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Last update Wed Jun 7 00:54:00 2006

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

Overloadable Unary Operators
op opfunc
-e opNeg
+e opPos
~e opCom
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 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:

int opEquals(Object o);
so that every class object has an opEquals().

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.

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().

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

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.opCmp(null))
which will fail if opCmp() is a virtual function.

Rationale

The reason for having both opEquals() and opCmp() is that:

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 = a.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: Array index overloading currently does not work for the lvalue of an op=, ++, or -- operator.

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);
}

Future Directions

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