CODEWITHSUNDEEP
c#pointersmemory-managementreferenceref
Secret
Secret

Reputation: 2647

C# ref is it like a pointer in C/C++ or a reference in C++?

I'm working with the ref and don't understand clearly "Is it like a pointer as in C/C++ or it's like a reference in C++?"

Why did I ask such a weak question as you thought for a moment? Because, when I'm reading C#/.NET books, msdn or talking to C# developers I'm becoming confused by the following reasons:

Upvotes: 42

Views: 54288

Answers (6)

ario soleimani
ario soleimani

Reputation: 1

in c# you can check run unsafe in your project properties and then you can run this code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Exercise_01
{
    public struct Coords
    {
        public int X;
        public int Y;
        public override string ToString() => $"({X}, {Y})";
    }
    class Program
    {


        static unsafe void Main(string[] args)
        {
            int n = 0;
            SumCallByRefPointer(1, 2, &n);
            Console.Clear();
            Console.WriteLine("call by refrence {0}",n);

            n = 0;
            SumCallByValue(3, 4, n);
            Console.WriteLine("call by Value {0}", n);

            n = 0;
            SumCallByRef(5, 6, ref n);
            Console.WriteLine("call by refrence {0}", n);

            Pointer();
            Console.ReadLine();
        } 


        private static unsafe void SumCallByRefPointer(int a, int b, int* c)
        {
            *c = a + b;
        }

        private static unsafe void SumCallByValue(int a, int b, int c)
        {
            c = a + b;
        } 

        private static unsafe void SumCallByRef(int a, int b, ref int c)
        {
            c = a + b;
        }


        public static void Pointer()
        {
            unsafe
            {
                Coords coords;
                Coords* p = &coords;
                p->X = 3;
                p->Y = 4;
                Console.WriteLine(p->ToString());  // output: (3, 4)
            }
        }
    }
}

Upvotes: -1

clarkewu
clarkewu

Reputation: 11

This seems like a disposing/eventing nightmare. If I have an object who's events are registered for and pass it into a function by reference and that reference is then reallocated, the dispose should be called or the memory will be allocated until the program is closed. If the dispose is called everything registered to the objects events will no longer be registered for and everything it is registered for will no longer be registered for. How would someone keep this straight? I guess you could compare memory addresses and try to bring things back to sanity if you don't go insane.

Upvotes: 1

fredoverflow
fredoverflow

Reputation: 263118

A ref in C# is equivalent to a C++ reference:

  • Their intent is pass-by-reference
  • There are no null references
  • There are no uninitialized references
  • You cannot rebind references
  • When you spell the reference, you are actually denoting the referred variable

Some C++ code:

void foo(int& x)
{
    x = 42;
}
// ...
int answer = 0;
foo(answer);

Equivalent C# code:

void foo(ref int x)
{
    x = 42;
}
// ...
int answer = 0;
foo(ref answer);

Upvotes: 25

Maciej
Maciej

Reputation: 7961

C# has no equvalent of C++ pointers and works on references. ref adds a level of indirection. It makes value type argument a reference and when used with reference type it makes it a reference to a reference.

In short it allows to carry any changes to a value type outside a method call. For reference type it allows to replace the original reference to a totally different object (and not just change object content). It can be used if you want to re-initialize an object inside a method and the only way to do it is to recreate it. Although I would try avoid such an approach.

So to answer your question ref would be like C++ reference to a reference.

EDIT

The above is true for safe code. Pointers do exist in unsafe C# and are used in some very specific cases.

Upvotes: 1

Billy ONeal
Billy ONeal

Reputation: 106539

In C#, when you see something referring to a reference type (that is, a type declared with class instead of struct), then you're essentially always dealing with the object through a pointer. In C++, everything is a value type by default, whereas in C# everything is a reference type by default.

When you say "ref" in the C# parameter list, what you're really saying is more like a "pointer to a pointer." You're saying that, in the method, that you want to replace not the contents of the object, but the reference to the object itself, in the code calling your method.

Unless that is your intent, then you should just pass the reference type directly; in C#, passing reference types around is cheap (akin to passing a reference in C++).

Learn/understand the difference between value types and reference types in C#. They're a major concept in that language and things are going to be really confusing if you try to think using the C++ object model in C# land.

The following are essentially semantically equivalent programs:

#include <iostream>

class AClass
{
    int anInteger;
public:
    AClass(int integer)
        : anInteger(integer)
    {  }

    int GetInteger() const
    {
        return anInteger;
    }

    void SetInteger(int toSet)
    {
        anInteger = toSet;
    }
};

struct StaticFunctions
{
    // C# doesn't have free functions, so I'll do similar in C++
    // Note that in real code you'd use a free function for this.

    static void FunctionTakingAReference(AClass *item)
    {
        item->SetInteger(4);
    }

    static void FunctionTakingAReferenceToAReference(AClass **item)
    {
        *item = new AClass(1729);
    }
};

int main()
{
    AClass* instanceOne = new AClass(6);
    StaticFunctions::FunctionTakingAReference(instanceOne);
    std::cout << instanceOne->GetInteger() << "\n";

    AClass* instanceTwo;
    StaticFunctions::FunctionTakingAReferenceToAReference(&instanceTwo);
    // Note that operator& behaves similar to the C# keyword "ref" at the call site.
    std::cout << instanceTwo->GetInteger() << "\n";

    // (Of course in real C++ you're using std::shared_ptr and std::unique_ptr instead,
    //  right? :) )
    delete instanceOne;
    delete instanceTwo;
}

And for C#:

using System;

internal class AClass
{
    public AClass(int integer)
        : Integer(integer)
    {  }

    int Integer { get; set; }
}

internal static class StaticFunctions
{
    public static void FunctionTakingAReference(AClass item)
    {
        item.Integer = 4;
    }

    public static void FunctionTakingAReferenceToAReference(ref AClass item)
    {
        item = new AClass(1729);
    }
}

public static class Program
{
    public static void main()
    {
        AClass instanceOne = new AClass(6);
        StaticFunctions.FunctionTakingAReference(instanceOne);
        Console.WriteLine(instanceOne.Integer);

        AClass instanceTwo  = new AClass(1234); // C# forces me to assign this before
                                                // it can be passed. Use "out" instead of
                                                // "ref" and that requirement goes away.
        StaticFunctions.FunctionTakingAReferenceToAReference(ref instanceTwo);
        Console.WriteLine(instanceTwo.Integer);
    }
}

Upvotes: 44

Akash Kava
Akash Kava

Reputation: 39916

Every reference in C# is pointer to objects on heap as pointer in C++ and ref of C# is same as & in C++

The reason ref should be avoided is, C# works on fundamental that method should not change the object passed in parameter, because for someone who does not have source of method may not know if it will result in loss of data or not.

String a = "  A  ";
String b = a.Trim();

In this case I am confident that a remains intact. In mathematics change should be seen as an assignment that visually tells is that b is changed here by programmer's consent.

a = a.Trim();

This code will modify a itself and the coder is aware of it.

To preserve this method of change by assignment ref should be avoided unless it is exceptional case.

Upvotes: 3

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