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The ++ (increment) operator is a unary operator that increases the value of a mutable variable. In Kotlin, it is not a primitive-only operation but rather syntactic sugar for the inc() operator function combined with a reassignment. Because Kotlin treats the increment operation as a reassignment, the operand must be declared as a mutable variable (var), not a read-only reference (val).

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Evaluation Modes

The operator exhibits different evaluation semantics based on its position relative to the operand:
  • Prefix (++a): The variable is incremented first, and the expression evaluates to the new (incremented) value.
  • Postfix (a++): The expression evaluates to the original value of the variable, and the variable is incremented afterward.
var prefixVar = 10
val prefixResult = ++prefixVar 
// prefixVar is 11, prefixResult is 11

var postfixVar = 10
val postfixResult = postfixVar++ 
// postfixVar is 11, postfixResult is 10

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Compiler Translation

Under the hood, Kotlin translates the ++ operator into a call to the inc() function followed by an assignment back to the original variable. Prefix Translation: 
// Source code
val b = ++a

// Conceptual compiler translation
a = a.inc()
val b = a
Postfix Translation: To preserve the original value for the expression while ensuring the getter of a is evaluated exactly once, the compiler uses a temporary reference. 
// Source code
val b = a++

// Conceptual compiler translation
val a0 = a       // 1. Evaluate getter once and store original reference
a = a0.inc()     // 2. Call inc() on the original reference and reassign to 'a'
val b = a0       // 3. The expression evaluates to the original reference

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Operator Overloading and Mutability Contract

Because ++ is resolved via operator overloading, it can be applied to any custom class by defining an inc() member or extension function prefixed with the operator keyword. The function must return a type that is assignable to the original variable. Crucially, the inc() function must never mutate the existing object; it must always return a new instance. If inc() mutates the internal state and returns this, the postfix operator will violate its semantic contract. In the postfix translation (val a0 = a; a = a0.inc(); val b = a0), if a0.inc() mutates a0, the temporary variable a0 will reflect the mutated state, causing the postfix expression to incorrectly evaluate to the new value instead of the original. 
class Point(val x: Int, val y: Int) {
    // The operator function MUST return a new instance.
    // Mutating state and returning 'this' breaks postfix semantics.
    operator fun inc(): Point {
        return Point(x + 1, y + 1)
    }
}

var p = Point(0, 0)
p++ // Translates to: val p0 = p; p = p0.inc(); p0

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Thread Safety

The ++ operator in Kotlin is not atomic. Whether applied to primitives or custom objects, the read-modify-write cycle (a = a.inc()) is subject to race conditions in a multithreaded environment. For atomic increments, Kotlin relies on the underlying platform’s concurrency utilities (e.g., AtomicInteger.incrementAndGet() on the JVM), which bypass the ++ operator syntax entirely.
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