C++ Playground Guide

Open the C++ Playground to compile and run small C++20 programs without installing a toolchain.

Read the default result

The initial program sums {2, 4, 6, 8} with a range-based for and prints sum: 20. Clang warnings and errors appear before the program output. Because the runner passes -fno-exceptions, avoid throw, try, and standard-library calls that rely on exceptions in this environment.

Find prime numbers

Choose Find prime numbers. It prints:

Primes: 2 3 5 7 11 13 17 19 23 29

The predicate rejects integers below 2, then tests divisors up to d * d <= n. Any larger factor would have a matching smaller factor. Change n <= 30 to extend the range.

Classes and containers

OOP: bank account defines a class with deposit and withdraw methods and a private balance_ field. Depositing 100 and withdrawing 30 prints Balance: 70. Examples also use std::vector, std::string, std::map and std::sort for sorting, binary search and letter counting. These containers compile without exceptions as long as you do not depend on throwing paths like at() or std::stoi on invalid input.

Prepare input and read it

The Read standard input example fills the input field with Ilham:

#include <iostream>
#include <string>

int main() {
    std::string name;
    if (!std::getline(std::cin, name)) {
        std::cout << "No input provided.\n";
        return 1;
    }
    std::cout << "Hello, " << name << "!\n";
    return 0;
}

It prints Hello, Ilham!. std::getline reads through the newline and removes it. Provide another line for each additional read. If the program reports no input, add the missing answer and Run again.

Star patterns and errors

Centered pyramid prints:

    *
   ***
  *****
 *******
*********

It builds each row from std::string(height - row, ' ') and std::string(2 * row - 1, '*'). Copy output preserves the spacing. Other examples cover arrays, sorting, binary search, letter frequency and hollow or inverted pyramids.

Syntax errors, missing includes and template errors come from Clang and appear in the output. Stop terminates a stuck program. Runtime assets are fetched from jsDelivr; an unavailable CDN prevents execution, not editing. The plain editor works without Monaco, and no C++ formatter is bundled.

References

Questions you might have

How do I read input?: Fill Standard input before Run, one answer per line, then use std::getline or std::cin. std::getline drops the trailing newline for you. If the stream is empty, getline returns false. This is prepared input, not a live terminal.
Why does the first Run take longer?: The Clang compiler, linker and sysroot download from jsDelivr on the first Run — roughly 26 MiB compressed. Runtime loading has a 90-second timeout, followed by a separate 30-second execution limit. Later Runs in the same page reuse the loaded toolchain.
Can I use exceptions, threads or the full standard library?: No. The WASI libc++ is built without exceptions or threads, and this runner compiles with -fno-exceptions. Code that relies on throwing or catching will not link. Most of the standard library for console programs, including string, vector and map, works.
Does selecting an example erase my code?: Yes. C++ examples are complete programs with their own main, so clicking one replaces the editor and pauses Auto-run. Copy anything you want to keep first.
Is this a security sandbox?: No. A Worker prevents direct DOM access and can be terminated, but compiled code can make network requests through WASI. The tool itself does not upload or persist code or input. Each Run starts fresh; output is limited to 500 rows.