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What Does "Overloaded"/"Overload"/"Overloading" Mean?
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Closed 9 years ago.
I've found resources that say method overloading is the ability for a language to use the same method with a different outcome, depending on context.
Somehow, when I read other definitions, I fell like that's not the whole definition. Is there more to method overloading?
That's just a very general way of describing it. Method overloading allows you to use a single method name, but "overload it" (provide more than one version) depending on "context" (which is typically the type or number of arguments passed in). Since each method is separate, they can cause a "different outcome".
For example, using C#, you can write:
void Foo() // Version with no arguments
{
}
void Foo(int arg) // Version with a single int
{
}
void Foo(string arg1, double arg2) // Version with string and double parameters
{
}
First, you should know what is signature in programming. A signature of a function is its representation; the name of a function and its parameters determine its signature.
Overloading means changing the signature of a function to meet our needs.
Have a look at the following example:
int sum(int x, int y, int z) {
return x + y + z;
}
int sum(int x, int y) {
return x + y;
}
Now, the function sum() can be called through two different ways: Either you can call it with two arguments or you can call it with three arguments. you have changed its signature to meet your needs. Instead of writing a separate function for two arguments, you put the load on the same function, that is why this is known as overloading.
It's where you have a multitude of methods of the same name with different parameters.
public void kittens(String paws) {
}
public void kittens(String paws, boolean tail) {
}
Both can be called independently to one another with either
kittens("fat");
or
kittens("thin", true);
The context in this case is determined by the argument signature of the method, i.e. the number and type of the arguments.
For example:
methodName(int x, int y, int w, int h)
methodName(Vector x)
The first method implementation would be an alternative to:
methodName(new Vector(x, y, w, h))
Is the ability of some languages to create methods/function with the same name but that differ for input / otput parameters.
A classical example is the class constructor overloading example in Java:
//Constructor without parameters
public User() {
}
//Constructor with only one parameter
public User(String username)
{
this.username = username;
}
//Constructor with two parameters
public User(String username, int age)
{
this.username=username;
this.age=age;
}
You have a class with different constructors that accept different parameters, as you see they differ in their signature.
Related
I'm currently going through the paper Extensibility for the Masses. Practical Extensibility with Object Algebras by Bruno C. d. S. Oliveira and William R. Cook (available many places on the internet - for example here: https://www.cs.utexas.edu/~wcook/Drafts/2012/ecoop2012.pdf).
On page 10, they write:
Adding new data variants is easy. The first step is to create new classes Bool
and Iff in the usual object-oriented style (like Lit and Add):
class Bool implements Exp {...}
class Iff implements Exp {...}
The implementation of Exp is, it seems, left as an exercise to the reader. It's not clear to me, however, how Exp is defined in this part of the paper. My question is:
How should Bool and Iff be implemented?
Here's what I've tried:
First defintion of Exp
Early in the paper, the Exp interface is defined like this:
interface Exp {
Value eval();
}
Here, Value is defined by another interface:
interface Value {
Integer getInt();
Boolean getBool();
}
The paper, however, quickly departs from this definition of Exp in favour of a Visitor-based definition.
Possible implementation of Bool
Based on that definition, how should one implement the Bool class?
Something like this seems like a start:
class Bool implements Exp {
boolean x;
public Bool(boolean x) { this.x = x; }
public Value eval() {
return new VBool(x);
}}
The question, however, becomes how to properly implement Value?
The paper only shows this:
class VBool implements Value {...}
The implementation doesn't seem total to me:
class VBool implements Value {
boolean x;
public VBool(boolean x) { this.x = x; }
public Boolean getBool() {
return new Boolean(x);
}
public Integer getInt() {
// What to return here?
}
}
As my above attempt shows, it's not clear what to return from getInt. I suppose I could return null or throw an exception, but that would imply that my implementation is partial.
In any case, this first definition of Exp seems only to exist as a motivating example in the paper, which then proceeds to define a better alternative.
Second definition of Exp
On page 4 the paper redefines Exp as an Internal Visitor:
interface Exp {
<A> A accept(IntAlg<A> vis);
}
Where IntAlg<A> is another interface:
interface IntAlg<A> {
A lit(int x);
A add(A e1, A e2);
}
So far things seem clear, until we get to implementing Bool and Iff...
Possible implementation of Bool
How should we implement the proposed Bool class based on this definition of Exp?
class Bool implements Exp {
boolean x;
public Bool(boolean x) { this.x = x; }
public <A> A accept(IntAlg<A> vis) {
// What to return here?
