Difference between Pointers and Reference in C++

Pointers and Reference looks similar but there are some difference between both of them.

POINTER

1. Its not necessary to initialize the pointer at the time of declaration. Like
   
   Code:

      
   int a = 10;
   int *P = &a; //It is not necessary 
   

   Another way is :
   
   Code:

   int a  = 10;
   int *P;
   P =  &a;  
   

2. You can create the array of Pointer.
   
3. You can assign NULL to the pointer like
   
   
   Code:

   int *P = NULL; //Valid

4. You can use pointer to pointer.

REFERENCE

1. Its necessary to initialize the Reference at the time of declaration. Like
   
   Code:

   int &a = 10;
   int &a;   //Error here but not in case of Pointer.

2. You can not create the Array of reference.
   
3. You can not assign NULL to the reference like
   
   
   Code:

   int &a = NULL; //Error

4. You can not use reference to reference.

 

Thanks.

 

good one.....but what would you say if i write in .NET or even in JAVA
FileStream fs; // (or using any other class)
What is FileStream -> a pointer or a reference ?
Can you elaborate it for other platforms ?

 

Thank you sir.

 

Reference is the other name given to the same variable. Pointer is new variable created which can contain address of another variable.

Try an experiment

Code:

# include<iostream>
using namespace std;
int main()
{

int i = 90; // variable
int &j = i;  // reference 
int *P;   //  pointer
P =&i;

cout<<endl<< "address of variable :i" <<&i;

cout<< endl << "address of reference: j" <<&j;
cout<<endl<<"address of pointer : P"<<&P;
return 0;
}
OUTPUT:
 address of variable i: i0012FEDC
address of reference j: 0012FEDC
address of pointer p: 0012FEE0

so you see the address of i and reference j is same , address of pointer P is different.

 

nice one

 

In C++ a reference variable is internally implemented as a constant pointer, and that is why it is necessary to initiallize a reference variable during declaration... as it is treated as a constant variable (pointer) internally.

 

That is incorrect, zaka. A reference is a second name (alias) for an object. You can't name something that doesn't exist, therefore you have to specify the object being aliased when you declare the reference. When a reference is passed as an argument, a copy of the objects address is passed. This is one fewer levels of indirection than when a pointer to the object is passed.

 

pointer is variable which holds the address of another variable ,another function and incomplete in C.
incomplete means ----------void *p;
it's a generic pionter ,we can not allocate memory by dynamically with this .
size of pointer is depending on the memory model.

 

very appropriately describe the difference between pointer and reference. I find it very valuable

 

In C#, the object of a Class

1. is a reference ? or
2. is a STACK to HEAP pointer ?

the objects shouldn't go into the HEAP ? if they are references of the class instances.

 

Thank you good difference...

 

pointer is not necessary to initialize the pointer at the time of declaration and reference to initialize the Reference at the time of declaration .Ok thank you.

 

Good

 

A reference is (usually) exactly the same as a pointer, from
an implementation and performance point of view. They are,
in that sense, pure syntactic sugar. The only difference is
that in some cases a reference can be optimized away.
I'm not saying they are a bad thing, they're good for what
they are designed for. It's just incorrect to suggest that
they offer a performance advantage over pointers.

If you still disagree, consider this C++ and its assembly code
(from g++ using -S flag):

Code:

#include <iostream>
using namespace std;

void f( int i, int& r, int* p ) {
    ++i;
    ++r;
    ++*p;
}

int main() {
    int i = 0, r = 1, p = 2;
    cout<< i <<' '<< r <<' '<< p <<endl;
    f( i, r, &p );
    cout<< i <<' '<< r <<' '<< p <<endl;
}

I've condensed the assembly for brevity. ## comments are mine.

Code:

...
__Z1fiRiPi:  ## f's mangled name: Z1(arbitrary) f(our name) iR(int&) iP(int*) i(int)
pushl	%ebp
movl	%esp, %ebp        
incl	8(%ebp)           ## inc value at stack offset 8     (i)
movl	12(%ebp), %eax    ## load pointer at stack offset 12 (r)
incl	(%eax)            ## inc indirect (dereference)
movl	16(%ebp), %eax    ## same as for r                   (p)
incl	(%eax)
popl	%ebp
ret
...
movl	$0, -4(%ebp)      ## Load the variables with 0, 1, and 2.
movl	$1, -8(%ebp)      ## Note their constant addresses (stack offsets).
movl	$2, -12(%ebp)     ## i is -4, r is -8, p is -12 (relative to ebp)
...
leal	-12(%ebp), %eax   ## push p's address
pushl	%eax
leal	-8(%ebp), %eax    ## push r's address (same as for p)
pushl	%eax
pushl	-4(%ebp)          ## push i (value)
call	__Z1fiRiPi        ## call f
...

 

You all are correct.
One main Difference is that Extra memory is taken by Pointer( Size depends on machine architecture i.e. 32-bit machine will be 32 bit size of pointer) But in case of reference is Only Alias for that object.

Some book written that reference is just like a constant pointer. But it's not correct. Just reference 's behabiour is like constant pointer.

 

One More use of it is to cleanup Code in place of pointer.......

 

Thanks to all.