Ever wanted to define a class, maybe a vector class, and make it possible to write code like<\/p>\n\n\n\n
dim v1 as new Vector(1,1)\ndim v2 as new Vector(2,2)\n\nif v1 > v2 then\n \/\/ do something when v1 > v2\nelseif v1 < v2 then\n \/\/ do something when v1 < v2\nelse\n \/\/ do something when v1 = v2\nend if<\/code><\/pre>\n\n\n\nPerhaps you have some other kind of class you’d like to define that has some kind of custom mechanism to compare itself to other instances – or even other data types.<\/p>\n\n\n\n
If so you need to know how to implement operator_compare<\/p>\n\n\n\n
First off operator_compare can ONLY be implemented in a Class. You cannot extend existing classes and add it to them. <\/p>\n\n\n\n
It’s a method, or several methods, you add to a class that will be called whenever you use the comparison operators =, <, >, <=, >= and <> are used. Note that this has implications for your code if you are using = to check is one instance is the same instance as another. In that case you should probably use IS rather than =.<\/p>\n\n\n\n
Beyond that operator_compare is pretty straight forward.<\/p>\n\n\n\n
You define the method with that name, and the parameter is whatever type you wish to compare your instance, the “self” instance, to. So you can have many as each signature would be different. The return value is and integer that is<\/p>\n\n\n\n
- < 0 means SELF is “less than” the passed parameter<\/li>
- = 0 means SELF is “equal to” the passed parameter<\/li>
- > 0 means SELF is “greater than” the passed parameter<\/li><\/ul>\n\n\n\n
and you get to define what less than<\/em>, equal to<\/em>, and greater than<\/em> mean.<\/p>\n\n\n\nSo suppose our Vector class was defined as follows<\/p>\n\n\n\n
Class Vector\n protected x as double\n protected y as double\n\n Public Sub Constructor(x as double, y as double)\n Self.x = x\n Self.y = y\n End Sub\n\n Function Operator_compare(other as vector) as Integer\n Dim a, b As Integer\n a = Self.x ^ 2 + Self.y ^ 2\n b = other.x ^ 2 + other.y ^ 2\n If a > b Then Return 1\n If a = b Then Return 0\n If a < b Then Return -1\n End Function\nEnd Class<\/code><\/pre>\n\n\n\nAnd now our original code from way back would work<\/p>\n\n\n\n
dim v1 as new Vector(1,1)\ndim v2 as new Vector(2,2)\n\nif v1 > v2 then\n \/\/ do something when v1 > v2\nelseif v1 < v2 then\n \/\/ do something when v1 < v2\nelse\n \/\/ do something when v1 = v2\nend if<\/code><\/pre>\n\n\n\nNow you could make Vectors compare themselves to Strings if you really wanted. I’m not sure what exactly that might mean – but you could. You might try implementing this as <\/p>\n\n\n\n
Public Function Operator_compare1(other As string) as Integer\n Dim myRepresentation As String = \"(\" + Str(x,\"-######.00\") + \",\" + Str(y,\"-######.00\") + \")\"\n \n If myRepresentation = other Then Return 0\n \n return -1\nEnd Function<\/code><\/pre>\n\n\n\nAgain I’m not sure what this might truly mean but we’ll indicate that unless they are the exact same string the “self” is < the string value. Suppose you try to test this with<\/p>\n\n\n\n
Dim v1 As New Vector(1,1)\n\nIf v1 = \"(foo)\" Then\n Break\nElse\n Break\nEnd If<\/code><\/pre>\n\n\n\nAnd now you Vectors can be compared to Strings. With extra overloads of Operator_compare you can make your classes compare themselves to all sorts of your other classes & types.<\/p>\n\n\n\n
Enjoy !<\/p>\n","protected":false},"excerpt":{"rendered":"
Ever wanted to define a class, maybe a vector class, and make it possible to write code like Perhaps you have some other kind of class you’d like to define that has some kind of custom mechanism to compare itself to other instances – or even other data types. If so you need to know … <\/p>\n