C++ Associativity

When reading c++ code, find interesting.
If input 0 then num1 is 5 else num1 is input number.

#include    

     int
 main ( )
 {
     int num1;
     std::cin >> num1;
     num1 || (num1 = 5);
     std::cout << num1 << std::endl;
     return 0;
 }

If input 0 then num1 is 0 else num1 is always 5.

#include    

     int
 main ( )
 {
     int num1;
     std::cin >> num1;
     num1 && (num1 = 5);
     std::cout << num1 << std::endl;
     return 0;
 }

interesting.