Simple assignment

In total = 20;, the value 20 is stored in total. In total = a + b;, the right-hand expression is calculated first and its result is converted to the type of total for storage. The left side must be a valid destination; 20 = total; is invalid.

Compound assignment

1. +=: total += 5 adds 5 to total.
2. -=: total -= 5 subtracts 5.
3. *=: total *= 5 multiplies by 5.
4. /=: total /= 5 divides by 5 using the relevant arithmetic types.
5. %=: total %= 5 stores the integer remainder.
6. Bitwise forms: &=, |=, ^=, <<= and >>= combine a bit operation with assignment.

Worked example

Starting with int total = 10;, total += 5; gives 15. Next, total *= 2; gives 30. Finally, total -= 4; gives 26. For a simple variable, total += 5 has the usual effect of total = total + 5. A compound assignment evaluates its left operand only once.

Important points

= assigns; == compares. Assignment operators group from right to left: a = b = 0 stores zero in b and then a, when their types permit it. Do not divide by zero. Check whether a conversion loses a fraction or whether arithmetic exceeds the type range. A const-qualified object cannot be assigned a new value through that object.

Reference table

Key points

• = assigns; == compares.
• The stored result is converted to the destination type.