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Last update Mon Apr 6 02:40:07 2009

# Operator Overloading

Overloading is accomplished by interpreting specially named struct and class member functions as being implementations of unary and binary operators. No additional syntax is used.

- Unary Operator Overloading
- Binary Operator Overloading
- Function Call Operator Overloading
- Array Operator Overloading
- Assignment Operator Overloading
- Forwarding
- Future Directions

## Unary Operator Overloading

Overloadable Unary Operators
op | opfunc
--- | ---
-e | `opNeg`
+e | `opPos`
~e | `opCom`
*e | `opStar`
e++ | `opPostInc`
e-- | `opPostDec`
cast(type)e | `opCast`

Given a unary overloadable operator op and its corresponding class or struct member function name opfunc, the syntax:

    op a

where a is a class or struct object reference, is interpreted as if it was written as:

    a.opfunc()

### Overloading ++e and --e

Since ++e is defined to be semantically equivalent to (e += 1), the expression ++e is rewritten as (e += 1), and then checking for operator overloading is done. The situation is analogous for --e.

### Examples

1.     class A { int opNeg(); }
       A a;
       -a;	// equivalent to a.opNeg();

2.     class A { int opNeg(int i); }
       A a;
       -a;	// equivalent to a.opNeg(), which is an error

### Overloading cast(type)e

The member function e.opCast() is called, and the return value of opCast() is implicitly converted to type. Since functions cannot be overloaded based on return value, there can be only one opCast per struct or class. Overloading the cast operator does not affect implicit casts, it only applies to explicit casts.

    struct A
    {
        int opCast() { return 28; }
    }

    void test()
    {
        A a;

        long i = cast(long)a;   // i is set to 28L
        void* p = cast(void*)a; // error, cannot implicitly
    			    // convert int to void*
        int j = a;		    // error, cannot implicitly convert
    			    // A to int
    }

## Binary Operator Overloading

Overloadable Binary Operators
op | commutative? | opfunc | opfunc_r
--- | --- | --- | ---
+ | yes | `opAdd` | `opAdd_r`
- | no | `opSub` | `opSub_r`
* | yes | `opMul` | `opMul_r`
/ | no | `opDiv` | `opDiv_r`
% | no | `opMod` | `opMod_r`
& | yes | `opAnd` | `opAnd_r`
| | yes | `opOr` | `opOr_r`
^ | yes | `opXor` | `opXor_r`
<< | no | `opShl` | `opShl_r`
>> | no | `opShr` | `opShr_r`
>>> | no | `opUShr` | `opUShr_r`
~ | no | `opCat` | `opCat_r`
== | yes | `opEquals` | -
!= | yes | `opEquals` | -
< | yes | `opCmp` | -
<= | yes | `opCmp` | -
> | yes | `opCmp` | -
>= | yes | `opCmp` | -
= | no | `opAssign` | -
+= | no | `opAddAssign` | -
-= | no | `opSubAssign` | -
*= | no | `opMulAssign` | -
/= | no | `opDivAssign` | -
%= | no | `opModAssign` | -
&= | no | `opAndAssign` | -
|= | no | `opOrAssign` | -
^= | no | `opXorAssign` | -
<<= | no | `opShlAssign` | -
>>= | no | `opShrAssign` | -
>>>= | no | `opUShrAssign` | -
~= | no | `opCatAssign` | -
in | no | `opIn` | `opIn_r`

Given a binary overloadable operator op and its corresponding class or struct member function name opfunc and opfunc_r, and the syntax:

    a op b

the following sequence of rules is applied, in order, to determine which form is used:

1. The expression is rewritten as both:

       a.opfunc(b)
       b.opfunc_r(a)

   If any a.opfunc or b.opfunc_r functions exist, then overloading is applied across all of them and the best match is used. If either exist, and there is no argument match, then it is an error.
2. If the operator is commutative, then the following forms are tried:

       a.opfunc_r(b)
       b.opfunc(a)

3. If a or b is a struct or class object reference, it is an error.

#### Examples

1.     class A { int opAdd(int i); }
       A a;
       a + 1;	// equivalent to a.opAdd(1)
       1 + a;	// equivalent to a.opAdd(1)

2.     class B { int opDiv_r(int i); }
       B b;
       1 / b;	// equivalent to b.opDiv_r(1)

3.     class A { int opAdd(int i); }
       class B { int opAdd_r(A a); }
       A a;
       B b;
       a + 1;	// equivalent to a.opAdd(1)
       a + b;	// equivalent to b.opAdd_r(a)
       b + a;	// equivalent to b.opAdd_r(a)

4.     class A { int opAdd(B b);  int opAdd_r(B b); }
       class B { }
       A a;
       B b;
       a + b;	// equivalent to a.opAdd(b)
       b + a;	// equivalent to a.opAdd_r(b)

5.     class A { int opAdd(B b);  int opAdd_r(B b); }
       class B { int opAdd_r(A a); }
       A a;
       B b;
       a + b;	// ambiguous: a.opAdd(b) or b.opAdd_r(a)
       b + a;	// equivalent to a.opAdd_r(b)

### Overloading == and !=

Both operators use the `opEquals()` function. The expression `(a == b)` is rewritten as `a.opEquals(b)`, and `(a != b)` is rewritten as `!a.opEquals(b)`.

The member function `opEquals()` is defined as part of Object as:

    bool opEquals(Object o);

so that every class object has a default `opEquals()`. But every class definition which will be using == or != should expect to need to override opEquals. The parameter to the overriding function must be of type `Object`, not the type for the class.

