Here is an example of how to add two variables in C++:

#include <iostream>

int main() {
  int x = 5;
  int y = 7;
  int sum = x + y;

  std::cout << "The sum of x and y is: " << sum << std::endl;

  return 0;
}

This code defines two variables x and y and assigns them the values 5 and 7, respectively. It then calculates the sum of x and y and stores the result in a third variable sum. Finally, it prints the value of sum to the console using the cout stream from the iostream library.