Compound Assignment Operators

We often apply an operator to an object and then assign the result to that same object. As an example, consider the sum program from § 1.4.2 (p. 13):

int sum = 0;
// sum values from 1 through 10 inclusive
for (int val = 1; val <= 10; ++val)
    sum += val; //  equivalent to sum = sum + val

This kind of operation is common not just for addition but for the other arithmetic operators and the bitwise operators, which we cover in § 4.8 (p. 152). There are compound assignments for each of these operators:

 +=   -=   *=   /=   %=         // arithmetic operators
<<=  >>=   &=   ^=   |=         // bitwise operators; see § 4.8 (p. 152)

Each compound operator is essentially equivalent to

a = a op b;

with the exception that, when we use the compound assignment, the left-hand operand is evaluated only once. If we use an ordinary assignment, that operand is evaluated twice: once in the expression on the right-hand side and again as the operand on the left hand. In many, perhaps most, contexts this difference is immaterial aside from possible performance consequences.