Increment/decrement operators
Increment/decrement operators are unary operators that increment/decrement the value of a variable by 1.
They can have postfix form:
expr ++
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expr --
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As well as the prefix form:
++ expr
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-- expr
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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 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
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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
#include <stdlib.h> #include <stdio.h> int main(void) { int a = 1; int b = 1; printf("\n"); 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); // Reset a and b. a = 1; b = 1; printf("\n"); 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
- 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
| Common operators | ||||||
|---|---|---|---|---|---|---|
| assignment | increment decrement |
arithmetic | logical | comparison | member access |
other |
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a = b |
++a |
+a |
!a |
a == b |
a[b] |
a(...) |