Operator Overloading

If you want to support operators that are not natively supported by a certain type, you can achieve this through operator overloading.

To overload an operator for a type, you can define a function with the operator's name for that type. When an instance of this type uses the operator, the corresponding operator function will be automatically called.

The definition of an operator function is similar to that of a regular function, with the following differences:

- The operator modifier must be added before the func keyword when defining an operator function.
- The number of parameters in the operator function must match the requirements of the corresponding operator (see Appendix Operators for details).
- Operator functions can only be defined within class, interface, struct, enum, and extend.
- Operator functions have the semantics of instance member functions, so the static modifier is prohibited.
- Operator functions cannot be generic functions.

Additionally, it's important to note that overloaded operators do not change their inherent precedence and associativity (see Appendix Operators for details).

There are two ways to define operator functions:

1. For types that can directly contain function definitions (including struct, enum, class, and interface), operator functions can be defined directly within them to overload operators.
2. Use the extend approach to add operator functions, thereby overloading operators for these types. For types that cannot directly contain function definitions (i.e., types other than struct, class, enum, and interface) or types whose implementations cannot be modified (such as third-party defined struct, class, enum, and interface), this is the only available method (see Extensions).

The conventions for parameter types in operator functions are as follows:

1. For unary operators, the operator function takes no parameters, and there are no requirements on the return type.

2. For binary operators, the operator function takes exactly one parameter, and there are no requirements on the return type.

The following example demonstrates the definition and usage of unary and binary operators:

The - operator negates the x and y member variables of a Point instance and returns a new Point object. The + operator sums the x and y member variables of two Point instances and returns a new Point object.

Now, the unary - operator and binary + operator can be used directly on instances of Point:

3. The index operator ([]) has two forms: value retrieval (let a = arr[i]) and value assignment (arr[i] = a). These are distinguished by the presence or absence of a special named parameter value. Overloading the index operator does not require overloading both forms simultaneously; you can overload only the assignment form or only the retrieval form.

For the value retrieval form of the index operator [], the parameter sequence inside the brackets corresponds to the non-named parameters of the operator overload. There can be one or more parameters of any type. No other named parameters are allowed. The return type can be any type.

For the value assignment form of the index operator [], the parameter sequence inside the brackets corresponds to the non-named parameters of the operator overload. There can be one or more parameters of any type. The expression on the right side of = corresponds to the named parameter of the operator overload. There must be exactly one named parameter, and its name must be value. It cannot have a default value, and value can be of any type. The return type must be Unit.

Note that value is just a special marker; you do not need to use named parameter syntax when calling the index operator for assignment.

Notably, immutable types (except enum) do not support overloading the assignment form of the index operator.

4. The function call operator (()) overload function can have input parameters and return values of any type. Example:

You cannot use this or super to call the () operator overload function. Example:

For enumeration types, when both the constructor form and the () operator overload function form are applicable, the constructor form takes precedence. Example:

The following table lists all overloadable operators (ordered by precedence from highest to lowest):

| Operator | Description |
|:--------------------|:----------------------|
| () | Function call |
| [] | Indexing |
| ! | NOT |
| - | Negative |
| * | Power |
| | Multiply |
| / | Divide |
| % | Remainder |
| + | Add |
| - | Subtract |
| << | Bitwise left shift |
| >> | Bitwise right shift |
| < | Less than |
| <= | Less than or equal |
| > | Greater than |
| >= | Greater than or equal |
| == | Equal |
| != | Not equal |
| & | Bitwise AND |
| ^ | Bitwise XOR |
| | | Bitwise OR |

Important notes:

> Note:
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> - If any binary operator other than relational operators (<, <=, >, >=, ==, and !=) is overloaded for a type, and the return type of the operator function matches the type of the left operand or is a subtype of it, then the type supports the corresponding compound assignment operator. If the return type does not match the left operand's type and is not a subtype, using the corresponding compound assignment operator will result in a type mismatch error.
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> - The Cangjie programming language does not support custom operators. Defining operator functions other than those listed in the above table is not allowed.
> - For a type T, if T already natively supports certain overloadable operators, attempting to redefine operator functions with the same signature via extension will result in a redefinition error. For example, overloading arithmetic operators, bitwise operators, or relational operators with the same signature for numeric types, overloading relational operators with the same signature for Rune, or overloading logical operators, equality, or inequality operators with the same signature for Bool will all trigger redefinition errors.
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