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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.
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";
}sort is O(n log n) and works on any random-access range; max_element returns an iterator, so dereference it.
#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"; }The : w(w), h(h) part is a member initialiser list — it initialises members before the constructor body runs.
error: 'cout' was not declared in this scope
Why: Missing include or namespace.
Fix: Add #include <iostream> and either using namespace std; or write std::cout.
undefined reference to `main'
Why: No main function was found.
Fix: Every program needs int main() { }.
no matching function for call to ...
Why: Argument types do not match any overload.
Fix: Read the first candidate line in the error — it shows the expected parameter types.
| Concept | Syntax |
|---|---|
| Include everything | #include <bits/stdc++.h> |
| Vector | vector<int> v = {1, 2, 3}; |
| Map | map<string, int> m; m["a"] = 1; |
| Sort | sort(v.begin(), v.end()); |
| Range for | for (auto &x : v) { } |
| Fast IO | ios::sync_with_stdio(false); |
Yes — the GCC-style catch-all header works, which is handy for competitive programming practice.
Yes: vector, map, set, queue, stack, algorithm and the rest of the standard library are all usable.
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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