What is the difference between an integer a and *(&a) ?

Question

I am not getting the difference between an integer a and *(&a) ? I mean *(&a) returns the value contained in the address of a and it's the same as a , no? So why the use of pointers in this case?

The only reason one could possibly write this is to prevent a from being stored exclusivley in a register (since it's address is taken, that is not possible).This has implications when the variable is used without initialization; it makes that less dangerous. Cf. http://stackoverflow.com/a/11965368/3150802. — Peter - Reinstate Monica, Nov 05 '15 at 05:55

haccks · Answer 1 · 2015-11-05T06:19:02.513

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What is the difference between an integer a and *(&a) ?

No difference. int a; declares a as a variable of an int type. &a is the address of a is of type int *, i.e. pointer type and therefore it can be dereferenced. So, *(&a) will give the value of a.

edited Nov 05 '15 at 06:19

answered Nov 05 '15 at 05:50

haccks

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score 2 · Answer 2 · answered Nov 05 '15 at 05:56

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As long as a is a variable, there's no difference, they're identical.

However, if you use that expression in a macro for instance, it will fail with a compiler error for temporary objects (*&(x+1)) or literals (*&5). Perhaps there's a reason for making that distinction in code.

answered Nov 05 '15 at 05:56

Blindy

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*Perhaps there's a reason for making that distinction in code*: What's that? – haccks Nov 05 '15 at 06:19
@haccks If the macro evaluates the expression twice you want to prevent it from being used on something that may have side effects. – Peter - Reinstate Monica Nov 05 '15 at 06:34
@haccks, I have no idea, but I've seen lots of weird little quirks people use in their libraries. Boost is enough to make your teeth hurt. Besides, this is the only use I can think of for the construct in question. – Blindy Nov 05 '15 at 06:43

score 0 · Answer 3 · answered Nov 05 '15 at 06:07

Take this as an example,

int a = 1;
int *ptr = &a;

ptr points to some address in memory, and the address of ptr itself, is different in memory.

Now, take a look at below drawing,

// *ptr = *(&a) 
// Get the adress of `a` and dereference that address.
// So, it gives the value stored at `a`

memory address of        a -> 0x7FFF1234
value stored at          0x7FFF1234 = 1

memory address of        ptr -> 0x7FFF4321
value stored at          0x7FFF4321 = &a = 0x7FFF1234

                                           +-------+
           +------------>  0x7FFF1234 ---->|a = 1  |
           |                               +-------+
           |              +-----------------+ 
           |              | ptr =           |    
           +--*ptr<-------| 0x7FFF1234 [&a] |<--- 0x7FFF4321       
                          +-----------------+

So, there is no difference between both of them, i.e., int a and *(&a) are same.

score 0 · Answer 4 · answered Nov 05 '15 at 06:32

More to the point, you wouldn't normally write *(&a), or *&a. This could arise as a result of macros or something, but you would normally never actually write something like that explicitly. You can take it to extremes: *&*&*&*&*&*&a works just as well. You can try it for yourself:

#include <stdio.h>

int main()
{
    int     a = 123;
    int     b = *&*&*&*&*&*&a;

    printf("%d\n", b);

    return 0;
}

What is the difference between an integer a and *(&a) ?

4 Answers4