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An Insight to References in C++

, 18 Apr 2006
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How C++ Compiler handles References

Introduction

I choose to write about references in C++ because I feel most of the people have misconceptions about references. I got this feeling because I took many C++ interviews and I seldom get correct answers about references in C++.

What is meant by references in C++? A reference is generally thought of as an aliasing of the variable it refers to. I hate the definition of references being an alias of a variable in C++. In this article, I will try to explain that there is nothing known as aliasing in C++.

Background

Both in C and in C++, there are only two ways by which a variable can be accessed, passed, or retrieved. The two ways are: 

  1. Accessing/passing variable by value
  2. Accessing/Passing variable by address - In this case pointers will come into the picture

There is no 3rd way of accessing/passing variables. A reference variable is just another pointer variable which will take its own space in memory. The most important thing about the references is that it's a type of pointer which gets automatically dereferenced (by compiler). Hard to believe? Let's see....

A Sample C++ Code using References

Lets write a simple C++ code which will use references:

#include <iostream.h>
int main()
{
    int i = 10;   // A simple integer variable
    int &j = i;   // A Reference to the variable i
    
    j++;   // Incrementing j will increment both i and j.

    // check by printing values of i and j
    cout<<  i  <<  j  <<endl; // should print 11 11

    // Now try to print the address of both variables i and j
    cout<<  &i  <<  &j  <<endl; 
    // surprisingly both print the same address and make us feel that they are
    // alias to the same memory location. 
    // In example below we will see what is the reality
    return 0;
}

References are nothing but constant pointers in C++. A statement int &i = j; will be converted by the compiler to int *const i = &j; i.e. References are nothing but constant pointers. They need initialization because constants must be initialized and since the pointer is constant, they can't point to anything else. Let's take the same example of references in C++ and this time we will use the syntax that the compiler uses when it sees references.

A Sample C++ Code using References (Compiler Generated Syntax)

#include <iostream.h>
int main()
{
    int i = 10;   		// A simple integer variable
    int *const j = &i;   	// A Reference to the variable i
    
    (*j)++;   		// Incrementing j. Since reference variables are 
			// automatically dereferenced by compiler

    // check by printing values of i and j
    cout<<  i  <<  *j  <<endl; // should print 11 11
    // A * is appended before j because it used to be reference variable
    // and it should get automatically dereferenced.
    return 0;
}

You must be wondering why I skipped the printing of address from the above example. This needs some explanation. Since reference variables are automatically dereferenced, what will happen to a statement like cout << &j << endl;. The compiler will convert the statement into cout << &*j << endl; because the variable gets automatically dereferenced. Now &* cancels each other. They become meaningless and cout prints the value at j which is nothing but the address of i because of the statement int *const j = &i;.

So the statement cout << &i << &j << endl; becomes cout << &i << &*j << endl; which is similar to printing the address of i in both the cases. This is the reason behind the same address being displayed while we try to print normal variables as well as reference variables.

A Sample C++ Code using Reference Cascading

Here we will try to look at a complex scenario and see how references will work in cascading. Let's follow the code below:

#include <iostream.h>
int main()
{
    int i = 10; // A Simple Integer variable
    int &j = i; // A Reference to the variable
    // Now we can also create a reference to reference variable. 
    int &k = j; // A reference to a reference variable
    // Similarly we can also create another reference to the reference variable k
    int &l = k; // A reference to a reference to a reference variable.

    // Now if we increment any one of them the effect will be visible on all the
    // variables.
    // First print original values
    // The print should be 10,10,10,10
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;
    // increment variable j
    j++; 
    // The print should be 11,11,11,11
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;
    // increment variable k
    k++;
    // The print should be 12,12,12,12
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;
    // increment variable l
    l++;
    // The print should be 13,13,13,13
    cout<<  i  <<  ","  <<  j  <<  ","  <<  k  <<  ","  <<  l  <<endl;
    return 0;
}

A sample C++ Code Using Reference Cascading (Compiler Generated Syntax)

Here we will see if we won't depend upon the compiler to generate constant pointers in place of reference and auto dereferencing the constant pointer, we can achieve the same results.

