How does Multiple C++ Threads execute on a class method

Question

let's say we have a c++ class like:

class MyClass
{
   void processArray( <an array of 255 integers> )
   {
     int i ;
     for (i=0;i<255;i++)
     {
        // do something with values in the array
     }
   }
}

and one instance of the class like:

MyClass myInstance ;

and 2 threads which call the processArray method of that instance (depending on how system executes threads, probably in a completely irregular order). There is no mutex lock used in that scope so both threads can enter.

My question is what happens to the i ? Does each thread scope has it's own "i" or would each entering thread modify i in the for loop, causing i to be changing weirdly all the time.

Answer 1

i is allocated on the stack. Since each thread has its own separate stack, each thread gets its own copy of i.