How to add element to C++ array?

There is no way to do what you say in C++ with plain arrays. The C++ solution for that is by using the STL library that gives you the std::vector.

You can use a vector in this way:

#include <vector>

std::vector< int > arr;

arr.push_back(1);
arr.push_back(2);
arr.push_back(3);