Where can I write a c++ code

Answer 1

You can use the code playground here in SoloLearn. If you are referring to offline compilers, then you can download IDEs on your desktop/laptop and even your phone/tablet.

https://www.sololearn.com/discuss/335845/?ref=app
https://www.sololearn.com/discuss/1165656/?ref=app
https://www.sololearn.com/discuss/1410647/?ref=app
https://www.sololearn.com/discuss/738719/?ref=app
https://www.sololearn.com/discuss/1646083/?ref=app
https://www.sololearn.com/discuss/608805/?ref=app
https://www.sololearn.com/discuss/701020/?ref=app
https://www.sololearn.com/discuss/410578/?ref=app

Answer 2

In the Sololearn code playground

On the website choose "Code playground"
Click "New code"
Click "HTML/CSS/JS" and choose c++

On Android
Choose {}
Click on +
Choose c++ as language