An assignment operation assigns the value of the right-hand operand to the storage location named by the left-hand operand. Therefore, the left-hand operand of an assignment operation must be a modifiable l-value. After the assignment, an assignment expression has the value of the left operand but is not an l-value.
assignment-expression :
conditional-expression
unary-expression assignment-operator assignment-expression
assignment-operator : one of
= *= /= %= += –= <<= >>= &= ^= |=
The assignment operators in C can both transform and assign values in a single operation. C provides the following assignment operators:
| Operator | Operation Performed |
| = | Simple assignment |
| *= | Multiplication assignment |
| /= | Division assignment |
| %= | Remainder assignment |
| += | Addition assignment |
| –= | Subtraction assignment |
| <<= | Left-shift assignment |
| >>= | Right-shift assignment |
| &= | Bitwise-AND assignment |
| | = | Bitwise-inclusive-OR assignment |
| ^= | Bitwise-exclusive-OR assignment |
In assignment, the type of the right-hand value is converted to the type of the left-hand value, and the value is stored in the left operand after the assignment has taken place. The left operand must not be an array, a function, or a constant. The specific conversion path, which depends on the two types, is outlined in detail in “Type Conversions”.
The simple-assignment operator assigns its right operand to its left operand. The value of the right operand is converted to the type of the assignment expression and replaces the value stored in the object designated by the left operand. The conversion rules for assignment apply (see “Assignment Conversions”).
double x;
int y;
x = y;
In this example, the value of y is converted to type double and assigned to x.
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 may 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. (For more information, see “The #define Directive”.)