> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Dart Postfix Decrement
The `--` operator is a unary arithmetic decrement operator in Dart that subtracts exactly `1` from its numeric operand. It mutates the underlying variable in place and requires the operand to be a non-final, non-const variable.
The operator functions in two distinct evaluation contexts based on its placement relative to the operand: **prefix** and **postfix**.
## Prefix Decrement (`--var`)
In the prefix position, the operator performs a pre-decrement. Dart subtracts `1` from the operand *before* evaluating the expression. The expression resolves to the newly decremented value.
```dart theme={"dark"}
int a = 5;
int b = --a;
// Evaluation sequence:
// 1. a is decremented by 1 (a becomes 4)
// 2. The expression resolves to the new value of a (4)
// 3. b is assigned 4
```
## Postfix Decrement (`var--`)
In the postfix position, the operator performs a post-decrement. Dart evaluates the expression using the current value of the operand, and subtracts `1` from the operand *after* the evaluation is complete.
```dart theme={"dark"}
int x = 5;
int y = x--;
// Evaluation sequence:
// 1. The expression resolves to the current value of x (5)
// 2. y is assigned 5
// 3. x is decremented by 1 (x becomes 4)
```
## Technical Constraints and Behavior
* **Type Requirements:** The operand must be of type `num` (`int` or `double`), or a custom class that implements the `-` operator.
* **Immutability:** Applying `--` to a `final` or `const` variable results in a compile-time error, as the operator fundamentally requires reassignment.
* **Desugaring:** Dart does not allow direct overriding of the `--` operator. Instead, the Dart compiler treats `a--` or `--a` as syntactic sugar for `a = a - 1`.
* **Custom Classes:** Because of the desugaring mechanism, if you want to use the `--` operator on an instance of a custom class, you must override the binary `-` operator. The class must also be assigned to a mutable variable (`var` or explicit type, not `final`).
```dart theme={"dark"}
class Counter {
final int value;
Counter(this.value);
// Overriding the binary '-' operator enables the '--' operator
Counter operator -(int amount) => Counter(value - amount);
}
void main() {
Counter c = Counter(10);
c--; // Implicitly executes: c = c - 1;
}
```
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