Structs and unions (hereafter just called structs) can provide a member function:

    bool opEquals(S s)

or:

    bool opEquals(S* s)

where `S` is the struct name, to define how equality is determined.

If a struct has no opEquals function declared for it, a bit compare of the contents of the two structs is done to determine equality or inequality.

Note: Comparing a reference to a class object against null should be done as:

    if (a is null)

and not as:

    if (a == null)

The latter is converted to:

    if (a.opEquals(null))

which will fail if `opEquals()` is a virtual function.

### Overloading <, <=, > and >=

These comparison operators all use the `opCmp()` function. The expression `(a op b)` is rewritten as `(a.opCmp(b) op 0)`. The commutative operation is rewritten as `(0 op b.opCmp(a))`

The member function `opCmp()` is defined as part of Object as:

    int opCmp(Object o);

so that every class object has a `opCmp()`.

`opCmp` for structs works analogously to `opEquals` for structs:

    struct Pair
    {
        int a, b;
        int opCmp(Pair rhs)
        {
            if (a!=rhs.a) return a-rhs.a;
            return b-rhs.b;
        }
    }

If a struct has no `opCmp()` function declared for it, attempting to compare two structs is an error.

#### Rationale

The reason for having both `opEquals` and `opCmp` is that:

- Testing for equality can sometimes be a much more efficient operation than testing for less or greater than.
- Having an `opCmp` defined in `Object` makes it possible to make associative arrays work generically for classes.
- For some objects, testing for less or greater makes no sense. This is why `Object.opCmp` throws a runtime error. `opCmp` must be overridden in each class for which comparison makes sense.

The parameter to opEquals and opCmp for class definitions must be of type `Object`, rather than the type of the particular class, in order to override the `Object.opEquals` and `Object.opCmp` functions properly.

## Function Call Operator Overloading f()

The function call operator, (), can be overloaded by declaring a function named opCall:

    struct F
    {
        int opCall();
        int opCall(int x, int y, int z);
    }

    void test()
    {   F f;
        int i;

        i = f();		// same as i = f.opCall();
        i = f(3,4,5);	// same as i = f.opCall(3,4,5);
    }

In this way a struct or class object can behave as if it were a function.

## Array Operator Overloading

### Overloading Indexing a[i]

The array index operator, [], can be overloaded by declaring a function named opIndex with one or more parameters. Assignment to an array can be overloaded with a function named opIndexAssign with two or more parameters. The first parameter is the rvalue of the assignment expression.

    struct A
    {
        int opIndex(size_t i1, size_t i2, size_t i3);
        int opIndexAssign(int value, size_t i1, size_t i2);
    }

    void test()
    {   A a;
        int i;

        i = a[5,6,7];	// same as i = a.opIndex(5,6,7);
        a[i,3] = 7;		// same as a.opIndexAssign(7,i,3);
    }

In this way a struct or class object can behave as if it were an array.

Note: To use array index overloading with the op=, ++, or -- operators, have the opIndex function return a reference type. This reference is then used as the lvalue for those operators.

### Overloading Slicing a[] and a[i .. j]

Overloading the slicing operator means overloading expressions like `a[]` and `a[i .. j]`. This can be done by declaring a function named opSlice. Assignment to a slice can be done by declaring opSliceAssign.

    class A
    {
        int opSlice();		 		  // overloads a[]
        int opSlice(size_t x, size_t y);		  // overloads a[i .. j]

        int opSliceAssign(int v);			  // overloads a[] = v
        int opSliceAssign(int v, size_t x, size_t y); // overloads a[i .. j] = v
    }

    void test()
    {   A a = new A();
        int i;
        int v;

        i = a[];		// same as i = a.opSlice();
        i = a[3..4];	// same as i = a.opSlice(3,4);

        a[] = v;		// same as a.opSliceAssign(v);
        a[3..4] = v;	// same as a.opSliceAssign(v,3,4);
    }

## Assignment Operator Overloading

The assignment operator `=` can be overloaded if the lvalue is a struct aggregate, and `opAssign` is a member function of that aggregate.

The assignment operator cannot be overloaded for rvalues that can be implicitly cast to the lvalue type. Furthermore, the following parameter signatures for `opAssign` are not allowed:

    opAssign(...)
    opAssign(T)
    opAssign(T, ...)
    opAssign(T ...)
    opAssign(T, U = defaultValue, etc.)

where T is the same type as the aggregate type A, is implicitly convertible to A, or if A is a struct and T is a pointer to a type that is implicitly convertible to A.

## Forwarding

Providing a struct or class member function `opDot` enables the forwarding of any names not found in the struct's scope to be forwarded to the return type of the `opDot` function. In other words:

    struct T {
      ...
      S opDot() { ... }
    }
    T t;
    ...
    t.m

is rewritten as:

    t.opDot().m

if m does not exist as a member of the struct T.

The members .sizeof, .init, .offsetof, .alignof, .mangleof and .stringof are not forwarded to `opDot`.

    struct S {
        int a, b, c;
    }

    struct T {
        S s;
        int b = 7;

        S* opDot() {
            return &s;    // forwards to member s
        }
    }

    void main() {
        T t;
        t.a = 4;
        t.b = 5;
        t.c = 6;
        assert(t.a == 4);
        assert(t.b == 5);    // T.b overrides S.b
        assert(t.c == 6);
        assert(t.s.b == 0);
        assert(t.sizeof == 4*4); // sizeof T, not sizeof S
    }

## Future Directions

The operators `! && || ?:` and a few others will likely never be overloadable.

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