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:
“Ice is an interesting subject for contemplation. They told me that they had some in the ice-houses at Fresh Pond five years old which was as good as ever. Why is it that a bucket of water soon becomes putrid, but frozen remains sweet forever? It is commonly said that this is the difference between the affections and the intellect.”
—Henry David Thoreau (1817–1862)
“There is all the difference in the world between the criminal’s avoiding the public eye and the civil disobedient’s taking the law into his own hands in open defiance. This distinction between an open violation of the law, performed in public, and a clandestine one is so glaringly obvious that it can be neglected only by prejudice or ill will.”
—Hannah Arendt (1906–1975)
“... as women become free, economic, social factors, so becomes possible the full social combination of individuals in collective industry. With such freedom, such independence, such wider union, becomes possible also a union between man and woman such as the world has long dreamed of in vain.”
—Charlotte Perkins Gilman (1860–1935)
“The structure was designed by an old sea captain who believed that the world would end in a flood. He built a home in the traditional shape of the Ark, inverted, with the roof forming the hull of the proposed vessel. The builder expected that the deluge would cause the house to topple and then reverse itself, floating away on its roof until it should land on some new Ararat.”
—For the State of New Jersey, U.S. public relief program (1935-1943)