#17 union

#17 union C Notes Vol-3 by Saurabh Shukla www.mysirg.com

#17 union By Saurabh Shukla | 2017©mysirg.com

union 

union is a keyword in C language.

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The syntax of union is very similar to structure.

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It is also used to create custom data type.

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A union is a special data type available in C that allows storing different data types in the same memory location.

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You can define a union with many members, but only one member can contain a value at any given time.

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Unions provide an efficient way of using the same memory location for multiplepurpose.

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A union is a variable that may hold (at different times) objects of different types and sizes, with the compiler keeping track of size and alignment requirements.

Defining union

Syntax of union is same as the syntax of structure.

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union { Type variableName; Type variableName; ... }; union is a keyword. is any name of your choice. It becomes the name of user defined data type. Type is any data type

Remember to put semicolon at the end of union body. You can define a union either inside a function block or outside a function. Former is known as local definition and later is known as global definition of union. Locally defined unions can only be used in the body of block. Globally defined unions can be used anywhere in the program.

Example union mydata {

#17 union C Notes Vol-3 by Saurabh Shukla www.mysirg.com

int a; float b; char c; }; Declaring variables of union

You can declare union variables either along with the union definition or afterwards. union mydata { int a; float b; char c; }m;

The variable is m of type mydata created along with the union definition. You can also declare variables any time after defining structure. union mydata { int a; float b; char c; }; int main () { union mydata u1; …

}

The variable u1 or m will be large enough to hold the largest of the three types; the specific size is implementation dependent. Anyone of these types may be assigned to u1 or m and then used in expressions, so long as the usage is consistent: the type retrieved must be the type most recently stored. It is the programmer’s responsibility to keep track of which tye is currently stored in a union; the results are implementation dependent if something is stored as one type and extracted as another.

#17 union C Notes Vol-3 by Saurabh Shukla www.mysirg.com

Accessing union variables

To access union member variables you have to use dot (.) operator (member access operator).The syntax is same as you used in structures. union mydata { int a; float b; char c; }; int main () { union mydata u1; u1.a=5; printf(“int value = %d”,u1.a);

u1.b=3.5; printf(“ \nfloat value = %f”,u1.b); u1.c=’A’; printf(“ \nchar value = %c”,u1.c);

}

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You can store only one value at a time in u1. It shares the same memory space to store value of mentioned type. So you can access value which is most recently stored.

#17 union C Notes Vol-3 by Saurabh Shukla www.mysirg.com

References: YouTube video links 

Lecture 17union in C o

https://youtu.be/IQ_SmLX_Ncc?list=PL7ersPsTyYt2Q-SqZxTA1D-melSfqBRMW

Exercise 1) List the difference between union and structure. 2) Define a union month with two variables, one can store month number another can store month name.