}}
There's no way to conjure an A value out of thin air, so one has to interact with vis in order to produce an A value. The vis parameter, however, only defines lit and add methods.
The lit method requires an int, which isn't available in Bool.
Likewise, add requires two A values, which are also unavailable. Again, I find myself at an impasse.
Third definition of Exp?
Then, on page 8, the paper shows this example:
int x = exp(base).eval();
Here, exp(base) returns Exp, but which definition of eval is this?
Apparently, Exp still (or again?) has an eval method, but now it returns int. Does it look like this?
interface Exp {
int eval();
}
The paper doesn't show this definition, so I may be misunderstanding something.
Possible implementation of Bool
Can we implement Bool and Iff with this definition of Exp?
class Bool implements Exp {
boolean x;
public Bool(boolean x) { this.x = x; }
public int eval() {
// What to return here?
}}
Again, it's not clear how to implement the interface. One could, of course, return 0 for false and 1 for true, but that's just an arbitrary decision. That doesn't seem appropriate.
Is there a fourth definition of Exp that I'm missing? Or is there some other information in the paper that's eluding me?
BTW, I apologise if I've made mistakes in my attempts. I don't normally write Java code.
#Mark. Let me try to clarify the confusion points that you have.
Definition of Exp
The definition of Exp that we are assumming in page 10 is:
interface Exp {
Value eval();
}
which is presented earlier in the paper in Figure 1. The alternative version of Exp with visitors can be ignored. We used it merely to talk about visitors and the relationship with Object Algebras, and it does not play a role in later parts of the paper.
Also, regarding the code in page 8, I think there is a typo in the paper. You are right: it seems that there we were assuming an eval method that returns int. When we were writing the paper we were using multiple versions of Exp, and probably we used the code for another version there by mistake. The code in page 8, adapted to the representation in the paper, should be:
int x = exp(base).eval().getInt();
The Choices for Value
The code that we intended to have for the implementation of the Value class uses exceptions, similarly to your own attempt, and it is indeed partial as presented in the paper. In the paper, the point that we were making is about the extensibility and type-safety with respect to the source expressions. For the values, all we wanted was that values were rich enough to support the source expressions, and the assumption was that the values themselves were not extensible (later in Section 7 we briefly discuss extensible values). The representation that we choose in the paper was aimed at keeping the code simple, and avoid some distractions with dealing with error management (which are orthogonal to extensibility).
Agreeably this is not a great representation but, at least with the Java version at the time, it seemed like a reasonable compromise. In modern Java, I think the way to go would be to have Value modelled as an algebraic datatype. Java 16 supports pattern matching and sealed types, which can be used to essentially model algebraic datatypes in functional programming. So you could model Value with something like:
sealed interface Value {}
record VInt(int x) implements Value {}
record VBool(boolean b) implements Value {}
In retrospect, to avoid confusions like the one you're having, I think it would have been better to have presented the paper simply using the interface:
interface Exp {
int eval();
}
and to use an if construct à la C, where integers play the role of booleans. This would have avoided distractions with the representation of Value in the first place.
The Representation of Values in the Paper
In any case, getting back to the representation of the paper, and also to your point about partiality, I show a minimal, but complete implementation of the code next. For illustrating that partiality is not a fundamental issue, I also switch from using unchecked exceptions to checked exceptions.
First we can define values as follows:
interface Value {
// You can choose checked exceptions or unchecked exceptions.
// In the paper we opted for unchecked exceptions.
// But here, I show that you can also use checked exceptions,
// if you want to ensure that you deal with exception cases.
