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Unions

union

• A union is a derived data type – like a structure.
• But, members of a union share the same storage space.
• union number {

int x;

double y;

};

union number a;

• If double occupies 8 bytes and int x occupies 4 bytes, the variable a occupies only 8 bytes (not 16 bytes).
• In the variable a you can store either an int or a double.

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union

• union number {

int x; double y;

} a;

a.x = 5;

printf(“%d”, a.y); /* you will see some garbage on the screen, need not be 5.0 */

• Language does not provide with automatic type conversion.
• So, you should somehow remember which member can be accessed.

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You should remember which member has the value !

• enum flag {first, second};

typedef enum flag Flag;

union number {

int x; double y;

};

struct Numb{

Flag f;

union number a;

};

struct Numb n;

n.a.x = 5;

n.f = first;

​

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Intialization

• In a declaration, a union may be initialized only with a value of the same type as the first union member.
• union number {

int x; double y;

};

union number a = {10}; /* OK */

union number b = {10, 1.43}; /* error */

​

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Operations with unions

• Operation that can be performed are same as operations allowed with structures.
• Assignment to the same typed unions.
• Taking its address with &
• Accessing its members.
• Using sizeof operator to find a union’s size

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