Increment and decrement operators

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Increment and decrement operators are unary operators that increase or decrease their operand by one.

They are commonly found in imperative programming languages. C-like languages feature two versions (pre- and post-) of each operator with slightly different semantics.

In languages syntactically derived from B (including C and its various derivatives), the increment operator is written as ++ and the decrement operator is written as --. Several other languages use inc(x) and dec(x) functions.

The increment operator increases, and the decrement operator decreases, the value of its operand by 1. The operand must have an arithmetic or pointer data type, and must refer to a modifiable data object. Pointers values are increased (or decreased) by an amount that makes them point to the next (or previous) element adjacent in memory.

In languages that support both versions of the operators:

• The pre-increment and pre-decrement operators increment (or decrement) their operand by 1, and the value of the expression is the resulting incremented (or decremented) value.
• The post-increment and post-decrement operators increase (or decrease) the value of their operand by 1, but the value of the expression is the operand's value prior to the increment (or decrement) operation.

In languages where increment/decrement is not an expression (e.g., Go), only one version is needed (in the case of Go, post operators only).

Since the increment/decrement operator modifies its operand, use of such an operand more than once within the same expression can produce undefined results. For example, in expressions such as x - ++x, it is not clear in what sequence the subtraction and increment operations should be performed. Such expressions generally invoke undefined behavior, and should be avoided.

In languages with typed pointers like C, the increment operator steps the pointer to the next item of that type -- increasing the value of the pointer by the size of that type. When a pointer (of the right type) points to any item in an array, incrementing (or decrementing) makes the pointer point to the "next" (or "previous") item of that array. Thus, incrementing a pointer to an integer makes it point to the next integer (typically increasing the pointer value by 4);^[1] incrementing a pointer to a structure of size 106 bytes makes it point to the next structure by increasing the pointer value by 106.^[2]