Renaming (aliasing) array elements C

Question

Not sure what is "good practice" or considered more "correct". I have an array, I want to access individual elements by names other than arrayname[]. I could use #defines or pointers, probably other ways also.

Example:

#define value1 myarray[1]

int myarray[SIZE];
value1 = 5;

or

int myarray[SIZE];
int *ptr;
ptr = &myarray[1];
*ptr = 5;

Seems to me the #define route is simpler and uses less memory, but could bring up a bunch of issues I am not aware of. Any insight would be great, I like to keep my code following the general accepted standards wherever possible.

*Edit:Maybe there is a better way altogether. My end goal is to get an array that will be sent out a peripheral port. However the data is comprised of very different data sets, and a single array name would not be representative of the data being assigned. My memory is quite limited so I would like to avoid double storing each value.

I feel that, if a value needs an identity distinct from all of the other values in an array, that value shouldn't be in an array in the first place. Can you provide a more concrete example that demonstrates the utility of your idea? — Kevin, Dec 14 '12 at 20:44
This will certainly become a nightmare to maintain. Why do you want to do this? — Rutix, Dec 14 '12 at 20:58

score 5 · Accepted Answer · answered Dec 14 '12 at 20:49

I propose a third option, by example:

#define VALUE1_IDX  1

int myarray[SIZE];
myarray[VALUE1_IDX] = 5;

The advantage of this over #1 is that it's still obvious that you're making use of myarray, and the advantage over #2 is, like you said, avoiding pointers. I would suspect, but haven't verified, that with optimization there is going to be no extra memory usage with option #2, despite that it seems intuitive that it would.

I think the best solution will vary with the situation, and I think either of your two options could be defensible in the right context.

Edit: To echo Kevin's comment, it's also first worth checking if it's possible to avoid using an array when you're not handling the values collectively. I realize that there are certainly situations where this does not apply, such as if you read in a long message and want to simply pull out a couple of key values.

This seems quite promising, and pretty obvious not sure why i didn't think of it. Keeping the array name general but using a descriptive index value would get me the result i desire. Thanks for your input. — abrand, Dec 14 '12 at 21:20

score 2 · Answer 2 · answered Dec 14 '12 at 21:20

2

Why not references?

int A[3];  
int& a1=A[1];

answered Dec 14 '12 at 21:20

Leonid Volnitsky

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score 1 · Answer 3 · answered Dec 14 '12 at 20:50

1

unions are another way to alias variables. But don't take this as a recommendation. They are dangerous (unportable) if you rely on the aliasing. You should only use them to optimize memory usage, for variables which you don't need at the same time.

answered Dec 14 '12 at 20:50

Sebastian

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A union won't let you alias a preexisting variable. Also using a union to make to objects alias each other can be (depending on the types) a violation of the strict aliasing rule and also is questionable due to the rule about only one member of a union being 'active' at a time. – bames53 Dec 14 '12 at 23:38

score 1 · Answer 4 · answered Dec 14 '12 at 21:49

enum {SIZE = 100};

struct Mapping {
  int unused1;
  int value1;
  int unused2;
};

Mapping& AsMapping( int(&array)[SIZE] ) {
  return *reinterpret_cast<Mapping*>(&array);
}

int arr[SIZE];
AsMapping(arr).value1 = 5;
Mapping& values = AsMapping(arr);
values.value1 = 5;

This requires next to no memory, and optimizers should be able to erase everything. Meanwhile, it is reasonably maintainable.

Setting up the alignment on your structs may be a requirement that you have to be careful of.

score 0 · Answer 5 · edited May 23 '17 at 12:08

0

You could do something like this

typedef enum
{
    foo = 0;
    // ... more named values
    bar = SIZE;
} Thing;

int value[SIZE];
value[foo] = 5;

Using an enum as an array index

edited May 23 '17 at 12:08

Community

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1

answered Dec 14 '12 at 21:00

Kane Anderson

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score 0 · Answer 6 · answered Dec 14 '12 at 22:28

The non-define method allow you a degree of type correctedness that is not available from the second.

Furthermore the actual code generated by any sane compiler will be identical for both.

All in I'd waaay prefer the 2nd as it is instantly obvious what is going on. The other bugger is that the #define could cause problems outside of the scope of the area you are actually dealing with.

Its much more sensible using the latter as it means it is less likely that someoen else will come along and mess it up at a later occasion ...

Renaming (aliasing) array elements C

6 Answers6

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