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Assignment Operators

Unlike many other programming languages, C treats value assignment as operation (represented by an operator) rather than instruction.

Simple Assignment Operator

For a common value assignment, a simple assignment operator (=) is used:

expression1 = expression2

The expression1 is an object (memory location) to which the value of expression2 is assigned. Operand expression1 has to be lvalue and expression2 can be any expression. The assignment expression itself is not lvalue.

If expression1 and expression2 are of different types, the result of the expression2 will be converted to the type of expression1, if necessary. Refer to Type Conversions for more information.

Compound Assignment Operators

C allows more comlex assignments by means of compound assignment operators. The syntax of compound assignment operators is:

expression1 op= expression2

where op can be one of binary operators +, -, *, /, %, &, |, ^, <<, or >>.

Thus, we have 10 different compound assignment operators: +=, -=, *=, /=, %=, &=, |=, ^=, <<= and >>=. All of them associate from right to left. Spaces separating compound operators (e.g. + =) will generate error.

Compound assignment has the same effect as

expression1 = expression1 op expression2

except the lvalue expression1 is evaluated only once. For example, expression1 += expression2 is the same as expression1 = expression1 + expression2.

Assignment Rules

For both simple and compound assignment, the operands expression1 and expression2 must obey one of the following rules:

1. expression1 is of qualified or unqualified arithmetic type and expression2 is of arithmetic type.
2. expression1 has a qualified or unqualified version of structure or union type compatible with the type of expression2.
3. expression1 and expression2 are pointers to qualified or unqualified versions of compatible types and the type pointed to by left has all qualifiers of the type pointed to by right.
4. Either expression1 or expression2 is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void. The type pointed to by left has all qualifiers of the type pointed to by right.
5. expression1 is a pointer and expression2 is a null pointer constant.