#include <iostream.h>
int main()
{
    int i = 10;         // A Simple Integer variable
    int *const j = &i;     // A Reference to the variable
    // The variable j will hold the address of i

    // Now we can also create a reference to reference variable. 
    int *const k = &*j;     // A reference to a reference variable
    // The variable k will also hold the address of i because j 
    // is a reference variable and 
    // it gets auto dereferenced. After & and * cancels each other 
    // k will hold the value of
    // j which it nothing but address of i

    // Similarly we can also create another reference to the reference variable k
    int *const l = &*k;     // A reference to a reference to a reference variable.
    // The variable l will also hold address of i because k holds address of i after
    // & and * cancels each other.

    // so we have seen that all the reference variable will actually holds the same
    // variable address.

    // Now if we increment any one of them the effect will be visible on all the
    // variables.
    // First print original values. The reference variables will have * prefixed because 
    // these variables gets automatically dereferenced.

    // The print should be 10,10,10,10
    cout<<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;
    // increment variable j
    (*j)++; 
    // The print should be 11,11,11,11
    cout<<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;
    // increment variable k
    (*k)++;
    // The print should be 12,12,12,12
    cout<<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;
    // increment variable l
    (*l)++;
    // The print should be 13,13,13,13
    cout  <<  i  <<  ","  <<  *j  <<  ","  <<  *k  <<  ","  <<  *l  <<endl;
    return 0;
}

A Reference Takes its Own Space in Memory

We can see this by checking the size of the class which has only reference variables. The example below proofs that a C++ reference is not an alias and takes its own space into the memory.

#include <iostream.h>

class Test
{
    int &i;   // int *const i;
    int &j;   // int *const j;
    int &k;   // int *const k; 
};

int main()
{    
    // This will print 12 i.e. size of 3 pointers
    cout<<  "size of class Test = "  <<   sizeof(class Test)  <<endl;
    return 0;
}

Conclusion

I hope that this article explains everything about C++ references. However I'd like to mention that C++ standard doesn't explain how reference behaviour should be implemented by the compiler. It's up to the compiler to decide, and most of the time it is implemented as a constant pointer.

Additional Notes to Support this Article

In the discussion forums for this article, people were having concerns that References are not constant pointers but aliases. I am writing one more example to support this fact. Look carefully at the example below:

#include <iostream.h>

class A
{
public:
	virtual void print() { cout<<"A.."<<endl; }
};

class B : public A
{
public:
	virtual void print() { cout<<"B.."<<endl; }
};

class C : public B
{
public:
	virtual void print() { cout<<"C.."<<endl; }
};

int main()
{
	C c1;
	A &a1 = c1;
	a1.print(); // prints C

 	A a2 = c1;
	a2.print(); // prints A
	return 0;
}

The example using references supports the virtual mechanism, i.e. looking into the virtual pointer to get the handle to correct function pointer. The interesting thing here is how the virtual mechanism is supported by the static type which is simply an alias. Virtual mechanism is supported by dynamic information which will come into the picture only when a pointer is involved. I hope this will clarify most of the doubts.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Deepak Kumar Gupta
Architect
India India
I love programming and started (in 1995)even before I got some professional education on the same. Since then I've worked on IP network stack (written IPv6 stack and next generation TCP Stack), VoIP, IP Security (IKE / IPSec).
My preferred programming language is C++ and I always explore the ways to improve the A.I based systems using enhanced algorithms and data structures.

Comments and Discussions

 
QuestionPointer to a reference. PinmemberAnitesh Kumar4-Jun-12 1:52 
AnswerRe: Pointer to a reference. PinmemberIshanB9-Jul-12 4:18 
GeneralReference Variable Disadvantages PinmemberArchangelOfTheElectronicWorld23-Jan-08 23:11 
GeneralWords fail me Pinmembertrelliot27-Apr-06 15:29 
Generalreference != const pointer PinmemberRob Hemstede19-Apr-06 5:50 
GeneralRe: reference != const pointer PinmemberIftahh19-Apr-06 22:51 
GeneralRe: reference != const pointer PinmemberGatik G19-Apr-06 23:18 
GeneralRe: reference != const pointer PinmemberRob Hemstede19-Apr-06 23:54 
GeneralRe: reference != const pointer Pinmembernutty26-Apr-06 3:20 
GeneralJust an alias Pinmembermaihem19-Apr-06 2:56 
GeneralGreat topic, style points (2) PinmemberShawn Poulson19-Apr-06 2:17 
GeneralRe: Great topic, style points (2) PinmemberIftahh19-Apr-06 22:55 
GeneralConfusing article Pinmemberjefito15-Mar-06 3:43 
GeneralRe: Confusing article PinmemberGatik G15-Mar-06 18:14 
GeneralRe: Confusing article PinmemberIlya Lipovsky16-Mar-06 4:38 
GeneralRe: Confusing article PinmemberIlya Lipovsky16-Mar-06 6:46 
GeneralRe: Confusing article PinmemberGatik G16-Mar-06 19:13 
GeneralRe: Confusing article Pinmemberjefito21-Mar-06 13:18 
GeneralRe: Confusing article PinmemberGatik G22-Mar-06 1:06 
Hi jef,
 
