The compound-assignment operators combine the simple-assignment operator with another binary operator. Compound-assignment operators perform the operation specified by the additional operator, then assign the result to the left operand. For example, a compound-assignment expression such as
expression1 += expression2
can be understood as
expression1 = expression1 + expression2
However, the compound-assignment expression is not equivalent to the expanded version because the compound-assignment expression evaluates expression1 only once, while the expanded version evaluates expression1 twice: in the addition operation and in the assignment operation.
The operands of a compound-assignment operator must be of integral or floating type. Each compound-assignment operator performs the conversions that the corresponding binary operator performs and restricts the types of its operands accordingly. The addition-assignment (+=) and subtraction-assignment (–=) operators can also have a left operand of pointer type, in which case the right-hand operand must be of integral type. The result of a compound-assignment operation has the value and type of the left operand.
#define MASK 0xff00
n &= MASK;
In this example, a bitwise-inclusive-AND operation is performed on n and MASK, and the result is assigned to n. The manifest constant MASK is defined with a #define preprocessor directive.