Why is order of expressions in if statement important

Question

Suppose I have an IF condition :

if (A || B) 
    ∧
    |
    |
   left
{ 
   // do something  
}

Now suppose that A

Answer 1

I put it on the left ... what am I gaining ? run time ?

Because || operator in C++ uses short-circuit evaluation.
i.e: B is evaulated only if A is evaluated to a false.

However, note that in C++ short-circuit evaluation is guaranteed for "built in" data types and not custom data types.