As per the standard result is undefined. On x86 architecture it will return the value of the EAX register.
EAX register will contain the result of the last executed instruction. Answer depends upon the platform which you were using.
But mostly it returns the result of the last executed instruction
using namespace std;
cout << '\n' << express();
cout << '\n' << donothing();
cout << '\n' << declarevariable();
} OUTPUT: 10 UNPREDICTABLE 9