Click here to Skip to main content
15,506,757 members
Articles / Programming Languages / C++
Article
Posted 9 Mar 2006

Stats

150.4K views
36 bookmarked

An Insight to References in C++

Rate me:
Please Sign up or sign in to vote.
3.52/5 (46 votes)
18 Apr 2006CPOL4 min read
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:

C++
#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)

C++
#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:

C++
#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.

C++
#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.

C++
#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:

C++
#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)


Written By
Architect
India India
A programmer by heart since 1998. Written code in C++, Java, JavaScript, Python & Ruby, Worked on Stack Development to Web Development. Data Specialist with SQL and NoSQL DBs

Comments and Discussions

 
QuestionPointer to a reference. Pin
Anitesh Kumar4-Jun-12 2:52
Anitesh Kumar4-Jun-12 2:52 
AnswerRe: Pointer to a reference. Pin
IshanB9-Jul-12 5:18
IshanB9-Jul-12 5:18 
GeneralReference Variable Disadvantages Pin
ArchangelOfTheElectronicWorld24-Jan-08 0:11
ArchangelOfTheElectronicWorld24-Jan-08 0:11 
GeneralWords fail me Pin
trelliot27-Apr-06 16:29
trelliot27-Apr-06 16:29 
Generalreference != const pointer Pin
Rob Hemstede19-Apr-06 6:50
Rob Hemstede19-Apr-06 6:50 
GeneralRe: reference != const pointer Pin
Iftahh19-Apr-06 23:51
Iftahh19-Apr-06 23:51 
GeneralRe: reference != const pointer Pin
itsdkg20-Apr-06 0:18
itsdkg20-Apr-06 0:18 
GeneralRe: reference != const pointer Pin
Rob Hemstede20-Apr-06 0:54
Rob Hemstede20-Apr-06 0:54 
GeneralRe: reference != const pointer Pin
nutty26-Apr-06 4:20
nutty26-Apr-06 4:20 
GeneralRe: reference != const pointer Pin
jefito9-Apr-21 3:39
jefito9-Apr-21 3:39 
GeneralJust an alias Pin
maihem19-Apr-06 3:56
maihem19-Apr-06 3:56 
GeneralGreat topic, style points (2) Pin
Shawn Poulson19-Apr-06 3:17
Shawn Poulson19-Apr-06 3:17 
GeneralRe: Great topic, style points (2) Pin
Iftahh19-Apr-06 23:55
Iftahh19-Apr-06 23:55 
GeneralConfusing article Pin
jefito15-Mar-06 4:43
jefito15-Mar-06 4:43 
GeneralRe: Confusing article Pin
itsdkg15-Mar-06 19:14
itsdkg15-Mar-06 19:14 
GeneralRe: Confusing article Pin
Ilya Lipovsky16-Mar-06 5:38
Ilya Lipovsky16-Mar-06 5:38 
GeneralRe: Confusing article Pin
Ilya Lipovsky16-Mar-06 7:46
Ilya Lipovsky16-Mar-06 7:46 
GeneralRe: Confusing article Pin
itsdkg16-Mar-06 20:13
itsdkg16-Mar-06 20:13 
GeneralRe: Confusing article Pin
jefito21-Mar-06 14:18
jefito21-Mar-06 14:18 
So let me get this straight. You claim that '6 out of 10 people will say references are alias', yet you cannot be bothered to define what an alias is, when asked (leaving aside the dubiousness of your '6 out of 10' claim). You claim not to care what aliases are, yet you make a big deal in your article about aliases, and that references are not aliases. Strange. You have introduced a term that is not defined by the Standard, and then proceed to make all kinds of claims about developers beliefs. Please define what an alias is, and then we can talk about whether references are aliases.

To reiterate, the Standard says two things about references -- that they are the names of objects, and that they may or may not take up space (you have read the Standard, haven't you?). Beyond that, it doesn't say anyting about implementation, since reference is a semantic concept. You claim that they always take up space ("there is no way you can have an alias in binary without occupying space"), and that is incorrect. If you force them to take up space, as when including them in a class, then sure. But when you use them as with parameter passing, then there is no need for them to take up any memory space at all. They may, for example be passed to a function in a register, and never make it to a memory location, depending on the underlying hardware architecture, and the cleverness of the compiler.

I think that it's a good thing to try to understand what goes on 'under the hood', but again, not everyone actually needs that information. You do, with the environment that you use, but many Windows programmers, for example, who are library clients (and not library designers) do not.

Again, I repeat: a reference is not a const pointer, although it may be implemented by an underlying mechanism that behaves like one. Even if it were, it would be no guarantee that it exists in memory, much like the fact that there is no guarantee that any constant that you use actually exists in memory.

I also believe that you are incorrect about my etiquette here. I read your article thinking that that I would gain some insight into references. I did not. Instead, I find an article that I found extremely confusing and unhelpful, due to introduction of undefined concepts, over-generalization from one particular implementation to all computers, and claims that go directly against what the Standard says, and and claims against what my experience tells me. It doesn't matter whether I pay for this article or not -- open forum etiquette allows me to question your claims, and ask that you back them up. Spreading confusion about C++ concepts does not do service to the audience of worldwide developers.

Jeff
GeneralRe: Confusing article Pin
itsdkg22-Mar-06 2:06
itsdkg22-Mar-06 2:06 
GeneralRe: Confusing article Pin
jefito23-Mar-06 8:30
jefito23-Mar-06 8:30 
GeneralRe: Confusing article Pin
Ilya Lipovsky24-Mar-06 10:16
Ilya Lipovsky24-Mar-06 10:16 
GeneralRe: Confusing article Pin
jefito24-Mar-06 10:58
jefito24-Mar-06 10:58 
GeneralRe: Confusing article Pin
Ilya Lipovsky24-Mar-06 11:52
Ilya Lipovsky24-Mar-06 11:52 
GeneralRe: Confusing article Pin
Matthias Becker2-Apr-06 9:45
Matthias Becker2-Apr-06 9:45 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.