C++ Online Compiler

About the C++ online compiler

C++ gives you C's control plus classes, templates and the Standard Template Library. It is the default language of competitive programming because STL containers and algorithms let you write correct solutions quickly. This online C++ compiler builds and runs your program with the standard library available.

Practise vectors, maps, sorting, iterators, classes with constructors and destructors, references versus pointers, and templates. Compiler errors in C++ are long — the guide below shows how to read the first line and ignore the noise.

• →Competitive programming practice with STL containers
• →Learning object-oriented C++ for university coursework
• →Testing sort/lower_bound/accumulate behaviour
• →Comparing pass-by-value against pass-by-reference

C++ sample programs you can run right now

Copy any snippet into the editor above and press Run. Each one is short on purpose — retype it from memory afterwards.

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

int main() {
  vector<int> v = {42, 7, 91, 13};
  sort(v.begin(), v.end());
  for (int x : v) cout << x << " ";
  cout << "\nMax: " << *max_element(v.begin(), v.end()) << "\n";
}

#include <iostream>
using namespace std;

class Rect {
  int w, h;
public:
  Rect(int w, int h) : w(w), h(h) {}
  int area() const { return w * h; }
};

int main() { cout << Rect(4, 5).area() << "\n"; }

Common C++ errors and how to fix them

C++ syntax cheatsheet

C++ compiler FAQs

Keep going after the compiler

Running snippets builds speed; the 17-chapter course builds understanding. Work through the C++ chapters, take the quiz, then claim your certificate.

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C++ tutorial: five steps from blank editor to working program

This walkthrough covers the practical C++ subset used in coursework and competitive programming: iostream, vectors and strings, references, structs and classes, and the STL algorithms.

1. 1. iostream instead of printf

   cin and cout handle types automatically, so there are no format specifiers to get wrong. '\n' is cheaper than endl because endl also flushes.

   #include <iostream>
   using namespace std;
   
   int main() {
       int marks = 87;
       cout << "Marks: " << marks << "\n";
       return 0;
   }

2. 2. Prefer vector and string over raw arrays

   vector grows on demand and knows its own size; string handles memory for you. Together they remove most memory bugs beginners hit in C.

   #include <iostream>
   #include <vector>
   using namespace std;
   
   int main() {
       vector<int> v = {5, 3, 9};
       v.push_back(1);
       for (int x : v) cout << x << " ";
       cout << "\nsize: " << v.size() << "\n";
       return 0;
   }

3. 3. Pass by reference to avoid copies

   A & parameter lets the function work on the caller's object. Add const when the function only reads it — that is the standard signature for large containers.

   #include <iostream>
   #include <vector>
   using namespace std;
   
   int sum(const vector<int> &v) {
       int t = 0;
       for (int x : v) t += x;
       return t;
   }
   
   int main() { cout << sum({1, 2, 3}) << "\n"; }

4. 4. Group data with structs and classes

   struct and class differ only in default access. Constructors initialise members, and member functions keep the related logic beside the data.

   #include <iostream>
   using namespace std;
   
   struct Student {
       string name;
       int marks;
       bool passed() const { return marks >= 50; }
   };
   
   int main() {
       Student s{"Diya", 72};
       cout << s.name << " " << s.passed() << "\n";
   }

5. 5. Let the STL do the work

   sort, find, count, accumulate and max_element cover most of what you would otherwise hand-write, and they are already optimised.

   #include <iostream>
   #include <vector>
   #include <algorithm>
   using namespace std;
   
   int main() {
       vector<int> v = {4, 1, 9, 2};
       sort(v.begin(), v.end());
       cout << *max_element(v.begin(), v.end()) << "\n";
       for (int x : v) cout << x << " ";
   }

Practice exercises with solutions

Try each one in the editor above before opening the solution — the struggle is where the learning happens.

#include <iostream>
using namespace std;

int main() {
    int a, b;
    cin >> a >> b;
    cout << a + b << " " << a - b << " " << a * b << "\n";
}

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

int main() {
    vector<pair<string,int>> v = {{"Aarav",82},{"Diya",91}};
    sort(v.begin(), v.end(), [](auto &a, auto &b){ return a.second > b.second; });
    for (auto &p : v) cout << p.first << " " << p.second << "\n";
}

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

int main() {
    string text = "code play code learn";
    istringstream in(text);
    map<string,int> freq;
    string w;
    while (in >> w) freq[w]++;
    for (auto &p : freq) cout << p.first << ": " << p.second << "\n";
}

Why learn C++?

C++ gives you C-level control plus containers and algorithms, which is why competitive programmers and game engineers pick it.

Learning it teaches both manual resource management and modern abstractions, so moving to Rust, Java or C# afterwards is straightforward.

What C++ is used for

• →Game engines and graphics (Unreal, Godot)
• →Competitive programming and Olympiad work
• →High-frequency trading and simulation software
• →Desktop applications and browser internals

Ready for structured practice? The 17-chapter C++ course takes these ideas one at a time, with a quiz and a certificate at the end.