Structures & Unions

What is the difference between a structure and Union

well, the books will tell you that they are very similar i.e. how to declare them, how to access the variables even declaring them

e.g.

struct example{
char var1;
char var2;
};

union next_eg{
char var3;
char var4;
};

struct example test;
union next_eg test2;

test.var1 = 'a';
test.var2 = 'b';

test2.var3 = 'c';
test2.var4 = 'd';

however, struct test has the size of 2 char whereas union test2 has the size of 1 char only.

this is because a union will take the size of the largest variable.

hence, if there were a int declared in the union then the size of that union will be 2 bytes, since int is 2 bytes long and char is 1 byte long.

refer to above example.



this also means that for a struct, you can use the variables at any time. for the union you can only use 1 of them at any one time.

clear?

it appears that the benefit of using unions is that if you only use 1 variable at any time, then you will save space.

feedback is welcome.

>>however, struct test has the size of 2 char whereas union test2 has the size of 1 char only.

Not totally true as the struct or union may be padded or 'byte aligned'. You can never assume sizeof(STRUCT)==sum(sizeof(elements of STRUCT)).

>>hence, if there were a int declared in the union then the size of that union will be 2 bytes, since int is 2 bytes long and char is 1 byte long.

Again not totally true on all OS and systems. int's being 4 bytes, UNICODE strings 2bytes ect.


Uses of unions are limited in my experience.

One good use is to return from a function.

Code:

---

typedef union
{
    int          iErrorCode;
    RETURN_BMP  BMPResultValues;// a structure
    RETURN_JPG  JPGResultValues;// a structure
    RETURN_TIFF  TIFFResultValues;// a structure
} UNION_EXAMPLE;

UNION_EXAMPLE  My_Function(int iData, ect);

---

If the function generates an error it can return ONLY the error in int form (iErrorCode) or if it works it can return ONE whole filled structure.
It allows a function to act differently depending on the data sent to it.

The variable filled remains the same allowing some more 'reusablility' (if that is a word).