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;Read more about this topic: Union (computer Science)
Famous quotes containing the words difference between, difference, union and/or structure:
“Man is the only animal that laughs and weeps; for he is the only animal that is struck with the difference between what things are and what they ought to be.”
—William Hazlitt (1778–1830)
“I waited and worked, and watched the inferior exalted for nearly thirty years; and when recognition came at last, it was too late to alter events, or to make a difference in living.”
—Ellen Glasgow (1873–1945)
“You can no more keep a martini in the refrigerator than you can keep a kiss there. The proper union of gin and vermouth is a great and sudden glory; it is one of the happiest marriages on earth, and one of the shortest-lived.”
—Bernard Devoto (1897–1955)
“Women over fifty already form one of the largest groups in the population structure of the western world. As long as they like themselves, they will not be an oppressed minority. In order to like themselves they must reject trivialization by others of who and what they are. A grown woman should not have to masquerade as a girl in order to remain in the land of the living.”
—Germaine Greer (b. 1939)