Increment/decrement operators

 

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Increment/decrement operators are unary operators that increment/decrement the value of a variable by 1.

They can have postfix form:

expr ++
expr --

As well as the prefix form:

++ expr
-- expr

The operand expr of both prefix and postfix increment or decrement must be a modifiable lvalue of integer type (including _Bool and enums), real floating type, or a pointer type. It may be cvr-qualified, unqualified, or atomic.

The result of the postfix increment and decrement operators is the value of expr.

The result of the prefix increment operator is the result of adding the value 1 to the value of expr: the expression ++e is equivalent to e += 1. The result of the prefix decrement operator is the result of subtracting the value 1 from the value of expr: the expression --e is equivalent to e -= 1.

Increment operators initiate the side-effect of adding the value 1 of appropriate type to the operand. Decrement operators initiate the side-effect of subtracting the value 1 of appropriate type from the operand. As with any other side-effects, these operations complete at or before the next sequence point.

int a = 1;
int b = a++; // stores 1+a (which is 2) to a
             // returns the old value of a (which is 1)
             // After this line, b == 1 and a == 2
a = 1;
int c = ++a; // stores 1+a (which is 2) to a
             // returns 1+a (which is 2)
             // after this line, c == 2 and a == 2

Post-increment or post-decrement on any atomic variable is an atomic read-modify-write operation with memory order memory_order_seq_cst.

(since C11)

See arithmetic operators for limitations on pointer arithmetic, as well as for implicit conversions applied to the operands.

Notes

Because of the side-effects involved, increment and decrement operators must be used with care to avoid undefined behavior due to violations of sequencing rules.

Increment/decrement operators are not defined for complex or imaginary types: the usual definition of adding/subtracting the real number 1 would have no effect on imaginary types, and making it add/subtract i for imaginaries but 1 for complex numbers would have made it handle 0+yi different from yi.

Unlike C++ (and some implementations of C), the increment/decrement expressions are never themselves lvalues: &++a is invalid.

Example

Run this code

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int a = 1;
    int b = 1;
 
    printf("original values: a == %d, b == %d\n", a, b);
    printf("result of postfix operators: a++ == %d, b-- == %d\n", a++, b--);
    printf("after postfix operators applied: a == %d, b == %d\n", a, b);
    printf("\n");
 
    // Reset a and b.
    a = 1;
    b = 1;
 
    printf("original values: a == %d, b == %d\n", a, b);
    printf("result of prefix operators: ++a == %d, --b == %d\n", ++a, --b);
    printf("after prefix operators applied: a == %d, b == %d\n", a, b);
}

Output:

original values: a == 1, b == 1
result of postfix operators: a++ == 1, b-- == 1
after postfix operators applied: a == 2, b == 0
 
original values: a == 1, b == 1
result of prefix operators: ++a == 2, --b == 0
after prefix operators applied: a == 2, b == 0

References

• C23 standard (ISO/IEC 9899:2024):

• 6.5.2.4 Postfix increment and decrement operators (p: TBD)

• 6.5.3.1 Prefix increment and decrement operators (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 6.5.2.4 Postfix increment and decrement operators (p: TBD)

• 6.5.3.1 Prefix increment and decrement operators (p: TBD)

• C11 standard (ISO/IEC 9899:2011):

• 6.5.2.4 Postfix increment and decrement operators (p: 85)

• 6.5.3.1 Prefix increment and decrement operators (p: 88)

• C99 standard (ISO/IEC 9899:1999):

• 6.5.2.4 Postfix increment and decrement operators (p: 75)

• 6.5.3.1 Prefix increment and decrement operators (p: 78)

• C89/C90 standard (ISO/IEC 9899:1990):

• 3.3.2.4 Postfix increment and decrement operators

• 3.3.3.1 Prefix increment and decrement operators

See also

Operator precedence

Common operators
assignment	increment decrement	arithmetic	logical	comparison	member access	other
a = b a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b	++a --a a++ a--	+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b	!a a && b a || b	a == b a != b a < b a > b a <= b a >= b	a[b] *a &a a->b a.b	a(...) a, b (type) a a ? b : c sizeof _Alignof (since C11)

C++ documentation for Increment/decrement operators