The updated "question" is still not a question.
Quote:
i want to write code in visual c++ to make an alram in cross web platform
It is uncommon to write web applications in C++ because there are very few libraries for it. You may however use any web development framework provided it has a C++ interface or you implement it yourself.
But the main problem - when it should run on different platforms - is that C/C++ applications use system specific library implementations (the CRT containing the startup code and the standard C and C++ library implementations). So you have to build such an application for different target platforms. That means, while your sources might be cross platform, the generated executable is not.