Difference b/w Unions and Structures...

xelios

What is the difference b/w Unions and Structures...

Mohsin Ali

CPallini

Basically the members of a union share the same memory address (while the members of a struct have distinct memory addresses). The above implies, for instance:

union MyUnion
{
int i;
float f;
};

struct MyStruct
{
int i;
float f;
};

void main()
{
MyUnion u;
MyStruct s;
u.i = 5; // this also changes u.f
s.i = 5; // this doesn't change s.f
}

and, conceptually, sizeof(MyStruct)=sizeof(int)+sizeof(float) while sizeof(MyUnion)= max(sizeof(int), sizeof(float)) :)

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Alan Balkany

With unions all members start at the same base address so they overlap. With structures, the members are "stacked", i.e. each one starts in the byte after the previous one ends.

toxcct

question before answering : did you bother searching the web/docs/msdn/google/whatever before asking that dumb question asked and answered tons of times ? the difference between unions and structures (and classes if in C++) is that all the members of the union share the same memory address. in other words, it means that they all start at the same address, and them, it also means that the size of the union equals the size of its largest member. so, the other question which usually comes after such an answer is : "but what can it be used for, then ?". good question Sherlock. in fact, it's not used much in everyday programming, especially in management applications. I used to use it when I was coding for hardware, when some component had to share the same physical address. so to represent such a material in the software, a union was exactly what was needed.

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Hamid Taebi

I think you are new to c++ so I suggest to see www.cplusplus.com and on that site you can see different union and other helpful info.

toxcct

[Message Deleted]

sudhir_Kumar

While structure enables us treat a number of different variables stored at different in memory , a union enables us to treat the same space in memory as a number of different variables. That is a Union offers a way for a section of memory to be treated as a variable of one type on one occasion and as a different variable of a different type on another occasion. Union allocates the memory equal to the maximum memory required by the member of the union but structure allocates the memory equal to the total memory required by the members. Union allocates the memory equal to the maximum memory required by the member of the union but structure allocates the memory equal to the total memory required by the members. Example: union exforsys_t { char c; int i; float f; } exforsys; Defines three elements: exforsys.c exforsys.i exforsys.f Each one with a different data type. Since all of them are referring to the same location in memory, the modification of one of the elements will affect the value of all of them. We cannot store different values in them independent from each other.

Sudhir Kumar

Hamid Taebi

It was optional no coercion because I didnt use of link for that site. ;P

DQNOK

Alan Balkany wrote:

each one starts in the byte after the previous one ends.

Too specific to be the right answer for this newbie. One poster said "conceptually" the sizes of the members sum, which is correct *CONCEPTUALLY*. We should probably point out to the original poster that in a structure, the compiler is free to add padding after ANY member (including the last member) to insure the members align correctly. That is, a structure may contain unused bytes that are there only for alignment.

struct example{
   char c;  //one byte?
   long l;  //four bytes?
};
printf( "sizeof(example) = %d", sizeof(example) );

In most compilers, this will NOT print "sizeof(example) = 5" as might be expected, since the compiler must put padding after c to insure that l aligns properly. In most modern compilers this will likely print "sizeof(example) = 8".

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