Operator Overloading

One of
the new features we're adding to Visual Basic is called Operator Overloading. This feature allows the programmer to create
a class that knows what +, -, *, and other operators mean.

There are
two main components to operator overloading: consumption and declaration. (Actually, I've just described a major precept of language design. Although
they go hand-in-hand, the declaration form of a feature and the consumption
form of a feature are two intrinsically different concepts. The dichotomy
reveals itself even in .NET executables. When people talk about
"metadata", they are referring to declarations. When people talk
about intermediate language (IL) opcodes, they are
referring to consumption.)

Let's take
a look at consumption first with an example.The System.Drawing namespace contains the
types Point and Size. A new point can be
calculated by adding or subtracting a size from an already existing point. Programmatically, the + and – operators lend
themselves naturally to this calculation:

p1 As New Point(10, 10)

s1 As New Size(20, -10)

p3 As Point = p1 + s1

p4 As Point = p1 - s1



Point overloads the + and - operators, this code compiles and works as
expected, outputting:



How was
this done? When the compiler analyzes
the expression "p1 + s1", it discovers the type of "p1" is
Point and that Point defines Operator +. Operators are actually just functions with funny names, so the compiler
turns "p1 + s1" into a function call, equivalently written as:

p3 As Point = System.Drawing.Point.op_Addition(p1, s1)

This is the key to understanding
overloaded operators
: they are just functions. An expression like "a + b" is
really a call to the + operator with two arguments: "+(a,
b)". Because member names cannot
contain symbols like "+", the compiler uses a special name behind the
scenes: "op_Addition(a, b)". And as
functions, the compiler uses normal overload resolution rules to pick the
correct overload (in exactly the same way the compiler uses these rules to pick
the correct overload of Console.WriteLine, which at
last count had 19 overloads!).

you can guess what the declarations of overloaded operators look like; they're an awful
lot like functions:

Shared Operator
Left As Point, ByVal Right As
Size) As Point

New Point(Left.X + Right.Width, Left.Y + Right.Height)


Shared Operator
Left As Point, ByVal Right As
Size) As Point

New Point(Left.X – Right.Width, Left.Y – Right.Height)


Visual Basic compiler will allow you to overload several operators, summarized
in the table below:


+ (unary)


- (unary)














= (comparison only)









IsTrue and IsFalse are
special and I'll cover those in a future entry. But quickly, you can overload conversions between types using the CType operator (instead of overloading the = assignment
operator). And IsTrue
and IsFalse are for overloading AndAlso
and OrElse which are themselves
not directly overloadable.

A few
rules greatly simplify the design and use of operator overloading: each operator declaration must be Public and
Shared, and must be declared in the same type as one of its operands. And some operators, like <= and >=,
must be pair-wise declared, i.e., a declaration of one requires an identical
declaration of the other. You'll find,
as I did, that these simple rules make understanding operator overloading much,
much easier.

More info
to follow...