Difference between reference and const pointer

[mailtokannan]

Can someone tell me the difference between reference and const pointer. Not in the way its used, but the way it is implemented.

My understanding

Reference - Always referes to the same location, but the content can be changed.

Const Pointer - Cannot change the location it points to, but can change the value.

Is that right?

So...

Thanks and Regards
Kannan S

[Andreas Masur]

1. Reference - Can change the referenced object
2. Const-Reference - Can not change the referenced object
3. Const-Pointer - Can change the adress it is pointing to but not the object
4. Pointer-Const - Can change the object but no the address it is pointing to
5. Const-Pointer-Const - Can neither change the address nor the object

CFoo Instance;

CFoo       &ReferenceToInstance      = Instance;       // 1.
const CFoo &ConstReferenceToInstance = Instance;       // 2.
const CFoo *pConstPointer            = &Instance;      // 3.
CFoo       *const pPointerConst      = &Instance;      // 4.
const CFoo *const pConstPointerConst = &Instance       // 5.

[Gabriel Fleseriu]

Originally posted by mailtokannan
Can someone tell me the difference between reference and const pointer. Not in the way its used, but the way it is implemented.

My understanding

Reference - Always referes to the same location, but the content can be changed.

Const Pointer - Cannot change the location it points to, but can change the value.

Is that right?

So...

Thanks and Regards
Kannan S

It is irrelevant how references are implemented by a particular compiler. They is likely to be implemented as pointers (or addresses), on the machine code level. But the Standard doesn't specify this, AFAIK.

The difference between a reference and a pointer const is conceptually, at the level of the C++ programming language, despite they behaving very alike.

A reference is the object itself and cannot be resettled (made to referre to another object) no matter what. A pointer const is simply a pointer the compiler consideres to be const.

Thake this piece of code, for example:

int a = 10;
int &r = a;
int * const p = &a;
const_cast<int *>(p) = &a + 1;

First of all r and a are one and the same object. They refere to the same memory location, as opposed to p, which referes to a memory location that contains the address of a.

The last line of code shows that you actually can change the address p points to -- at least in a way the compiler will accept. Executing the last line of code has undefined results, because, depending on the context, the compiler may choose to place p in a read-only memory area. The example is purely academical, and is a no-no for the daily coding practice.