int intValue() throws Exception;
boolean boolValue() throws Exception;
}
class VInt implements Value {
int x;
public VInt(int x) {
this.x = x;
}
public int intValue() throws Exception {
return x;
}
public boolean boolValue() throws Exception {
throw new Exception();
}
public String toString() {
return Integer.valueOf(x).toString();
}
}
class VBool implements Value {
boolean b;
public VBool(boolean b) {
this.b = b;
}
public int intValue() throws Exception {
throw new Exception();
}
public boolean boolValue() throws Exception {
return b;
}
public String toString() {
return Boolean.valueOf(b).toString();
}
}
Then you can write the code with for Object Algebras as follows:
// Integer Expressions
interface IntAlg<Exp> {
Exp lit(int x);
Exp add(Exp e1, Exp e2);
}
interface Exp {
Value eval() throws Exception;
}
class IntEval implements IntAlg<Exp> {
public Exp lit(int x) {
return () -> new VInt(x);
}
public Exp add(Exp e1, Exp e2) {
return () -> new VInt(e1.eval().intValue() + e2.eval().intValue());
}
}
// Boolean Expressions
interface BoolAlg<Exp> extends IntAlg<Exp> {
Exp bool(boolean b);
Exp iff(Exp e1, Exp e2, Exp e3);
}
class BoolEval extends IntEval implements BoolAlg<Exp> {
public Exp bool(boolean b) {
return () -> new VBool(b);
}
public Exp iff(Exp e1, Exp e2, Exp e3) {
return () -> e1.eval().boolValue() ? e2.eval() : e3.eval();
}
}
public class Main {
public static <Exp> Exp m(IntAlg<Exp> alg) {
return alg.add(alg.lit(4),alg.lit(3));
}
public static <Exp> Exp m2(BoolAlg<Exp> alg) {
return alg.iff(alg.bool(false),m(alg),alg.bool(true));
}
public static void main(String[] args) {
Exp e = m(new IntEval());
try {
System.out.println(e.eval());
} catch (Exception exp) {
System.out.println("Ops! There is an error...");
}
Exp e2 = m2(new BoolEval());
try {
System.out.println(e2.eval());
} catch (Exception exp) {
System.out.println("Ops! There is an error...");
}
}
}
As you can see in the definition of the main, we need to eventually deal with the exceptions. As a matter of fact, if you wanted to deal more nicely with errors, you should change the code in the interpreter to catch the exceptions there, and then give error messages like "You cannot add a boolean to an integer..." and so on. That does involve extra code (which is not so related to the main point of the technique), but it can be easily done.
Also note that I'm using lambdas to avoid some boilerplate with anonymous classes. In Java anonymous classes with a single method implementation can be written as a lambda.
If you do not like code using exceptions, and you'd prefer a more functional-style approach, you could also use Java's Optional class to model failure (just as you'd do in a functional language like Haskell, say). In such case, you could have the following:
// Values
interface Value {
Optional<Integer> intValue();
Optional<Boolean> boolValue();
}
// The expression interface
interface Exp {
Optional<Value> eval();
}
Where we now use Optional instead of making the operations partial. The Java code (without pattern matching) with this option will not be so nice, but maybe with the new pattern matching features it won't be that bad.
I have the following function in my OperationWalker:
public override void VisitDynamicInvocation(IDynamicInvocationOperation operation)
{
var memberReferenceOp = (IDynamicMemberReferenceOperation)operation.Operation;
switch (memberReferenceOp.Instance.Type)
{
case INamedTypeSymbol type:
{
var memberName = memberReferenceOp.MemberName;
var members = type.GetMembers(memberName);
if (members.Length > 1)
{
// WHAT DO I DO HERE ???
}
else
{
Result.Add((IMethodSymbol)members[0]);
}
break;
}
case IDynamicTypeSymbol dynamicType:
Unresolved.Add((operation.Syntax, memberReferenceOp.MemberName));
break;
}
}
I am clueless when a method on a normal type (non dynamic) is called with a dynamic parameter and there is a choice of target methods with the same name. E.g.:
class A
{
public void Get(int i){}
public void Get(string s){}
public void Get(object o){}
public void Get(double s, int precision){}
}
...
dynamic x = ...;
A a;
a.Get(x)
In this case any of the first 3 A.Get methods may be called, depending on the actual type of x. But not the fourth method.
Is there a way in Roslyn to get this information? Specifically in this example, I would like to get the symbols for first 3 Get methods.
The logic is non trivial, because one needs to take into account:
Default parameters, so just counting the arguments may not be enough
Type conversions
Visibility Scope
Number of arguments
Parameters may be passed using the named syntax in arbitrary order
Combining it all together we get non trivial logic. Is there anything in the SemanticModel or anywhere else to help get the answer?
I figured it out and it is straightforward - SemanticModel.GetSymbolInfo. When there is exact match its Symbol property returns it. When there are multiple candidates, as may be the case when one of the passed arguments is dynamic, then the property CandidateSymbols holds all the options.
I have not tested it with extension methods, so it is possible there is a gap there.
For example I'm having a class with three overloaded methods like this:
class MyClass
{
int sum(int i)
{
// Method implementation.
}
int sum(string x)
{
// Method implementation.
}
int sum(object o)
{
// Method implementation.
