Union (computer Science) - Difference Between Union and Structure

Difference Between Union and Structure

A union is a class all of whose data members are mapped to the same address within its object. The size of an object of a union is, therefore, the size of its largest data member.

In a structure, all of its data members are stored in contiguous memory locations. The size of an object of a struct is, therefore, the size of the sum of all its data members.

This gain in space efficiency, while valuable in certain circumstances, comes at a great cost of safety: the program logic must ensure that it only reads the field most recently written along all possible execution paths. The exception is when unions are used for type conversion: in this case, a certain field is written and the subsequently read field is deliberately different.

An example illustrating this point is:

+-----+-----+ struct { int a; float b } gives | a | b | +-----+-----+ ^ ^ | | memory location: 150 154 | V +-----+ union { int a; float b } gives | a | | b | +-----+

Structures are used where an "object" is composed of other objects, like a point object consisting of two integers, those being the x and y coordinates:

typedef struct { int x; // x and y are separate int y; } tPoint;

Unions are typically used in situation where an object can be one of many things but only one at a time, such as a type-less storage system:

typedef enum { STR, INT } tType; typedef struct { tType typ; // typ is separate. union { int ival; // ival and sval occupy same memory. char *sval; } } tVal;