A union is a special data type available in C/C++ that enable user to store different data types in the same memory location. User can define a union with many members, but only one member can contain a value at any given time. Union thus provide an efficient way of using the same memory location for multiple types. Unions are quite similar to the structures in C but there is difference in memory allocation, memory required to store a union variable is the memory required for largest element of an union. Whereas, to store a structure variables, memory required is the sum of memory size of all members.
Union data type is quite useful in C. Unions in C are used for type-safe polymorphism. In C++'s use of classes & inheritance make the need of Union redundant.
Because of above mentioned properties Unions are mostly used for
1. Creating own Variant type
2. In embedded systems.
In the embedded realm, each bit of a register may mean different things. In such scenarios a union of a 1 byte char and a structure with 8 separate 1 bit bit-fields , allows you to either change one bit or the entire byte.
Similar examples can be found in many networking APIs for IPv4 address like
union ipv4Addr {
unsigned ipAddress;
char octets[4];
};
3. To cast pointer to an int and vice-versa
In the embedded realm, mostly the size of a pointer and an int are same. This is because both of them can fit into a CPU register. So if you want to store a value at a particular memory location, you can do something like this
typedef union {
int i;
int* iptr;
}intptr;
intptr p;
p.i = 256;
*(p.iptr)=5; /* put 5 a location 256 */
when we set p.i = 256; both the members of union has value 256, since each member share same location.
An example C code build using VS2010 IDE can be downloaded from here. In this code a student record in "data.txt" file is read and stored in linked list. Each node of list has a union to store student information. The requirement is that, at a time user want to display only one of either name, roll number, grades or result. So depending on what user want to display, data is stored in Node that store a only a particular type ( Variant ) and display only that particular type of data.
Union data type is quite useful in C. Unions in C are used for type-safe polymorphism. In C++'s use of classes & inheritance make the need of Union redundant.
Because of above mentioned properties Unions are mostly used for
1. Creating own Variant type
A variant can be thought of as a container that can hold a variable of almost any data type be it a numeric, float, string, or even an entire array. To create your own variant type, a union can be encapsulated within a structure along with a discriminator that indicates which of the fields of the union are valid.
typedef enum {EMPTY, CHAR, INT, UINT, FLOAT, DOUBLE, PTR} ValueType;
struct variant_t {
int mType;
union {
char charVal;
int intVal;
unsigned int uintVal;
float floatVal;
double doubleVal;
void* ptrVal;
};
};
it can be then used as,
/* Read a value to the variant from some record */
void getData(struct variant_t* v, int variantType)
{
v->mType = (ValueType)variantType;
switch(v->mType)
{
case CHAR:
v->charVal = ReadData(fptr);
break;
case DOUBLE:
v->doubleVal = ReadData(fptr);
break;
}
2. In embedded systems.
In the embedded realm, each bit of a register may mean different things. In such scenarios a union of a 1 byte char and a structure with 8 separate 1 bit bit-fields , allows you to either change one bit or the entire byte.
Similar examples can be found in many networking APIs for IPv4 address like
union ipv4Addr {
unsigned ipAddress;
char octets[4];
};
3. To cast pointer to an int and vice-versa
In the embedded realm, mostly the size of a pointer and an int are same. This is because both of them can fit into a CPU register. So if you want to store a value at a particular memory location, you can do something like this
typedef union {
int i;
int* iptr;
}intptr;
intptr p;
p.i = 256;
*(p.iptr)=5; /* put 5 a location 256 */
when we set p.i = 256; both the members of union has value 256, since each member share same location.
An example C code build using VS2010 IDE can be downloaded from here. In this code a student record in "data.txt" file is read and stored in linked list. Each node of list has a union to store student information. The requirement is that, at a time user want to display only one of either name, roll number, grades or result. So depending on what user want to display, data is stored in Node that store a only a particular type ( Variant ) and display only that particular type of data.