}
}
My question is when I call the sum method of MyClass by passing any value (integer, string or object) it should invoke only third method (with object type input parameter)
class MainClass
{
static void Main(string[] args)
{
MyClass obj = new MyClass();
obj.sum(10);
obj.sum("X")
}
}
You said "without type casting" but you can't, because you need some way to indicate to the compiler which version to call, and the runtime uses the type it sees to do that bit. Boxing the int as an object means the compiler will pick the object version
sum(1);//call int version
sum((object)1); //call object version
sum((string)(object)"1"); //call string version
sum((object)(int)(object)1); //call object version
First of all, let me say that if you sometimes want to call one version of the sum function when working with ints and sometimes want to call another, overloading probably isn't the right tool to use. Overloading works best when you are implementing conceptually the same operation for a number of different types, and you want the compiler to figure out automatically which function is the right one to call for each type; if you need more manual control over which function is called, you're probably better off using different names.
That said, if you're sure that this is what you want to do, you could implement the overloaded version for object in terms of another function in the public interface, as in:
class MyClass
{
int sum(int i)
{
// Method implementation.
}
int sum(string x)
{
// Method implementation.
}
int sum(object o)
{
sum_object(o);
}
int sum_object(object o)
{
// Method implementation for objects
}
}
Then, when you want to apply the object version to int and string objects, you just call sum_object directly instead.
I want to change the global variable in a function where a local variable of same is already present.
int x=10; //global variable
void fun1()
{
fun2(5);
}
void fun2(int x)
{
x=7; //here i want that this statement assigns the value 7 to the global x
}
Just qualify it with this. It's a pretty common pattern, particularly for constructors:
public class Player
{
private readonly string name;
public Player(string name)
{
this.name = name;
}
}
While I view it as acceptable if your parameter really is meant to be a new value for the field (potentially in a method which creates a new instance based on the current one and the new value for the single field, for example), I would try to avoid it in general, just from a readability perspective. Of course, the names of your private fields are an implementation detail, but when reading the code for the method, it's confusing to have two different concepts represented by the same variable name.
Rename the local parameter value.
Like Yuriy Vikulov said.
this.x for non-static variables
int x=10; //global variable
void fun1()
{
fun2(5);
}
void fun2(int lx)
{
x=7; //if you want 7
x=lx; //if you want the paramValue
}
this.x for non-static classes
NameClass.x for static variables
I'm new to C++/CLI and I'm wondering what is "best practice" regarding managed type data members. Declaring as handle:
public ref class A {
public:
A() : myList(gcnew List<int>()) {}
private:
List<int>^ myList;
};
or as a value:
public ref class B {
private:
List<int> myList;
};
Can't seem to find definitive advice on this.
When writing managed C++ code, I'm in favor of following the conventions used by the other managed languages. Therefore, I'd go with handles for class-level data members, and only use values (stack semantics) where you'd use a using statement in C#.
If your class member is a value, then replacing the object entirely means that the object would need a copy constructor defined, and not many .NET classes do. Also, if you want to pass the object to another method, you'll need to use the % operator to convert from List<int> to List<int>^. (Not a big deal to type %, but easy to forget, and the compiler error just says it can't convert List<int> to List<int>^.)
//Example of the `%` operator
void CSharpMethodThatDoesSomethingWithAList(List<int>^ list) { }
List<int> valueList;
CSharpMethodThatDoesSomethingWithAList(%valueList);
List<int>^ handleList = gcnew List<int>();
CSharpMethodThatDoesSomethingWithAList(handleList);
It all depends on the lifetime. When you have a private member which lives exactly as long as the owning class, the second form is preferable.
Personally, I would use the second form. I say this because I use frameworks that are written by other teams of people, and they use this form.
I believe this is because it is cleaner, uses less space, and is easier for the non-author to read. I try to keep in mind that the most concise code, while still being readable by someone with minimal knowledge of the project is best.
Also, I have not encountered any problems with the latter example in terms of readability across header files, methods, classes, or data files ...etc
Though I'm FAR from an expert in the matter, that is what I prefer. Makes more sense to me.
class AlgoCompSelector : public TSelector {
public :
AlgoCompSelector( TTree *tree = 0 );
virtual ~AlgoCompSelector(){ /* */ };
virtual void Init(TTree *tree);
virtual void SlaveBegin(TTree *tree);
virtual Bool_t Process(Long64_t entry);
virtual void Terminate();
virtual Int_t Version() const { return 1; }
void setAlgo( Int_t idx, const Char_t *name, TTree* part2, TTree* part3 );
void setPTthres( Float_t val );
void setEthres( Float_t val );
private:
std::string mAlgoName[2]; // use this for the axis labels and/or legend labels.
TTree *mPart1;
TTree *mPart2[2], *mPart3[2]; // pointers to TTrees of the various parts
TBranch *mPhotonBranch[2]; // Used branches
TClonesArray *mPhotonArray[2]; // To point to the array in the tree
for example