I know there is no such thing like Operator Overloading in Java and C#. A task is given to me by my teacher to achieve operator overloading in any of these languages. I don't know the deep concepts of these languages, only basic OOP. So can any one tell is there any other way to achieve this?
There is a thing called operator overloading in C#, check out this code snippet from MSDN:
public struct Complex
{
public int real;
public int imaginary;
public Complex(int real, int imaginary)
{
this.real = real;
this.imaginary = imaginary;
}
// Declare which operator to overload (+), the types
// that can be added (two Complex objects), and the
// return type (Complex):
public static Complex operator +(Complex c1, Complex c2)
{
return new Complex(c1.real + c2.real, c1.imaginary + c2.imaginary);
}
}
Full list of overloadable operators
As des has shown, C# does have operator overloading. Java, on the other hand, does not. The way Java compares that two objects are equal is done through overriding the method equals(Object), which is inherited from the base object java.lang.Object. Here's an example usage:
public class MyClass {
private int value;
#Override
public boolean equals(Object o) {
return o instanceof MyClass && ((MyClass)o).value == this.value;
}
}
Of course this is only a workaround for replicating overloading the == operator. For other operators, such as >= or <= there is nothing. However, you can use OO to sort of recreate it by using a common interface:
interface Overloadable<T> {
public boolean isGreaterThan(T other);
public boolean isLessThan(T other);
}
public class MyClass implements Overloadable<MyClass> {
private int value;
#Override
public boolean equals(Object o) {
return o instanceof MyClass && ((MyClass)o).value == this.value;
}
#Override
public boolean isGreaterThan(MyClass other) {
return this.value > other.value;
}
#Override
public boolean isLessThan(MyClass other) {
return this.value < other.value;
}
}
This is by no means real operator overloading because, well, you're not overloading the operators. It does however provide the ability to compare objects in the same way.
Related
My class looks something like this:
public class Testclass
{
public int myValue;
}
In another context I want to simply check the value of myValue against 0.
So I would write:
Testclass tc = new Testclass();
tc.myValue = 13;
if (tc.myValue == 0)
{
}
How would it be possible to simplify this so that the Testclass object knows when it is compared to a boolean? (or used as a boolean value) to write:
Testclass tc = new Testclass();
tc.myValue = 13;
if (tc)
{
}
To be more precise, the Testclass will be the result of another method that is included in a library, so the code would look like this:
anotherClass ac =new anotherClass();
// if (ac.AMethod().myValue == 0)
// should be
if (ac.AMethod())
{
}
Where AMethod would look like this:
public Testclass AMethod()
{
return new Testclass();
}
[Edit at 2016-04-13]:
Like Dennis wrote, i am using
public static implicit operator bool(TestClass value)
to get the "Boolean Value" of my class. To be a bit more precise and to stick more to my real application i would like to change the signature to
public static implicit operator UInt64(FlexComDotNetFehler fehler)
public static implicit operator Boolean(FlexComDotNetFehler fehler)
So these two methods of class FlexComDotNetFehler return the internal UInt64 field in the first case as the real representation as UInt64and in the second one as a Boolean value, that is true, when the UInt64 value is > 0.
But now, when i code
FlexComDotNetFehler x;
FlexComDotNetFehler y;
if (x == y)
where x and y are both of type FlexComDotNetFehler
the compiler cant know if it should use the Boolean or the UInt64 operator.
So i wrote
if ((UInt64)x != (UInt64)y)
but then those two type casts are greyed out.
#Ɖiamond ǤeezeƦ: thanks for your reformatting and editing. But I think now i got i right?
Greetings Wolfgang
BTW is there a playground where i can test the formatting and its output? And how do i send private messages to other users?
Define implicit cast operator for TestClass:
class TestClass
{
public int myValue;
public static implicit operator bool(TestClass value)
{
// assuming, that 1 is true;
// somehow this method should deal with value == null case
return value != null && value.myValue == 1;
}
}
Also think about converting TestClass from class to structure (see this reference). If you'll decide to convert it, avoid mutable structures.
you can use extension methods to implement method you can use any time not only for this class Testclass
public static class IntExtension
{
public static bool IsBool(this int number)
{
bool result = true;
if (number == 0)
{
result = false;
}
return result;
}
}
and then yo can
if ((ac.AMethod()).IsBool())
{}
Is there a way I can overload primitives, for example addition with doubles? I want to automatically round the doubles whenever an operation is performed. My current code is this:
class Test{
public static double operator +(double x, double y){
return Math.Round(x + y)
}
}
but there's an unfortunate error that says "One of the parameters of a binary operator must be the containing type".
