Hi,

Any one Explain the Question

Thanks
Sundar Raja

A union is a user-defined data or class type that, at any given time, contains only one object from its list of members (although that object can be an array or a class type).

For example, you need to store color data as four 8-bit unsigned char numbers. At the same time, you have to store this color data as one 32-bit unsigned integer. Union allows to use both representation(struct with 4 unsigned char and unsigned integer) at the same time, using same block of computer's memory.

The union is declared as:

union union-type-name { union-list } union-variable;
In this form, union-type-name is optional. However, if you want to use union type in several places, it is better to use another way of enum declaration:
union union-type-name { union-list };

//... (and somewhere below)

union union-type-name union-variable;
Of course, in the second case union-type-name cannon be omitted.

Example: declaring union type for color data

source code: C++

union u_color {
// first representation (member of union)
struct s_color {
unsigned char a, b, g, r;
} uc_color;

// second representation (member of union)
unsigned int i_color;
};
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After declaration, it is possible to use union-type-name as user-defined type.

Following example demonstrates basic union usage. It reads file which contains 32 bit unsigned ints and decodes it to four 8 bit RGBA (Red, Green, Blue, Alpha-Transparency) values.

source code: C++

#include <iostream>
#include <stdio.h>
using namespace std;

union u_color {
// first representation (member of union)
struct s_color {
unsigned char a, b, g, r;
} uc_color;

// second representation (member of union)
unsigned int i_color;
};

int main(void) {
FILE *f = fopen("color.dat", "rb");
if (f == NULL)
exit(1);

fseek(f, 0, SEEK_END);
size_t size = ftell(f);
// file size in bytes
fseek(f, 0, SEEK_SET);
size /= 4;
// file size in 32bit ints

for(size_t i=0; i<size; i++)
{
u_color clr;
//reading from tile to clr.i_color
fread(&clr.i_color, sizeof(unsigned int), 1, f);

// printing from clr.uc_color to output stream
cout << "R=" << int(clr.uc_color.r) << " ";
cout << "G=" << int(clr.uc_color.g) << " ";
cout << "B=" << int(clr.uc_color.b) << " ";
cout << "A=" << int(clr.uc_color.a) << endl;
}

fclose(f);
}