Assignment operators assign the value
of the right operand to the object designated by the left operand.
Syntax |
 |
assignment-expression ::=
conditional-expression
unary-expression assignment-operator assignment-expression
assignment-operator ::= one selected from the set
= *= /= %= += -= <<= >>= &= ^= |= |
Description |
|
Each assignment operator must have a modifiable lvalue as
its left operand. An assignment operator stores a value into the
left operand. The C language does not define the order of evaluation
of the left and right operands. For this reason, you should avoid
operations with side effects (such as
or ) if their
operands appear on both the left and right side of the assignment.
For example, you should not write an expression like the following
because the results depend on which operand is evaluated first.
Simple Assignment |
|
In simple assignment, the value of the right operand replaces
the value of the object specified by the left operand. If the source
and destination objects overlap storage areas, the results of the
assignment are undefined.
The left and right operands can be any of the following combinations:
Both arithmetic
If both of the operands are arithmetic types, the type of
the right operand is converted to the type of the left operand.
The converted value is then stored in the location specified by
the left operand.
Both structure/union
If both operands are structures or unions of the same type,
the right structure/union is copied into the left structure/union.
A union is considered to be the size of the largest member of the
union, and it is this number of bytes that is moved.
Left operand is a pointer to type T
In this case, the right operand can also be a pointer to type
T. The right
operand is then copied to the left operand.
The right operand can also be a null pointer constant or a
pointer to void.
A special case of pointer assignment involves the assignment
of a pointer to void
to another pointer. No cast is necessary to convert a "pointer to
void" to any
other type of pointer.
An assignment is not only an operation, it is also an expression.
Each operand must have an arithmetic type consistent with those
allowed by the binary operator that is used to make up the assignment
operator. You can use the +=
and -= operators
with a left operand that is a pointer type.
Compound Assignment |
|
Given the general assignment operator op=,
if used in the expression
the result is equal to the following assignment
except that the expression represented by A
is evaluated only once.
Therefore,
is very different from
because the latter statement causes the function f()
to be invoked twice.
Assignment operators are useful to reference complex subscript
operators. For example:
In this case, the subscript expression is evaluated only once.
Examples |
|
a <<= 1
Left shift a by 1 bit. |