I can buy your arguments only when if you can proof that compilers implements the references in any way, other than const pointers.
 
For your claim that in case of functions it might go into registers. This can be the case for simple variable also for efficiency reasons. Try to dig into assemble codes generated.
 
Finally, Standards may also not ask you how to implement virtual table in the memory but more or less it is done in a same way by most of the compilers (you can’t argue till you get proper functionality).
 
And finally aging I am sorry but I found your statement
 
Spreading confusion about C++ concepts does not do service to the audience of worldwide developers.
 
VERY outrageous. if you can prove that I am spreading confusion about C++ and audience can support that. I will withdraw this article very next day along with my written apology for this.

Regards
Gatik

GeneralRe: Confusing article Pinmemberjefito23-Mar-06 7:30 
GeneralRe: Confusing article PinmemberIlya Lipovsky24-Mar-06 9:16 
GeneralRe: Confusing article Pinmemberjefito24-Mar-06 9:58 
GeneralRe: Confusing article PinmemberIlya Lipovsky24-Mar-06 10:52 
GeneralRe: Confusing article PinmemberMatthias Becker2-Apr-06 8:45 
GeneralRe: Confusing article PinmemberGatik G5-Apr-06 18:56 
GeneralRe: Confusing article Pinmemberjefito12-Apr-06 6:03 
GeneralRe: Confusing article PinmemberGatik G18-Apr-06 19:34 
GeneralRe: Confusing article PinmemberKenGuru18-Apr-06 21:14 
GeneralRe: Confusing article PinmemberIftahh19-Apr-06 2:24 
GeneralRe: Confusing article PinmemberIlya Lipovsky28-Apr-06 11:54 
GeneralRe: Confusing article Pinmemberjefito1-May-06 7:32 
GeneralRe: Confusing article PinmemberIlya Lipovsky1-May-06 13:13 
GeneralRe: Confusing article Pinmemberjefito2-May-06 3:26 
GeneralRe: Confusing article PinmemberIlya Lipovsky2-May-06 6:15 
GeneralRe: Confusing article Pinmemberjefito2-May-06 8:00 
GeneralRe: Confusing article PinmemberIlya Lipovsky2-May-06 10:10 
GeneralRe: Confusing article Pinmemberjefito2-May-06 12:17 
GeneralRe: Confusing article PinmemberIlya Lipovsky2-May-06 13:09 
GeneralRe: Confusing article Pinmemberjefito3-May-06 11:21 
GeneralRe: Confusing article PinmemberIlya Lipovsky2-May-06 10:39 
GeneralRe: Confusing article Pinmemberjefito19-Apr-06 9:22 
GeneralRe: Confusing article PinmemberGatik G12-May-06 3:40 
GeneralRe: Confusing article Pinmemberjefito13-May-06 17:18 
GeneralRe: Confusing article PinmemberAndrone19-Apr-06 5:09 
GeneralCertain Inaccuracies Found Pinmemberilya l14-Mar-06 8:46 
GeneralRe: Certain Inaccuracies Found PinmemberPaulius Maruška14-Mar-06 12:41 
GeneralRe: Certain Inaccuracies Found PinmemberIlya Lipovsky14-Mar-06 15:15 
GeneralRe: Certain Inaccuracies Found PinmemberIlya Lipovsky14-Mar-06 15:28 
GeneralRe: Certain Inaccuracies Found PinmemberGatik G14-Mar-06 17:20 
GeneralRe: Certain Inaccuracies Found PinmemberIlya Lipovsky14-Mar-06 18:58 

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