The output will be 4. Here is how the logic works on your case:
Given :
a = 3
b = 2
Then :
b = a++
which means take a value to b and then increment the a value. so b value is same as a (before the increment), so with that statement, their value become:
b = 3 (same as a before increment)
a = 4 (the value change because we got increment)
Then evaluate the last statement:
++b => this means that we increment the value before we use the value. so in the end.
so,
cout << ++b; // will result: 4
It will produce 4 as output. Let us examine
int a=3;
int b=2;
b=a++; // b=a++; => b = a; a = a + 1 so b = 3
cout <<++b //++b is prefix operator so value of b shall be
// incremented before display
