation of key algorithms like searching, sorting, returning maximum/minimum value from a container, and so on. These algorithms can be accessed using the <algorithm> header.

The <algorithm> library in C++ offers many useful functions. Some of the most commonly used functions are given below −

• Sort
• Copy
• Find
• Max Element and Min Element
• For Each
• Swap

sort() in C++

sort() algorithm is used to sort the given data in ascending, descending or custom order (using comparator or lambda function).

Syntax

sort(start_iterator,end_iterator)
sort(start,end, comparator_function) //for custom sorting

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
using namespace std;

int main(){
   vector<int> v = {999, 1252, 3117, 122222 , 10, 88, 2, 9, 45, 82, 546, 42, 221 , -1};
   cout<<"Ascending order: ";
   sort(v.begin(),v.end());
   for(int i:v) cout<<i<<" ";
      cout<<endl;

   cout<<"Descending order: ";
   sort(v.begin(),v.end(),greater<int>());
   for(int i:v) cout<<i<<" ";
      cout<<endl;
   return 0;
}

Output

Ascending order: -1 2 9 10 42 45 82 88 221 546 999 1252 3117 122222 
Descending order: 122222 3117 1252 999 546 221 88 82 45 42 10 9 2 -1 

copy() in C++

copy() algorithm is used to copy the elements from one container to another.

Syntax

copy(start_iterator,end_iterator, destination_operator)

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
using namespace std;

int main(){
   vector<int> v = { 1, 2, 3, 4, 5 , 4 ,3 , 2, 1};
   vector<int> newvec(9);
   copy(v.begin(), v.end(), newvec.begin());

   for(int &i:newvec) cout<<i<<" ";
   cout<<endl;
   return 0;
}

Output

1 2 3 4 5 4 3 2 1 

find() in C++

find() algorithm is used to find a key element in a given range of elements.

Syntax

find (firstIterator, lastIterator, value);

Example

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main(){
   vector<int> v= { 1,3,5,2,3,1,55,41};

   // finding 5
   auto itr = find(v.begin(), v.end(), 55);
   if (itr != v.end()) {
      cout << *itr << " Element found !!!" << endl;
   }else {
      cout << "Element not present !!!" << endl;
   }

   return 0;
}

Output

55 Element found !!!

max_element() and min_element() in C++

max_element() and min_element() are used to find the maximum and minimum value in a given range of elements.

Syntax

max_element (firstIterator, lastIterator);
min_element (firstIterator, lastIterator);

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
using namespace std;

int main(){
   vector<int> v = {999, 1252, 3117, 122222 , 10, 88, 2, 9, 45, 82, 546, 42, 221 , -1};

   cout << "Maximum Element: " << *max_element(v.begin(),v.end())<<endl;
   cout<<"Minimum Element: "<<*min_element(v.begin(),v.end()) <<"\n";
   return 0;
}

Output

Maximum Element: 122222
Minimum Element: -1

for_each in C++

for_each() algorithm is used to apply a given operation or instruction to a range of elements in a container.

Syntax

for_each (firstIterator, lastIterator, unaryFunction);

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
using namespace std;

int main(){
   vector<int> vec1 = { 1, 2, 3, 4, 5 };

   for_each(vec1.begin(), vec1.end(), [](int& i){
      i++;
   });

   for(int i:vec1) cout<<i<<" ";
      cout<<endl;

   return 0;
}

Output

2 3 4 5 6 

swap() in C++

swap() algorithm is used to replace one element with another in place, hence the elements swap places.

Syntax

swap(container1,container2)

Example

#include <algorithm>
#include <vector>
#include <iostream>
using namespace std;

int main() {

   vector<int> vec1 = {1, 2, 3};
   vector<int> vec2 = {4, 5, 6};

   swap(vec1,vec2);
   cout<<"vec 1: "<<endl;
   for(int i:vec1) cout<<i<<" ";
      cout<<endl;

   cout<<"vec 2: "<<endl;
   for(int i:vec2) cout<<i<<" ";
      cout<<endl;
   return 0;
}

Output

vec 1: 
4 5 6 
vec 2: 1 2 3 

Iterators in C++ STL

Iterators can be considered as pointers that are used to iterate over containers sequentially. These iterators are included using the <iterator> header file in C++.

Each container has its own iterator. For avoiding confusion, we can use ‘auto’ keyword to define an iterator while traversing a container.

Iterators are of five types −

• Input Iterator
• Output Iterator
• Forward Iterator
• Bi-Directional Iterator
• Random Iterator

Example

#include <bits/stdc++.h>
using namespace std;

int main(){
   vector<int> myvec={1,5,2,4,3,6,6,9,8};
   set<int>set(myvec.begin(),myvec.end());

   for(auto itr:myvec) cout<<itr<<" ";
      cout<<endl;
   for(auto itr:set) cout<<itr<<" ";
      cout<<endl;

   return 0;
}

Output

1 5 2 4 3 6 6 9 8 
1 2 3 4 5 6 8 9