No, and this would be horrible. Users using your library would suddenly get different behaviors from their double variables!
You can write and use a wrapper object however:
public struct MyDouble
{
public double Value {get; set;}
public MyDouble(double initValue)
{
Value = initValue;
}
public static double operator +(MyDouble x, MyDouble y){
return Math.Round(x.Value + y.Value)
}
}
You can also make it castable to/from a double, among other options. This way users know they are using your object and won't be surprised when their math operations are rounded.
If you want to assign from a simple double, you would need to define an implicit operator, similar to that of Nullable<T> (source):
public static implicit operator MyDouble(double value) {
return new MyDouble(value);
}
You can't overload operators on primitive types. This would cause havoc in your codebase.
What you can do instead, is to create a simple wrapper around the primitive type, let's say RoundedDouble:
public struct RoundedDouble : IEquatable<RoundedDouble>, IComparable<RoundedDouble>
{
public readonly double Value;
public RoundedDouble(double value)
{
Value = Math.Round(value); // Or anything else
}
public static implicit operator RoundedDouble(double value)
{
return new RoundedDouble(value);
}
public static implicit operator double(RoundedDouble wrapper)
{
return wrapper.Value;
}
public int GetHashCode()
{
return Value.GetHashCode();
}
public bool Equals(object other)
{
if (other is RoundedDouble)
return ((RoundedDouble)other).Value == Value;
return false;
}
public string ToString()
{
return Value.ToString();
}
// Add your operators here, and implement the interfaces
}
This is a structure. It has the same value semantics as a double.
Extend it by adding the operators, and by implementing at least IEquatable<RoundedDouble> and IComparable<RoundedDouble>.
I have a problem with generic. When I try to use less operators in generic, their call is not happening. But it works with the method Equals.
That is a some test class:
public class Test
{
public int i;
static public Boolean operator ==(Test obj1, Test obj2)
{
Console.WriteLine("operator ==");
return obj1.i == obj2.i;
}
static public Boolean operator !=(Test obj1, Test obj2)
{
Console.WriteLine("operator !=");
return obj1.i != obj2.i;
}
public override bool Equals(object obj)
{
Console.WriteLine("operator equals");
return this == (Test)obj;
}
public override int GetHashCode()
{
Console.WriteLine("HashCode");
return 5;
}
}
And class Checker:
public class Checker
{
public Boolean TestGeneric<T>(T Left, T Right) where T : class
{
return Left == Right; //not work override operators
return Left.Equals(Right); //work fine
}
}
Small testing:
Test left = new Test() { i = 4 };
Test right = new Test() { i = 4 };
var checker = new Checker();
Console.WriteLine(checker.TestGeneric<Test>(left, right));
Console.ReadKey();
How I can use less operators in class Test from generic?
Overloaded operators are static methods, so they don't participate in polymorphism; they are resolved statically at compile time, based on the known type of the operands.
In a generic method, the compiler can't know that T will be Test (since it could actually be anything else), so it uses the most general definition of ==, which is reference comparison.
Note that if you add a constraint on the generic method to force T to be Test or a subclass of Test, it will work as expected, but of course it won't work anymore for other types...
This now works in C# 11 / .NET 7 (or above):
public class Test : IEqualityOperators<Test, Test, bool>
{ /* no changes except ^^^ addition */ }
public bool TestGeneric<T>(T Left, T Right) where T : IEqualityOperators<T, T, bool>
{
return Left == Right; // does what you expect
}
So the compiler will not allow me to overrload the == and != operators of my class. Here is what the class looks like:
public class Item
{
public string _name;
public double _weight;
public decimal _wholesalePrice;
public int _quantity;
public Item(string name, double weight, decimal wholesalePrice, int quantity)
{
_name = name;
_weight = weight;
_wholesalePrice = wholesalePrice;
_quantity = quantity;
}
public static override bool operator ==(Item left, Item right)
{
if (left._name == right._name)
{
return true;
}
return false;
}
public static override bool operator !=(Item left,Item right)
{
return !(left == right);
}
}
The compiler keeps telling me "The modifier 'override' is not valid for this item. At first I thought I might not have declared a base method as virtual, but my class does is not derived. Any ideas what's happening?
You cannot declare an override unless you have derived the class from a parent class. You also cannot declare override on a static method. Have you tried removing override all together? That seems to work for me...
public class Item
{
public string _name;
public double _weight;
public decimal _wholesalePrice;
public int _quantity;
public Item(string name, double weight, decimal wholesalePrice, int quantity)
{
_name = name;
_weight = weight;
_wholesalePrice = wholesalePrice;
_quantity = quantity;
}
public static bool operator ==(Item left, Item right)
{
if (left._name == right._name)
{
return true;
}
return false;
}
public static bool operator !=(Item left, Item right)
{
return !(left == right);
}
}
As a side note, if you override the == and != operators, it's also good practice to override the GetHashCode and Equals methods.
You are deriving your class from the class Object, which does not have an == or != operator. So you cannot override those operators.
In addition, you cannot override a static operator or method, you can only override instance methods.
Finally, note that override and overload are two very different things. An overload is where you have multiple definitions of methods with the same name but different signatures (eg. different parameters).
The short answer is that the syntax is public static bool operator ==(Item left, Item right) without the override keyword.
This is called operator overloading, not overriding.
You may think of == as a kind of static method (inside an imaginary "global" class) taking two parameters. When the compiler sees something like
xxx == yyy
it uses overload resolution to find out which == to use. This is analogous to
Meth(xxx, yyy)
where the compiler considers overloads like Meth(Object, Object), Meth(String, String), Meth(Item, Item) and finds out which of them (if any) fits best to the compile-time types of xxx and yyy.
This is just an anolgy, of course, but helps remembering why you include static and not override when you change the == operator.
My brain has turned to jelly, or I'm having an out of mind experience, or something. I'm tinkering with a class hierarchy that looks a bit like this:
My Money class looks like this:
public abstract class Money
{
public int Amount { get; set; }
public static bool operator ==(Money leftSide, Money rightSide)
{
// Money can only be equal if it is in the same currency.
if (leftSide.GetType() != rightSide.GetType()) return false;
return leftSide.Amount == rightSide.Amount;
}
public static bool operator !=(Money leftSide, Money rightSide)
{
// If the currencies are different, the amounts are always considered unequal.
if (leftSide.GetType() != rightSide.GetType()) return true;
return leftSide.Amount != rightSide.Amount;
}
public static Money operator *(Money multiplicand, int multiplier)
{
var result = multiplicand * multiplier;
return result;
}
public static Dollar Dollar(int amount)
{
return new Dollar(amount);
}
public static Franc Franc(int amount)
{
return new Franc(amount);
}
}
My Dollar operator * looks like this:
public static Dollar operator *(Dollar multiplicand, int multiplier)
{
var result = multiplicand.Amount * multiplier;
return new Dollar(result);
}
Now, if I run this test code, I get a Stack overflow (wahoo!)
{
Money fiveDollars = Money.Dollar(5);
Money timesTwo = fiveDollars*2;
}
I had expected that this would recursively call the subclass (Dollar) operator *, which would return a definite result since (Dollar * int) is defined non-recursively. Since this doesn't work, the alternative is that I have done something dumb. Why doesn't this work? What would be the right way to get this behaviour?
You seem to have left out .Amount
public static Money operator *(Money multiplicand, int multiplier)
{
var result = multiplicand.Amount * multiplier;
return result;
}
The problem is that you expect that you can override operators in derived classes and expect dynamic binding. This is not the way it works in C#. Operators are overloaded and the actual overload is chosen compile-time. This means that the following code is recursive and calls itself:
public static Money operator *(Money multiplicand, int multiplier)
{
var result = multiplicand * multiplier;
return result;
}
Another example where you can see the difference between operator overloading and method overriding is this:
int a = 5;
int b = 5;
Console.WriteLine(a == b); // true
Console.WriteLine(a.Equals(b)); // true
Console.WriteLine((object)a == (object)b); // false
Console.WriteLine(((object)a).Equals((object)b)); // true
In the third case, C# treats a and b as objects instead of integers, so it uses the default == operator that is used for objects: comparing references (in this case the references of boxed integers).
This can make it awkward to define operators on a class hierarchy where you want to redefine the operators in derived classes. It is especially awkward when the behavior depends on the combination of both operands, since C# (and most other OOP languages) lacks support for multiple dispatch. You can solve this by using the visitor pattern, but I think in this case you should reconsider if using subclasses for each currency is the best solution.